Development of localized electrochemical deposition process for the fabrication of on machine micro EDM electrode

... bio-MEMS micro component fabrication, especially in the area of fabrication of on- machine noncircular microelectrodes for micro- EDM process Moreover, from fabrication time and economic point of view... section describes different micro- EDM electrode fabrication process In the second section, fabrication process related to non-circular electrode fabrication and the role of LECD process in these... on) Fabrication processes involved like photolithography and non- conventional machining Micro EDM, an efficient solution for the fabrication of these micro parts Tool handling and fabrication

DEVELOPMENT OF LOCALIZED ELECTROCHEMICAL DEPOSITION PROCESS FOR THE FABRICATION OF ON-MACHINE MICRO-EDM ELECTRODE MOHAMMAD AHSAN HABIB NATIONAL UNIVERSITY OF SINGAPORE 2010 DEVELOPMENT OF LOCALIZED ELECTROCHEMICAL DEPOSITION PROCESS FOR THE FABRICATION OF ON-MACHINE MICRO-EDM ELECTRODE MOHAMMAD AHSAN HABIB (B.Sc. in Mechanical Engineering, BUET) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2010 Acknowledgements Acknowledgements First, I show my heartiest gratitude to the most gracious and the most merciful ALLAH (S.W.T.) who has given me the strength and ability to write this thesis; without His order and His help, it would have been impossible to end my project and write this doctoral thesis. I would like to express my deepest and heartfelt gratitude and appreciation to my supervisor Professor Mustafizur Rahman for his valuable guidance, continuous support and encouragement throughout my research work. His comments and advice during the research has contributed immensely towards the success of this work. In addition, his patient guidance and suggestions have also helped me in learning more. I also want to take this opportunity to show my sincere thanks to the National University of Singapore (NUS) for providing me a research scholarship and to Advanced Manufacturing Lab (AML) and Micro Fabrication Lab for the state of the art facilities and support, without which the present work would not be possible. My special thanks go to Dr. Tanveer Saleh from Mikrotools for his continuous mental and technical supports and suggestions. My thanks also go to Mr. Tan Choon Huat, Mr. Lim Soon Cheong, Mr. Lee Chiang Soon and Mr. Wong Chian Long from AML for their support. i Acknowledgements I would like to offer my appreciation for the support and encouragement during various stages of this research work to my lab mates and friends. My appreciation goes to Ms. Sze Wei Gan, Mr. Muhammad Pervej Jahan, Mr. Abu Bakar Md Ali Asad, Mr. Indraneel Biswas and many more. My heartfelt gratitude goes to my parents and my younger brother for their support, encouragement and best wishes as always by praying to ALLAH (S.W.T.) for my real success. Last but not the least, I would also like to convey my sincere gratitude to my loving wife for her inspiration and support throughout, as always, by praying to ALLAH (S.W.T.) for my success in the world and the hereafter. I shall be ever grateful to them for their kind support. ii Table of Contents Table of Contents Acknowledgements .......................................................................................................... i  Table of Contents .......................................................................................................... iii  List of Figures ............................................................................................................. viii  List of Tables ................................................................................................................ xv  Nomenclatures.............................................................................................................. xvi  Summary ................................................................................................................... xviii  Chatper 1  Introduction ............................................................................................... 1  1.1  Background of this study .................................................................................. 2  1.2  Role of micro-EDM in micro-feature fabrication ............................................ 5  1.3  Challenges involved in the fabrication of micro-feature using micro-EDM .... 5  1.4  Need for on-machine fabrication of micro-EDM non-circular electrode ......... 6  1.5  Significance of the research.............................................................................. 7  1.6  Research objectives .......................................................................................... 8  1.7  Organization of thesis ..................................................................................... 10  Chatper 2  2.1  Literature Review .................................................................................... 13  Micro-EDM Electrode fabrication process .................................................... 16  2.1.1  Reverse EDM (REDM) process .............................................................. 16  2.1.2  Rapid Prototyping (RP) process .............................................................. 18  2.1.3  Etching technology ................................................................................. 20  2.1.4  Conventional machining technology ...................................................... 20  iii Table of Contents 2.2  Micro-EDM complex electrode fabrication process ...................................... 21  2.2.1  LIGA process .......................................................................................... 21  2.2.2  Material deposition processes ................................................................. 22  2.3  Study of existing LECD process .................................................................... 23  2.3.1  Effect of different parameters ................................................................. 23  2.3.2  Process control system ............................................................................ 28  2.3.3  Process modeling .................................................................................... 29  2.4  Concluding Remarks ...................................................................................... 30  Chatper 3  Development and performance study of LECD process ......................... 33  3.1  Introduction .................................................................................................... 33  3.2  Concept of new LECD and EDM process...................................................... 34  3.3  Development of LECD and EDM combined setup ........................................ 35  3.3.1  Development of LECD sub-setup ........................................................... 37  3.3.2  Development of micro-EDM sub setup .................................................. 43  3.4  Performance study of the LECD process ....................................................... 44  3.4.1  Experimental plan and conditions ........................................................... 44  3.4.2  Effect of Plating Solution Concentration and Organic Additives ........... 49  3.4.3  Deposition height study .......................................................................... 50  3.4.4  Deposition microstructure study ............................................................. 51  3.5  Concluding remarks........................................................................................ 56  iv Table of Contents Chatper 4  Modeling for fabrication of micro electrodes by LECD ......................... 58  4.1  Introduction .................................................................................................... 58  4.2  Theory ............................................................................................................ 59  4.2.1  Concept of new LECD ............................................................................ 59  4.2.2  Mechanism of new LECD process.......................................................... 59  4.3  Simulation plan and Experimental setup ........................................................ 66  4.3.1  4.4  Simulation and experimental plan........................................................... 66  Effect of different LECD parameters ............................................................. 67  4.4.1  Effect of pulse voltage amplitude ........................................................... 69  4.4.2  Effect of pulse voltage frequency ........................................................... 71  4.4.3  Effect of pulse voltage duty ratio ............................................................ 73  4.4.4  Effect of electrode effective gap distance ............................................... 75  4.5  Concluding remarks........................................................................................ 77  Chatper 5  Control for LECD micro electrode fabrication process .......................... 79  5.1  Introduction .................................................................................................... 79  5.2  Determine of the initial growth height ........................................................... 80  5.2.1  Operating in the higher deposition region............................................... 80  5.2.2  Seal the leak for the electrolyte ............................................................... 81  5.2.3  Determination of limit of the initial growth by FLUENT analysis......... 82  5.3  Design of an open loop control system for LECD process ............................ 85  v Table of Contents 5.4  Design of a closed loop control system for LECD process ............................ 87  5.4.1  5.5  Controller gain optimization ................................................................... 94  Comparison of open and close loop implemented algorithm ......................... 95  5.5.1  Comparison on monitoring current density profile ................................. 96  5.5.2  Comparison of deposition height and its repeatability............................ 98  5.6  Concluding remarks...................................................................................... 100  Chatper 6  Performance analysis of LECD electrode in micro-EDM application.. 101  6.1  Introduction .................................................................................................. 101  6.2  Parameter influencing the micro-EDM process ........................................... 102  6.3  Experimental conditions and procedures...................................................... 103  6.3.1  EDM electrode, workpiece dielectric .................................................... 103  6.3.2  Experimental Procedure ........................................................................ 104  6.4  LECD electrode fabrication for micro-EDM ............................................... 106  6.5  Effect of electrode polarity ........................................................................... 107  6.6  Performance study of LECD electrode on high melting point material ....... 109  6.6.1  Effect of gap voltage ............................................................................. 110  6.6.2  Effect of capacitance ............................................................................. 111  6.7  Performance comparison of LECD electrode on various workpiece material 113  6.7.1  EDX spectrum analysis of the LECD electrode.................................... 113  6.7.2  Effect on MMR ..................................................................................... 116  vi Table of Contents 6.7.3  Effect on RWR ...................................................................................... 118  6.7.4  Effect on ASG ....................................................................................... 120  6.7.5  Effect on ATA ....................................................................................... 122  6.8  Comparative study of LECD electrode with circular electrode ................... 125  6.9  Performance comparison of LECD electrode and circular electrode for complex structure fabrication.................................................................................. 128  6.10  Concluding remarks...................................................................................... 130  Chatper 7  Conclusions, Contributions and Recommendations ............................. 132  7.1  Major findings .............................................................................................. 132  7.2  Research Contributions ................................................................................ 135  7.3  Limitations and recommendations ............................................................... 136  Chatper 8  Bibliography.......................................................................................... 139  List of publications...................................................................................................... 149  Appendix A: Solidworks design of LECD setup ........................................................ 151  vii List of Figures List of Figures Figure 1.1: Background and purpose of this study ......................................................... 4  Figure 2.1: Challenging areas for micro-EDM (Pham, et al. 2004).............................. 15  Figure 2.2: Three types of sacrificial electrode for on machine tool fabrication (Lim, et al. 2003) ........................................................................................................................ 17  Figure 2.3: (a) LECD process setup from literature study (b) new proposed setup design in order overcome the fabrication challenges .................................................... 32  Figure 3.1: (a) A simple illustration of a typical LECD setup arrangement (b) concept of the LECD setup......................................................................................................... 35  Figure 3.2: (a) Flow chart of setup development process (b) and (c) initially developed LECD setup and tank (d) modified LECD setup .......................................................... 36  Figure 3.3: Schematic diagram of LECD EDM combined process .............................. 38  Figure 3.4: Portion of X and Y shape mask fabricated by micro milling for LECD .... 38  Figure 3.5: (a) Improper positioning of the cathode and mask (b) mask is bent due to pressure of the cathode .................................................................................................. 39  Figure 3.6: Mask detecting software algorithm ............................................................ 41  Figure 3.7: Flowchart for close loop control LECD Setup ........................................... 42  Figure 3.8: (a) LECD and EDM setup (b) EDM operation is running (c) LECD operation is running ...................................................................................................... 43  Figure 3.9: SEM image of deposition (a) without and (b) with polishing .................... 46  Figure 3.10: Electrode polishing method before deposition ......................................... 46  Figure 3.11: Vickers Pyramid Diamond Indenter Indentation ...................................... 47  viii List of Figures Figure 3.12: SEM images of the deposited structure top and side view (a) and (b) with 0.04 g/l thiourea; (c) and (d) without thiourea .............................................................. 49  Figure 3.13: Deposition height for different deposition conditions (a) different applied voltage amplitude (b) frequency (c) duty ratio and (d) anode and cathode electrode gap ....................................................................................................................................... 51  Figure 3.14: Inhomogeneous structure; (a) and (b) penetration of the indenter for lower load and higher load, (c) and (d) indenter mark on workpiece for lower load and higher load ................................................................................................................................ 52  Figure 3.15: Deposition hardness for different deposition conditions (a) different applied voltage amplitude (b) frequency (c) duty ratio and (d) anode and cathode electrode gap ................................................................................................................. 53  Figure 3.16: Deposition microstructure at voltage frequency 100kHz, duty 0.33, electrode gap 350µm and amplitude level of (a) 1.0V (b) 1.5V (c) 1.6V (d) 1.8V ...... 54  Figure 3.17: Deposition microstructure at voltage amplitude 1.5V, duty 0.33, electrode gap 350µm and frequency level of (a) 70kHz (b) 85kHz (c) 100kHz (d) 130kHz ....... 55  Figure 3.18: Deposition microstructure at voltage amplitude 1.5V, frequency 100kHz, electrode gap 350µm and duty ratio level of (a) 0.2 (b) 0.33 (c) 0.4 (d) 0.5 ................ 55  Figure 3.19: Deposition microstructure at voltage amplitude 1.5V, frequency 100kHz, duty ratio 0.33 and electrode gap of (a) 350µm (b) 450µm (c) 600µm ........................ 56  Figure 4.1: (a) HP model of double later: φm, excess charge density on metal, φs excess charge density in solution (b) HP double layer: a parallel plate capacitor (c) Electrochemical cell upon application of a voltage pulse. ............................................ 60  ix List of Figures Figure 4.2: Applied pulse voltage in LECD and DL time constant effect (a) tc damping (b) tc < ton small damping (c, d) tc > ton , tc ton no ton strong damping .................. 61  Figure 4.3: (a) Showing the gap between the electrode and mask (b) SEM image showing the extra deposited material through the gap .................................................. 68  Figure 4.4: Effect of pulse voltage amplitude on deposition height (simulation and experimental)....................................................................................... 70  Figure 4.5: Effect of pulse voltage amplitude on deposition rate (simulation and experimental)....................................................................................... 71  Figure 4.6: Effect of pulse voltage frequency on deposition height (simulation and experimental)....................................................................................... 72  Figure 4.7: Effect of pulse voltage frequency on deposition rate (simulation and experimental)....................................................................................... 73  Figure 4.8: Effect of pulse voltage duty ratio on deposition height (simulation and experimental)....................................................................................... 74  Figure 4.9: Effect of pulse voltage duty ratio on deposition rate (simulation and experimental)....................................................................................... 75  Figure 4.10: Effect of gap distance on deposition height (simulation and experimental)....................................................................................... 76  Figure 4.11: Effect of gap distance on deposition rate (simulation and experimental)....................................................................................... 77  Figure 4.12: (a) LECD electrode side view (b) LECD electrode top view (c) Tree structure of deposited electrode side view (d) top view (improper deposition) ............ 78  Figure 5.1: Operating zone for LECD control .............................................................. 80  x List of Figures Figure 5.2: (a) Showing the gap between the electrode and mask (b) control is applied without initial growth height (c) control is applied after initial growth height ............. 81  Figure 5.3: Concept of FLUENT simulation ................................................................ 82  Figure 5.4: (a) flow analysis (b) grid inside the mask area (c) velocity for vertical grid line (d) velocity for the horizontal grid line .................................................................. 83  Figure 5.5: Surface plot for initial growth height for different flow rate and electrode gap ................................................................................................................................. 84  Figure 5.6: Tree structure of deposition due to force convection of electrolyte. (a) top view (b) side view ......................................................................................................... 85  Figure 5.7: Algorithm for open loop control................................................................. 86  Figure 5.8: Relation of deposition height and electrode gap ........................................ 88  Figure 5.9: Wiring diagram of a voice coil motor ........................................................ 89  Figure 5.10: Controller of the voice coil motor ............................................................ 90  Figure 5.11: Controller of the LECD process ............................................................... 91  Figure 5.12: Algorithm for close loop control .............................................................. 93  Figure 5.13: LECD system response for different proportional controller constant (a) K P = 4200 (overshoot) (b) K P = 4600 (undershoot) (c) K P = 4430 (optimize value) ....................................................................................................................................... 94  Figure 5.14: Current density profile from simulation and experimental result for the condition of 1.6V, 100 kHz, 0.33 duty and 350 µm electrode gap ............................... 96  Figure 5.15: Current density profile from open loop, close loop and without control for the condition of 1.6V, 100 kHz, 0.33 duty and 350 µm electrode gap ......................... 97  Figure 5.16: Deposition height for the open loop controller and close loop controller 98  Figure 5.17: Deposited structure for (a) open loop control (b) close loop control ....... 99  xi List of Figures Figure 6.1: Schematic diagrams of the RC type pulse generator used in this study ... 102  Figure 6.2: Measurement of (a) average spark gap (b) taper angle θ ......................... 105  Figure 6.3: (a) LECD electrode side view (b) LECD electrode top view (c) dimensions of LECD electrodes (c) EDX spectrum analysis of the LECD electrode top surface before micro EDM ...................................................................................................... 107  Figure 6.4: Effect of polarity on (a) MRR (b) RWR................................................... 109  Figure 6.5: (a) Entrance and (b) Exit side SEM image of micro hole with LECD electrode at different energy level of discharge energy .............................................. 109  Figure 6.6: Effect of gap voltage on (a) MRR (c) RWR; Effect of capacitance on (b) MRR (d) RWR ........................................................................................................... 111  Figure 6.7: Effect of gap voltage on (a) ASG (c) ATA; Effect of capacitance on (b) ASG (d) ATA .............................................................................................................. 112  Figure 6.8: LECD electrode top surface after micro-EDM on (a) stainless steel (b) copper (c) brass (d) aluminum .................................................................................... 113  Figure 6.9: EDX spectrum analysis of the LECD electrode top surface after microEDM on (a) stainless steel shown in Figure 5(a), (b) copper shown in Figure 5(b), (c) brass shown in Figure 5(c) and (d) aluminum shown in Figure 5(d) .......................... 114  Figure 6.10: Effect on MRR with the variation of voltage at a fixed capacitor value of (a) 100pf (c) 470pf (e) 2200pf. Effect on MRR with the variation of capacitor at a fixed voltage value of (b) 60V (d) 100V (f) 140V ...................................................... 117  Figure 6.11: Effect on RWR with the variation of voltage at a fixed capacitor value of (a) 100pf (c) 470pf (e) 2200pf. Effect on RWR with the variation of capacitor at a fixed voltage value of (b) 60V (d) 100V (f) 140V ...................................................... 119  xii List of Figures Figure 6.12: Effect on ASG with the variation of voltage at a fixed capacitor value of (a) 100pf (c) 470pf (e) 2200pf. Effect on ASG with the variation of capacitor at a fixed voltage value of (b) 60V (d) 100V (f) 140V ............................................................... 121  Figure 6.13: Effect on ATA with the variation of voltage at a fixed capacitor value of (a) 100pf (c) 470pf (e) 2200pf. Effect on ATA with the variation of capacitor at a fixed voltage value of (b) 60V (d) 100V (f) 140V ............................................................... 122  Figure 6.14: (a) Entrance and (b) Exit side SEM image of micro hole with LECD electrode at the discharge energy of 0.18µJ (voltage 60V and capacitance 100pf) .... 123  Figure 6.15: (a) Entrance and (b) Exit side SEM image of micro hole with LECD electrode at the discharge energy of 2.35µJ (voltage 100V and capacitance 470pf) .. 124  Figure 6.16: (a) Entrance and (b) Exit side SEM image of micro hole with LECD electrode at the discharge energy of 21.56µJ (voltage 140V and capacitance 2200pf) ..................................................................................................................................... 124  Figure 6.17: Circular copper electrode of equal LECD electrode cross sectional area ..................................................................................................................................... 125  Figure 6.18: (a) Entrance and (b) Exit side SEM image of micro hole with circular copper electrode at different energy level of discharge energy .................................. 125  Figure 6.19: Effect of gap voltage on (a) MRR (c) RWR; Effect of capacitance on (b) MRR (d) RWR ............................................................................................................ 126  Figure 6.20: Effect of gap voltage on (a) MRR (c) RWR; Effect of capacitance on (b) MRR (d) RWR ............................................................................................................ 127  Figure 6.21: (a) Circular copper micro shaft and its scanning direction (b) Entrance and exit of the micro hole fabricated by scanning EDM. ........................................... 129  xiii List of Figures Figure 6.22: Effect of gap voltage on (a) MRR (b) RWR for die sinking EDM and scanning EDM............................................................................................................. 129  Figure 6.23: (a) NUS shape deposited electrode (b) NUS shape hole was machined by NUS shape electrode with EDM discharge energy of 2.35µJ..................................... 130  Figure 7.1: New mask design for future research ....................................................... 137  Figure A.1: Schematic diagram of modified LECD setup designed in solidworks .... 151  Figure A.2: Solid works design for Outside Tank ...................................................... 152  Figure A.3: Solid works design for Inside Tank ......................................................... 152  Figure A.4: Solid works design for Mask ................................................................... 153  Figure A.5: Solid works design for Hole of Mask ...................................................... 153  xiv List of Tables List of Tables Table 3.1: Composition of the electrolyte ..................................................................... 44  Table 3.2: Properties of the LECD electrode material .................................................. 45  Table 3.3: Experimental Conditions ............................................................................. 48  Table 4.1: LECD parameter for simulation and experiments ....................................... 67  Table 6.1: Properties of the EDM workpiece material ............................................... 103  Table 6.2: Properties of the EDM oil 3 dielectric fluid............................................... 104  Table 6.3: Machining Parameters of RC Pulse generator micro-EDM for micro holes machining of LECD Electrode .................................................................................... 105  Table 6.4: The relative percentages of material from the EDX spectrum analysis of deposited structures shown in Figure 6.9 .................................................................... 115  xv Nomenclatures Nomenclatures F Load in kgf d Arithmetic mean of the two diagonals, d1 and d2 in mm for micro indenter HV Vickers hardness τC Time constant ton Pulse on time tC Double layer charging time ϕC Potential across double layer η Overpotential i Current density Z Localized electrochemical deposition rate ζ (t ) Reaction rate ϕ0 Pulse amplitude f Pulse frequency D Pulse duty ratio d gap Electrode gap distance i0 Exchange current density ρ Specific electrolyte resistivity α Leak factor n Stoichiometric number xvi Nomenclatures cDL Specific capacitance T Temperature F Faraday constant R Gas constant Hi Initial growth height Hflow Safe height for deposition from electrolyte flow M Cu Atomic weight of copper KP Close loop controller gain A Cross sectional area DCu Density of copper Km Motor constant m Motor mass b Frictional coefficient MRR Material removal rate RWR Relative wear ratio ASG Average spark gap ATA Average taper angle xvii Summary Summary Currently MEMS (Micro-Electro-Mechanical Systems) and bio-MEMS components are generally produced by semiconductor processing technologies, like photolithography on silicon substrate. The mechanical properties of silicon material are unsuited for the application like microsurgery, biotechnology, fluidics or hightemperature environments. Moreover, these processes require special and tremendously expensive facilities. On the other hand, due to tool wear and breakage problems, tool based machining process such as micro milling and drilling are not always suitable for micro-fabrication of MEMS and bio-MEMS structures. Among non-conventional machining processes, micro-EDM has some advantages over other processes in fabricating 3D microstructure. However, in micro-EDM besides other problems tool handling and tool preparation are of significant importance. This study shows an effective solution in order to overcome the above challenges by introducing LECD (localized electrochemical deposition) process for fabricating on-machine micro-EDM of non-circular electrodes. A new combined LECD and EDM experimental setup, which is mounted on a multiprocess machine, has been developed in this study. Non-circular electrodes are fabricated with the help of different shapes of mask. In this context, the non-conductive mask is placed between the anode and cathode, which is immersed in a plating solution of acidified copper sulfate. This non-conductive mask is fabricated by micro milling process. The LECD is achieved by applying pulse type voltage between the anode and cathode. In this setup, the cathode is placed above the anode and mask, so xviii Summary that the deposited electrode can be used directly for EDM without changing tool orientation. A performance study has been conducted for LECD process on its height of the deposited structure and its microstructure. Results showed that the deposition height and its microstructure vary with the change of the operating parameters. In addition, a set of mathematical equations have been derived using Faraday's laws of electrolysis and Butler-Volmer equation in order to model and simulate the deposition growth rate under current experimental conditions. Mathematical simulation results are validated by the experimental results. This model gave a clear indication of the optimized operating range for LECD process. In the next stage of this study, in order to increase the aspect ratio of the microstructure an open loop controller and a close loop feedback controller has been designed and implemented for LECD process. A performance evaluation between an open loop and a close loop has been conducted and better performances have been achieved from the close loop feedback controller. In the final stage of the study, performance of the LECD electrode has been evaluated by micro-EDM machining process on different workpiece materials and the results have been compared with pure copper circular electrodes. Results showed that LECD electrode is capable of machining non-circular 3D structure on wide range of materials. This study is expected to make a significant contribution in MEMS and bio-MEMS micro component fabrication, especially in the area of fabrication of on-machine noncircular microelectrodes for micro-EDM process. Moreover, from fabrication time and economic point of view this study will be a good guide for mass production of micro components. xix Introduction Chatper 1 Introduction In the 21st century, new micro-fabrication processes are being investigated worldwide to build micro electromechanical structures such as gears, springs, helices and columns. However, huge difficulties and challenges need to be solved in order to optimize the process operating parameters and to make them viable for the manufacturing industries. These optimization processes require simplifying the complex set of technical units into apparently straightforward units, theoretical predication as well as its experimental validation. In order to overcome the challenges, it requires proper understanding of the process requirements, setting the criteria for mechanical system, mechanical design, fabrication and assembly of the mechanical structure, developing electronic circuits and control systems. To develop the intelligence of the control unit, it is required to know the physics behind the process and the requirement and the capability of the machine in order to handle the process. Presently, lithography technology is commonly used in micro-fabrication or micromachining activities mainly to develop thin and thick film fabrication in semiconductor industries. Although this process is beneficial for mass production and miniaturization, equipment used in this process is expensive and it is applicable to limited material such as silicon. Moreover, due to tool wear and breakage problems, tool based machining process such as micro milling and drilling are not always suitable for micro-fabrication of MEMS (Micro-Electro-Mechanical Systems) structures. Another micro-fabrication method is non-conventional machining process; 1 Introduction like micro ultrasonic machining, laser beam machining, and focused ion beam machining, and micro electrical discharge machining (micro-EDM). Besides other techniques, micro-EDM has become one of the most accepted advanced manufacturing technologies in micro-level. This chapter will provide background of this study, a brief overview of the microfabrication process by micro-EDM and its challenging areas. Among all challenging areas, more attention will be given to micro-EDM on machine non-circular tool fabrication process by localized electrochemical deposition (LECD). In addition, the significance of this research work will be elaborated in this chapter followed by the objectives and the scope of this work and finally a brief overview on the organization of this research proposal. 1.1 Background of this study Now-a-days fabrication of products and its miniaturization with broad range of materials enable micro-systems technology to enhance health care, quality of life, to attain new technological breakthrough and to cover engineering applications with environment friendly & energy saving practices. Currently, state of the art fabrication techniques refer to the fabrication of components and parts for Micro-ElectroMechanical Systems (MEMS), sub miniature actuators & sensors, components for biomedical devices, high precision equipment, components for advanced communication technology, long micro-channels for lab-on-chips, shape memory alloy ‘stents’, fluidic graphite channels for fuel cell applications and many more (Corbett 2000) (Lang 1999) (Madou 1997) (Weck 1997). The more recent trends have 2 Introduction furnished that the drive has gone beyond the little earlier challenge of precision and minuteness in dimension to a new level where components of same precision and invisible dimensions are demanded to be machined on tough materials with lower cost. Semiconductor processing technologies like photolithography on silicon substrate are used for fabricating MEMS components (Meeusen 2003) (Schoth 2005). The material properties of silicon often do not meet the requirement of recent applications of these micro parts, because they require high quality structure and capability to withstand high strength. Such applications are in microsurgery, biotechnology, fluidics and environments of high-temperature (Kuo 2003). Moreover, photolithography technique is not capable of fabricating high aspect ratio microstructure (Okuyama 1998) (Rajurkar 2000). On the other hand, LIGA process (from the German: Lithographie Galvanformung und Abformung – a combination of lithography, electroplating and molding) can fabricate high aspect ratio components with sub-micron structure using the synchrotron radiation process and focused ion beam machining process. However, LIGA requires special and extremely expensive facilities like a synchrotron system and require fabrication of expensive masks, which are not economical for micro parts fabrication in laboratory scale and fabrication industries (Ananthakrishnan 2003) (Okuyama 1998). Non conventional micromachining technology such as micro-turning, micro-grinding, micro-EDM and micro-ECM (electro chemical machining) have many advantages in productivity, efficiency, flexibility and cost effectiveness and consequently these non conventional methods have been applied to a variety of substrates and materials to fabricate micro structures (Schoth 2005) (Fang 2006) (Li 2006) (Yu 1998) (Zhao 3 Introduction 2004). Among the non-conventional micromachining techniques, micro-EDM has provided an efficient solution for machining hard conductive materials and fabricating complex cross-sectional structures. In order to fabricate these complex cross-sectional structure effectively, non-circular electrode is required, which is one of the challenges in micro-EDM area. To overcome this challenge this study focused on the development of LECD process in order to fabricate non-circular electrodes. Figure 1.1 shows the background information behind this study. Micro parts fabrication for MEMS and Bio MEMS (microsurgery, biotechnology, fluidics and so on) Fabrication processes involved like photolithography and non- conventional machining Micro EDM, an efficient solution for the fabrication of these micro parts Tool handling and fabrication of non circular tool are challenges in Micro-EDM On-machine electrode fabrication by LECD can be a good solution to overcome these challenges Figure 1.1: Background and purpose of this study 4 Introduction 1.2 Role of micro-EDM in micro-feature fabrication Micro-EDM is a non-conventional, thermo-electric process in which the workpiece and electrode are separated by a specific small gap. A spark is applied between them to remove the material from the workpiece through melting and evaporation. These electrical discharges melt and vaporize tiny amounts of work material, which are then driven out and flushed away by the dielectric. It involves almost negligible amount of force interaction between the tool and workpiece and capable of machining wide range of conductive materials irrespective of toughness. Moreover, it has the ability to manufacture complex shapes with high accuracy. In addition, for manufacturing micro-features and parts with sub-micrometer size, micro-EDM plays a significant role in manufacturing industries. 1.3 Challenges involved in the fabrication of micro-feature using micro-EDM Until now, micro-EDM is usually performed using conventional EDM machines. However, due to product miniaturization as well as the recent improvements of these machines to accommodate these micro-manufacturing requirements, four major challenges have arisen; handling, electrode and workpiece preparation, machining process and measurement (Pham, et al. 2004). Among these, tool handling and electrode and workpiece preparation are most important challenges in microfabrication. For this reason, the focus of this thesis is to solve these two major issues. The tool handling can be solved by on-machine electrode fabrication process and non- 5 Introduction circular electrode can be fabricated by LECD process, which is most likely only process that can fabricate non-circular as well as circular electrode. 1.4 Need for on-machine fabrication of micro-EDM non-circular electrode Depending on the type of electrode and electrode/workpiece movement, many variations of micro-EDM technologies are used for manufacturing micro-features. These include micro-wire EDM, micro-EDM die-sinking, micro-EDM drilling, and micro-EDM milling (Masuzawa 2000) (Pham, et al. 2004). Micro-EDM die-sinking is a more effective technique than trajectory micro-EDM. In micro-EDM die-sinking, more than one electrode is required when fabricating high-accuracy micro-components (Kim 2005). Generally, those electrodes can be produced in advance by micro-milling, micro-turning, or micro-grinding (Masuzawa 2000). However, there are some limitations in the above techniques, such as the electrode clamping error that may result in degraded accuracy, long production times, and high production costs. To compensate for these limitations, on-machine electrode fabrication is required. In order to fabricate non-circular micro-EDM electrodes LIGA process is usually applied (Takahata 2000). Although the process can produce high aspect ratio structures with high resolution, it is rather expensive (Ananthakrishnan 2003). Moreover, the process requires special facilities and the maximum thickness is relatively small (Okuyama 1998). On the other hand, fabrication techniques related to material deposition such as low-pressure chemical vapor deposition (LPCVD) (Rausch 1993), laser-assisted chemical vapor deposition (LCVD) (Ishihara S. 1998), plasma-enhanced chemical 6 Introduction vapor deposition (PECVD) (Shizhi 1992), ultraviolet stereo lithography (Zhang 1999), spinning (Harley 2006), spraying (Hoyer 1996) and localized electrochemical deposition (LECD) (Madden 1996) (Habib 2009) are being used presently. Among all other techniques, LECD is a simple, inexpensive, reproducible, and damage-free fabrication process and capable of fabricating high aspect ratio metal structures. Moreover, various materials can be deposited using this technique, such as metal, metal alloys, conductive polymers and semiconductors on the micrometer and sub micrometer scale. 1.5 Significance of the research A new LECD experimental setup, which is mounted on a multi-process machine, has been developed in this study and that has the ability to do EDM after on-machine electrode fabrication of non-circular shape by electrochemical deposition. This newly developed setup of this present study may have significant impact on mass production in manufacturing industries. Some significant key features are stated below: • Due to on-machine fabrication of electrode, this process may fabricate noncircular electrodes at the same time it is able to reduce the tool-handling problem, which is one of the prime challenges in industries. In this regard, this study introduces a combined LECD and micro EDM setup, where non-circular shape of electrode can be fabricated in the LECD tank. This non-circularity of the electrode shape can be achieved by a non-conductive mask. 7 Introduction • In addition, the LECD non-circular EDM electrodes can be applied to fabricate different varieties of micro features for MEMS and Bio-MEMS equipments like microsurgery, biotechnology, micro scale fuel cells, micro scale pumps, micro fluidic systems and for working environments of high-temperature (Kuo 2003) (Liu 2004) as well as for micro-mold cavities fabrication. These micromold cavities require very precise machining of 3D structures on hard to machine workpiece materials (Asad 2007). • This developed process is economical for the industrial application, because it uses economical mask fabrication and it uses copper as anode material instead of platinum (Said 2003) (El-Giar 2000). To reduce the production cost copper is used instead of platinum in the current study. The effect of other materials is not discussed in this study. This is not the focus of the main study and therefore is outside the scope of this thesis. This study is limited to process development of LECD process. This is why, the effect of different mechanical properties of LECD structures are discussed in less detail. The process of fabricating the mask is very complicated and involves many engineering issues, but these are not central to this study and hence are beyond the scope of this thesis. 1.6 Research objectives This research mainly focuses to develop an elegant technology that will help to fabricate on-machine non-circular electrode in order to overcome the challenges in the field of micro-EDM. The specific objectives of this research were to: 8 Introduction 1. Develop a combined dual setup for LECD and EDM simultaneous operation. To fabricate non-circular shape of electrode, a nonconductive mask with different designs is used in this study. In order to reduce the tool-handling problem, the cathode is placed on the machine z-axis, which is above the anode electrode and the mask is placed in between them. 2. Investigate the influence of the concentration of the plating solution and the organic additives on the microstructure of the deposited electrode. In addition, performance of LECD process is evaluated by studying the growth of the deposited structure and the homogeneity of the deposited electrodes microstructure. In this study, four different parameters like voltage, frequency, duty ratio and gap between the electrodes are taken into consideration. The microstructure homogeneity of the deposited structure is evaluated by micro hardness testing. 3. Theoretical modeling and experimental investigation are conducted on the effect of different LECD parameters for fabricating variety of microstructures. In order to estimate the rate of deposition and the condition of the deposited structure, a set of mathematical relations is developed with the help of Faraday's laws of electrolysis and ButlerVolmer equation. Mathematical simulation results are validated by the experimental results. 9 Introduction 4. Develop a closed loop control system in order to increase the aspect ratio electrode. First, an open loop controller is developed and it is controlled by the growth rate equation, which is derived in the study. Open loop control may not be the best solution in these conditions; this is why a closed loop controller is also developed. In this controller, current is used as a feedback signal and deposition height is controlled by a voice coil motor. 5. The performance of LECD electrode is evaluated in this study by micro holes fabrication on high melting point material (austenitic stainless steel SUS 304) in terms of material removal rate, tool relative wear ratio, average spark gap and taper angle. Finally, the performance of the LECD electrode is also evaluated by a comparative study with a circular EDM electrode for fabrication of complex three-dimensional structure. 6. An overall comparative study is carried out on electrode fabricated by LECD in order to evaluate the performance in micro-EDM application. Machining is conducted by the LECD electrode on stainless steel, copper, aluminum and brass workpiece materials on different energy levels. The comparisons in this study are based on the analysis of material removal rate, tool relative wear ratio, average spark gap and average taper angle. 1.7 Organization of thesis This thesis comprises of seven chapters. Chapter 1 gives a brief overview of the background and concept of this study. Finally, significance of the research and the 10 Introduction objectives of this study are summarized. This chapter also outlines the organization of this dissertation. A comprehensive literature review is given in the Chapter 2, which categorized into three sections. First section describes different micro-EDM electrode fabrication process. In the second section, fabrication process related to non-circular electrode fabrication and the role of LECD process in these circumstances. Finally, extensive literature review on LECD process is discussed such as process parameter and their effect, process modeling and process control. Chapter 3 describes the development of LECD and micro-EDM combined setup. In addition, it describes the performance evaluation of LECD process by deposition growth study and microstructure homogeneity study. Chapter 4 describes the modeling and simulation of the growth of the LECD structure, which is developed with the help of Faraday's laws of electrolysis and Butler-Volmer equation. Moreover, the mathematical simulation results are compared by the experimental results and elaborate discussion are presented in this chapter. Chapter 5 presents the details design and development of an open loop and a close loop control algorithm for the control of LECD process. It also describes the implementation and the outcome of the control algorithm from the process. 11 Introduction Chapter 6 presents performance analysis of the LECD electrode on austenitic stainless steel (SUS 304) workpiece, a comparative study on four different workpiece materials, a performance comparative study of LECD electrode with circular copper electrode and a process comparative study die sinking EDM of LECD electrode with scanning EDM of a circular copper electrode in fabricating same holes or cavities. The conclusions and summary of the contributions are presented in chapter 7. In addition, some directions for future work related to this study are also presented. 12 Literature Review Chatper 2 Literature Review Micro-fabrication processes are being explored worldwide to build micro electromechanical structure for industries like aerospace, automotive, precision engineering and so on. These industries are frequently using mechanical tool-based micromachining and there have been considerable advances in the fabrication techniques, metrology and equipment technology (Chae 2005) (Dornfeld 2006). MEMS (Micro-Electro-Mechanical Systems) are a combination of mechanical elements, sensors, actuators and electronics on a common silicon substrate through the utilization of microfabrication technology. In recent years, many researchers are focusing on the development on fabrication techniques for MEMS. Integrated circuit (IC) fabrication processes are used to fabricate the MEMS electronic components and silicon micromachining processes are used to fabricate the micromechanical components, where either selected parts of the silicon wafer is etched away or new layers can be added to form mechanical and electromechanical devices (Zha 2006). In order to develop manufacturing techniques for MEMS components and to establish a suitable infrastructure of integrated circuit fabrication, billion and millions of dollars are invested for last few decades due to its demand in the world market. As it has been described in the introduction, most of these processes are silicon based fabrication process. Exorbitantly expensive semiconductor manufacturing facilities are used for fabricating MEMS based products, which has become a serious hindrance to commercialization. 13 Literature Review Miniaturization of products and launch of new technologies require variety of shapes including true 3D structures on almost every material such as metals, plastics & semiconductors. In order to fabricate this variety of structures, fabrication processes require moving parts and guiding structures and these are the demand for microfabrication processes (Rahman 2007). Precise micro fabrication is required for the fabrication of micro components such as micro scale fuel cells, micro scale pumps, micro fluidic systems (Weck 1997) (Liu 2004), as well as for the fabrication of micromold cavities for mass-production. However, the fabrication of micro-mold cavities require very precise machining of 3D structures on hard to machine workpiece materials (Asad 2007). Over the years, miniaturization in the area of micro-electro mechanical system (MEMS), and the applications of micro-features made of difficult-to-cut materials have made the micro-EDM an important and cost-effective manufacturing. Despite the number of publications appreciating the improved capabilities of micro-EDM, they are still not widely used and industrial acceptance of micro-EDM is considerably slow. This is mainly because; available machine tools and process characteristics are still not sufficiently dependable. Until recently, micro-EDM has tended to be performed using conventional EDM machines modified to accommodate the micro-manufacturing requirements and due to this lack of focused development for micro-EDM process, there exist significant number of challenges, which has been summarized in Figure 2.1 (Pham, et al. 2004). Among the challenging areas, micro-EDM process related issues are inherent to the process itself, which comes as a package with the advantages of 14 Literature Review micro-EDM, and it is practically impossible to get rid of them with the available technology and process knowledge. Figure 2.1: Challenging areas for micro-EDM (Pham, et al. 2004) The first section of this chapter provides an extended literature review of different electrode fabrication process such as reverse EDM, rapid prototyping, etching, machining and present challenges in these processes. The second section focuses on the review of micro-EDM non-circular electrode fabrication process and the role of localized electrochemical deposition (LECD) process in fabricating non-circular 15 Literature Review electrode. In the final section, extensive literature review on LECD process such as process parameter and their effect, process modeling and process control are discussed. 2.1 Micro-EDM Electrode fabrication process There are several methods of microelectrode fabrication for micro-EDM machine. This section explores those techniques to investigate the feasibility of those methods in the field of micro-EDM for fabricating micro feature. 2.1.1 Reverse EDM (REDM) process Kim, Park and Chu (2006) investigated reverse EDM to fabricate multiple electrodes with various shapes. In order to fabricate multiple electrodes on WC rods, they first prepared a copper plate on which micro holes were machined in advance. Then using reverse EDM process they fabricated the multiple electrodes on the WC rod and they machined micro holes on stainless steel workpiece with the array electrodes, which were fabricated by the reverse EDM. Finally, they determined optimum voltage and capacitance for that process. Lim, et al. (2003) showed on-machine microelectrode fabrication process with highaspect ratio. A cylindrical electrode was fabricated from an electrode thicker than the required diameter by micro-EDM process using a sacrificial electrode. Figure 2.2 shows that for this operation, they described different set-up of the sacrificial electrode 16 Literature Review like stationary sacrificial block, rotating sacrificial disk and guided running wire. They mentioned that the simplest way to machine a tool electrode is a stationary block. For the rotating sacrificial disk, the thickness of the rotating electrode was 0.5mm, and diameter 60mm and rotating speed of the disk electrode was about 90 rpm during tool fabrication. For guided running wire method, wire of diameter 0.07mm can be used as a sacrificial electrode. However, if there was a dimensional change in the sacrificial electrode, the diameter of the tool–electrode fabricated was usually unpredictable. Figure 2.2: Three types of sacrificial electrode for on machine tool fabrication (Lim, et al. 2003) Weng, et al. (2003) studied a multi-EDM grinding process to fabricate microelectrodes. Equipments such as a wire EDM machine and a traditional CNCEDM machine were used for machining microelectrodes. Rod electrodes of copper with diameter 3.0 mm were cut to be 0.15mm on wire-EDM machine at first step. EDM grinding process was used to grind microelectrodes to fine diameter below 20µm 17 Literature Review on a CNC-EDM machine at second step. For EDM grinding, rotating mechanisms are mounted on both the WEDM machine and the CNC-EDM machine. They concluded that microelectrode could be fabricated through this proposed multi-EDM process where, a single process may not achieve this desired micro-size. 2.1.2 Rapid Prototyping (RP) process Partt, et al. (1998) first introduced a rapid method for fabricating a precision electrical discharge machining (EDM) electrode. To fabricate precision micro-EDM electrodes, he suggested the following steps: a) creating a computer model of the electric discharge machining electrode; b) scaling the computer model to allow shrinkage; c) offsetting a portion of the scaled computer model in a direction normal to respective surfaces of the scaled model; d) fabricating master parts using the models made in steps (b) and (c) by a rapid prototyping technique; e) molding a flexible elastomer in the master parts to form a flexible mold; f) filling the flexible mold with an electrically conductive powder; g) cold isostatically pressing the electrically conductive powder filled mold of step (f) to form a solid electric discharge machining electrode; and h) removing the solid electric discharge machining electrode from the flexible mold. 18 Literature Review Tang, et al. (2005) applied rapid prototyping (RP) technology to fabricate an abrading tool which was used to abrade graphite EDM electrodes. In this process, the cost and cycle-time could greatly be reduced. During the abrading process, a graphite block was fixed on the worktable, which performs a circular translational motion driven by a double-eccentric mechanism, and a 3D form-abrading tool fixed on the slider feeds downward, realizing the abrading process. As a new process to fabricate EDM electrodes, it had also negative points. The main weakness of this technique was that the abrading accuracy of the electrode was limited by the eccentric radius of the translational worktable. However, the overall dimensional difference between abrading tool and electrode due to the presence of abrading gap can be compensated by subsequent EDM processes. Zhao, et al. (2003) showed selective laser sintering (SLS) was a suitable process to manufacture an EDM metal prototype directly. It was mentioned that electrode fabrication by SLS process, it was possible to achieve fine surface finish and low wear. They showed by a parametric experiment study that the wear rate of the electrode approaches to that of a general electrode, and the surface roughness of the cavity was acceptable at the same machining conditions. The preferable surface finish of cavity can be obtained using rough or semi-finish machining parameters with this kind of electrode. 19 Literature Review 2.1.3 Etching technology Weng (2004) fabricated multi-headed microelectrodes, which were machined by a combined sequence process of WEDG, ultrasonic-aided chemical etching and an electrochemical anodic etching procedure. Electrodes were continually machined by chemical etching and anodic electrochemical etching. During electrolysis, copper impurity produced on the anode was not easily removed from its matrix. An ultrasonic mechanism was utilized to agitate the ferric chloride solution to clean the surface impurity off the electrode. The removal rate of ultrasonic-aided chemical etching was higher than chemical etching. Lim, et al. (2003) used electrochemical etching as an alternative method for the simple and cheap fabrication of microelectrodes. Electrochemical etching was usually considered an appropriate method of producing sharp probes. However, it was possible for the entire shape of an electrode to be controlled under certain conditions such as the immersed length, the applied current, and the etching time. 2.1.4 Conventional machining technology Rahman, et al. (2001) and Lu, et al. (1999) used conventional material removal processes, such as turning, milling and grinding in order to fabricate microstructures by introducing a single point diamond cutter or very fine grit sized grinding wheels. These material removal processes could machine almost every material such as metals, plastics and semiconductors. There was also no limitation in machining shape, so that 20 Literature Review flat surfaces, arbitral curvatures and long shafts could be machined, which are required for the moving parts and guiding structures. On the other hand, Chae, et al. (2006) mentioned that electrode fabrication by micro machining has many challenges and limitations. 2.2 Micro-EDM complex electrode fabrication process From the review in previous section, it is clear that most of the fabrication techniques have some drawbacks and limitations in fabricating electrode. Specially, numerous challenges and limitations are present in electrode fabricated by micro machining (Chae 2006). In addition, most of the techniques involved in micro-EDM electrode fabrication process are not able to avoid clamping error in EDM machining. One of the main reasons is that not all these processes are fabricating on-machine electrodes. Although some mechanisms like RP reduce clamping error, the whole procedure is very slow relative to other procedures. Furthermore, it has the limitation in fabricating non-circular electrodes. The following sections will explore non-circular electrode fabrication processes. 2.2.1 LIGA process A high-aspect-ratio WC-Co microstructure has been fabricated by a new micro fabrication process which combines LIGA (from the German: Lithographie Galvanformung und Abformung – a combination of lithography, electroplating and molding) and micro-EDM (Takahata 1999) (Takahata 2000). LIGA fabricated 21 Literature Review electrodes of negative geometry are electroplated in a metal plate for use in the microEDM. The fabricated high-aspect-ratio WC-Co microstructures of 1 mm long with gear pattern has a variation of 4 µm in the outside diameter along its length and has high resistance to buckling and wear when used as mechanical components or tools. They showed an increase in machining efficiency from using multiple electrodes instead of a single electrode. However, their method for fabricating multiple electrodes has drawbacks. Although the LIGA process can produce high aspect ratio structures with high resolution, it is rather expensive (Ananthakrishnan 2003). Deep X-ray lithography using synchrotron radiation beam (LIGA process) can produce high-aspect ratio three-dimensional sub-micron structures with very high form accuracy. However, the process requires special facilities and the maximum thickness is relatively small (Okuyama 1998). 2.2.2 Material deposition processes Instead of using LIGA process, fabrication techniques related to material deposition such as low-pressure chemical vapor deposition (LPCVD) (Rausch 1993), laserassisted chemical vapor deposition (LCVD) (Ishihara S. 1998), plasma-enhanced chemical vapor deposition (PECVD) (Shizhi 1992), ultraviolet stereo lithography (Zhang 1999), spinning (Harley 2006), spraying (Hoyer 1996) and localized electrochemical deposition (LECD) (Madden 1996) (Habib 2009) are being used presently. Among all other techniques, LECD is a popular, cost-effective, reproducible, damage-free method and capable of fabricating non-circular electrodes directly (Hunter 1997) (Habib 2008). The detail of this process is discussed in the following section. 22 Literature Review 2.3 Study of existing LECD process About a decade ago, Madden and Hunter introduced LECD as a realistic technique for inexpensive free form micro-fabrication method (Hunter 1996). It has a huge prospective to afford solutions to a variety of challenges for the micro-fabrication of three-dimensional metal structures. In order to achieve the deposition, the electrode tip diameter of micrometer range was placed near a substrate in an electrolyte solution and an electric potential is applied between them (Hunter 1996) (Hunter 1997) (Jansson 2000). The structures were fabricated by the movements of the electrode with respect to the substrate position. A study on the effect of distance between electrode and substrate on deposition height was also carried out. The experiment shows that electrodeposition was constrained by limiting the extent of the electric field (and hence the distance between the electrode and substrate) which results in the deposition being localized. By using a glass insulated ultra-microelectrode (UME) tip (Heb 1997), the tip of an atomic force microscope (AFM) (Hunter 1997) (LaGraff 1994) and a micropipette electrode variety of microstructures were fabricated by LECD (Müller 2000). The following sections will investigate current LECD process details, like effect of experimental parameters, process control and modeling. 2.3.1 Effect of different parameters El-Giar, et al. (1997) (2000) demonstrated the fabrication of micro-meter scale copper structures for micromachining applications from different substrates in acid sulphate solutions by the process of LECD. Parameters affecting this setup were also 23 Literature Review investigated in this paper. Results from these experiments showed that fabrication of copper electrodes with micro-meter scale dimensions was possible. The factors affecting the deposited structure are electrolyte concentration (which affects structure morphology), addition agents (better structure obtained) as well as tip and substrate material (no significant changes in deposition rate). Optimum conditions for this set up was found to be an applied voltage of between 3.5 - 4.2V using a Cu2+ concentration of 0.25-1.0M and with the addition of an organic addition reagent. Schnupp, et al. (2000) studied different deposition parameters and conditions for photoresist. The detailed investigation of applied voltage, deposition time, and the composition of the aqueous photoresist solution to improve the resist properties like homogeneity and complete coverage of cavities for the PEPR 2400: 175 V at 25 s and solvent contents of 0.5% TEA, 1.7% OCT, and 1.75% NMP. This way, resist roughness, adhesion, and the complete coverage of etched cavities on a wafer 100 mm in diameter were optimized. Using these parameters, layers of 11 mm in thickness with a mean surface roughness of 250 nm were deposited. Electrochemical photoresist deposition was a promising technique in patterning three-dimensional structures in anisotropically etched silicon cavities. This technology allowed the fabrication of wafer through-hole interconnections, backside contacts to electrodes and transistors, and the development of new devices like conical coils and micro relays. 24 Literature Review Using the process of LECD, Yeo and Choo (2001) studied the effects of electrode rotation on the growth of the fabricated nickel micro-columns. Experiments were conducted on a set up whereby nickel microstructures were deposited onto copper cathodes from a nickel sulfamate plating solution, using a non-soluble anode electrode (Pt-Rh). Electrode rotation was introduced by the researchers in the hope of improving the growth rate and dimensional control of the nickel columns as well as countering the effects of inherent asymmetries on the electrode. The following results were obtained from the experiments conducted. Firstly, columns were more even and had smoother external surfaces with electrode rotation. Secondly, concentricity of the nickel columns was also improved with electrode rotation. Thirdly, electrode rotation produced nickel columns with a well defined hollow core. Lastly, effects of electrode rotation were repeatable as all nickel columns fabricated this way had hollow columns. Yeo, et al. (2002) reports on the effects of ultrasonic vibrations on the LECD process. Experiments were conducted on a set up whereby nickel microstructures were deposited onto copper cathodes from a nickel sulfamate plating solution, using a nonsoluble platinum counter electrode. The rate of deposition, concentricity and porosity of the nickel columns were investigated. The study showed that for rate of deposition, ultrasonic vibrations generally increase the growth rate and significant improvements were noted with ultrasonic frequencies beyond the system’s resonant frequency. For the case of geometric control; a column to tip ratio of 1:1 is achievable at ultrasonic vibrations above the resonant frequency. Furthermore the concentricity of the fabricated columns was improved with ultrasonic vibrations. Lastly, porosity of the 25 Literature Review deposited columns is determined by the ultrasonic vibration frequencies; from the least porous at resonant frequency to porous at any frequency higher than resonant frequency. Seol, et al. (2004) carried out experiments in this paper during LECD processes to determine the existence of a critical value for the distance between the microelectrode (cathode) and the substrate (anode). The LECD experiments were carried out in special setups that permitted X-Rays of the deposition profile of the structure produced by LECD. Analysis of the results showed that a critical value for the distance between cathode-anode exists and that this critical value is dependent upon the voltage applied. Furthermore, the morphology of the deposit changed from dense and uniform growth to porous and dramatic growth when the critical value was exceeded. Chung, et al. (2007) reports the effect of pulse current (DC) with different frequencies on the morphology and mechanical properties of nickel (Ni) films deposited by electroplating. The pulse frequency varies from 0 to 500 Hz while the duty cycle (Ton/Toff) is 1 during electroplating. In order to control the structure of the electrodeposits is to adjust the current density during dc plating or to apply a periodically changing current signal. In the first case, films of different grain sizes can be achieved, even in the nanometer range if sufficiently high current densities are applied. These nanostructured materials have recently attracted great attention due to the improvement produced specially in the mechanical response (e.g., wear resistance) 26 Literature Review of the films (Czerwinski 1998) (Gleiter 1989). In the case of a periodically changing current, i.e., periodic electrolysis, different morphologies can be obtained. It has been established that this kind of electrodeposition technique leads to beneficial morphological effects such as smoother, more uniform and more compact deposits (Popov 1999) (Mandich 2000) (Hu 2003). This being the case, if different structures showing suitable morphologies can be formed, different and maybe better mechanical responses will be found. Ibanez (2005) studied the influence of the electrodeposition parameters and applied current program and current density on the mechanical characteristics of the electrodeposited copper films. They described the relations between electrodeposition parameters and mechanical responses have been obtained for the electrodeposited thin copper films. The films with the best mechanical properties are those obtained with the dc and rectangular wave methods. On the contrary, the triangular and square wave methods lead to films with significantly worse characteristics. The hardness values obtained with the dc method were higher than those obtained by application of periodically changing current signals. However, concerning elasticity, the rectangular wave method produced better results. The adequate selection of the current density leads to deposits with even higher values of hardness and elasticity. It has also been found that the grain size of these deposits lies in the nanometer range, which permits advantage to be taken of the good properties of these kinds of nanostructures in opposition to the coarse grain materials. 27 Literature Review 2.3.2 Process control system Said (2004) demonstrated that the use of conventional feedback control in LED arrangement highly affects the repeatability of the process. The main deficiency of the conventional feedback loop was that the tip withdrawal attempts to match the speed of the growing deposit end by triggering the tip positioner at a constant speed upon a direct contact between the tip and the growing deposit. Such arrangement requires a careful estimation of the deposition growth rate and a corresponding adjustment in the tip withdrawal speed, but still may fail a complete deposition session due to changes in the growth rate altered by continuous modification in the substrate and already grown structures. The effect of the tip and deposit-surface relative velocities was studied through experimental investigation of deposit characteristics for the three possible cases where the tip withdrawal is; slower than, relatively equal to, and faster than the deposition rate. Best results were obtained when the tip withdrawal is relatively of the same magnitude of the deposition rate. To overcome difficulties encountered in the conventional feedback LED, an adaptive tip withdrawal technique has been developed and demonstrated. The technique operates by monitoring the deposition current gradient rather than the current value, and triggers tip positioning at a speed proportional to the detected current gradient. Yong, et al. (2003) presented an approach to electrochemical micromachining in which side-insulated electrode, micro gap control between the cathode and anode, and the pulsed current are synthetically utilized. A strategy for micro gap control was proposed based on the fundamental experimental behavior of electrochemical machining current 28 Literature Review with the gap variance. It consists of a pulse generator, voltage and power amplifiers, feedback circuit (sample resistance, signal converting and current amplifier circuit), and A/D and I/O interface circuits. Through the output enabling port, the output can be switched on or off at any moment easily by computer control. Based on the fundamental experimental behavior of electrochemical machining current with the gap variance, a strategy for micro gap control via following the current jump-up was presented, which constrained the machining gap within 10 and 20µm. Yeo, et al. (2000) investigated the deposition phenomena of LECD for Ni micro column structure by using open loop (without analog feedback) and close loop (with analog feedback) systems. 2.3.3 Process modeling Said (2003) investigated the feasibility of using LECD to fabricate high aspect ratio microstructures such as columns and lines. In addition, the paper attempts to verify a deposition model that was suggested that transport of depositing ions leading to deposit formation is mainly dominated by migration forces. This was achieved by comparing the proposed deposition model calculated by numerical simulation, with that of images obtained at different growth stages of the copper columns during the LECD process. Results showed that the proposed model holds true and can be used to analyze structures produced by this deposition process (LECD). Furthermore, it had been shown that LECD was capable of fabricating high aspect ratio microstructures. 29 Literature Review Said (2004) studied various interactions influencing the process mechanisms and rate were well understood through mass transfer modes as demonstrated by the agreement between simulation and experimental results of the deposition profile evolution under various process conditions. The various mechanisms controlling the process rate and characteristics were described by the mass transport of the depositing species, which had been represented by three models, depending on the LED arrangement used. Such as, tip withdrawal speed slower than the deposition rate, faster than the deposition rate, and equivalent to the deposition rate. 2.4 Concluding Remarks From the above discussion of the literature study following conclusions can be drawn: • Most of the techniques involved in micro-EDM electrode fabrication process are incapable of avoiding clamping error in EDM machining. One of the main reasons is that, not all these processes are fabricating on-line electrodes. However, rapid prototype process reduces the clamping error; the whole procedure is very slow relative to other procedures. • LIGA is capable to fabricate non-circular electrodes with high aspect ratio structures with high resolution. However, LIGA requires special and extremely expensive facilities like a synchrotron system and require fabrication of expensive masks, which in not economical fabrication of micro parts in laboratory scale and fabrication industries (Ananthakrishnan 2003) (Okuyama 1998) 30 Literature Review • In order to fabricate the non-circular electrode for micro-EDM, LECD process has the ability to minimize the existing problems. However, with current LECD process setup and procedure it is not possible to meet the challenges. This is because; most of the researches have used the copper as a cathode and the platinum as an anode electrode to fabricate a column structure in the LECD process, where the cathode is placed bottom of anode. Here, the first issue is use of platinum for mass production is not cost effective and the second issue is that if the cathode is placed as a substrate then it is very difficult to use the deposited metal for micro-EDM process. Previous studies show that the shape of the deposited metal by LECD process is always circular because it follows the shape of the anode and the shape of the anode was only circular. From the above concluding remarks of the literature studies, the key feature of the novel idea for fabricating on-machine electrode came in following manners: • The cathode electrode, where the metal will be deposited is located on the machine z-axis, which is above the anode electrode. In this way, the deposited electrode can be used directly in micro-EDM or some other applications. This helps to reduce the tool-handling problem. • In order to make the process cost effective for industrial mass production, copper is used for both anode and cathode material. • In order to bring the variation in shape to the deposited structure, a mask is placed between the anode and cathode. This mask is made of non-conductive materials and with various designs like ‘X’, ‘Y’ and ‘NUS’ are used to provide 31 Literature Review the pre-shape for the deposition structure. The reason of using shape ‘X’ and ‘Y’ is to represent non-circular shape electrode and ‘NUS’ is to represent noncircular multiple electrodes. The mask design will be prepared by micro milling process. Figure 2.3 show the structural difference between old LECD setup and new proposed setup. e− e− Cathode Anode Deposition CuSo4 Solution Cathode (a) e− Deposition Mask CuSo4 Solution Anode e− (b) Figure 2.3: (a) LECD process setup from literature study (b) new proposed setup design in order overcome the fabrication challenges 32 Development and performance study of LECD process Chatper 3 Development and performance study of LECD process 3.1 Introduction Madden and Hunter introduced localized electrochemical deposition (LECD) as a realistic technique for inexpensive free form micro-fabrication method and it has a huge prospect to afford solutions to a variety of challenges for the micro-fabrication of three-dimensional metal structures. In their process, to deposit the metal a cathode electrode tip was placed near an anode substrate in an electrolyte solution and an electric potential is applied between them (Madden 1996). By LECD, three- dimensional microstructure could be made easily. This process has advantages in fabrication time and cost compared with any other microfabrication methods. In addition, various materials such as metals, metal alloys, conductive polymer, and semiconductor can be used, and the application field is very wide. In near future, if micro systems are generalized in industry, when needs for micro parts of small quantity with various shapes are increasing, it is expected that micro electrochemical deposition is promising and powerful for micro structuring. In order to use these micro structures, fabricated by LECD process for further applications in manufacturing industries, on machine electrode fabrication is required. This chapter will present the new concept and development of LECD process combining with micro-EDM in order to fabricate on machine microelectrodes for micro-EDM. Moreover, this chapter will 33 Development and performance study of LECD process also present performance analysis of the LECD process and microstructure of the deposited electrodes along with experimental procedure and preparation. 3.2 Concept of new LECD and EDM process Electrochemical deposition in a predetermined and controlled area is known as Localized Electrochemical Deposition (LECD). When a metallic salt is dissolved in water, it dissociates to form positively charged ions. The solution that contains these charged ions is referred to as an electrolyte or a plating solution. By passing a sufficient amount of electric current through this electrolyte, one can reduce the metal ions to form solid metal. This process is most commonly referred to electroplating or electrochemical deposition. These positively charged ions can be achieved by dissolving metallic salt into water. The schematic diagram of LECD experimental setup is shown in figure 1. The acidic supper sulfate is used as an electrolyte and an anode is immersed in this electrolyte. Cathode is placed above the anode and between the anode and cathode; a non-conductive mask is located to create the complex shape of the deposition. A small constant gap is maintained between the anode and mask during deposition time. When both of the electrodes are conducted electrically, current will pass through the plating solution. The positively charged metal ions get (Cu2+) deposited as solid metal on the cathode through the non-conductive mask. A more realistic equation is Cu 2+ + 2e− → Cu ↓ (Cathode) 1 H2O → 2H+ + 2e− + O2 ↑ (Anode) 2 34 Development and performance study of LECD process   e−   Cathode Deposition Mask Anode Y CuSo4 Solution e− (a) (b) Figure 3.1: (a) A simple illustration of a typical LECD setup arrangement (b) concept of the LECD setup In this way, metal can be deposited on the cathode surface and electrode can be fabricated. In this process, the cross section of the electrode will be same as the mask. Finally, the deposited electrode can be used directly to micro EDM process without changing its orientation. 3.3 Development of LECD and EDM combined setup Based on the above concept a LECD setup was fabricated initially to validate the concept. After that, it was modified to a new one where all the necessary components for electrode fabrication and LECD control were implemented. Figure 3.2 shows the development stages of the LECD setup and it shows the initial setup and the modified setup. 35 Development and performance study of LECD process Figure 3.2: (a) Flow chart of setup development process (b) and (c) initially developed LECD setup and tank (d) modified LECD setup 36 Development and performance study of LECD process The development of the setup is divided into two sub-divisions. One is development of LECD sub-setup and another is development of micro-EDM sub-setup. Both of them are mounted on a 3-axes multi-process machine. It has the maximum travel range of 210mm (X) × 110mm (Y) × 110mm (Z). Each axis has optical linear scale with the resolution of 0.1 µm, and full closed feedback control ensures accuracy of sub-micron. In the next section, the details of the two sub-setups are discussed. 3.3.1 Development of LECD sub-setup The LECD sub-setup consists of two main parts: a cathode-electrode holder and a deposition tank (Figure 3.3). The electrode holder is attached to a voice coil motor, which has a resolution of 0.1 µm is fixed on the z-axis of the machine. Voice coil motor is capable of sensing the mask and maintaining a constant distance between the anode and cathode. It does so by giving the feedback motion after measuring the current of the system through the pico ammeter. The voice coil motor and pico ammeter are connected with PC by RS232 serial communication. The cathode electrode where deposition will take place is attached to this voice coil motor. The deposition tank consists of the anode electrode and the mask. In the deposition tank, a micrometer screw and two wedges are used to adjust the gap between the mask and anode. The mask is made from a non-conductive material like PMMA (Poly methyl methacrylate) because of its advantages over other materials; it has greater transparency, ease for fabrication, excellent alkaline and good acidic chemical resistance. The masks are machined in different kinds of cross-sectional shapes by using micro-milling process such as “X”, “Y” and “NUS” (Figure 3.4). The thickness 37 Development and performance study of LECD process of the mask was 250µm. The detailed dimensions of the mask are given in the appendix. Figure 3.3: Schematic diagram of LECD EDM combined process Figure 3.4: Portion of X and Y shape mask fabricated by micro milling for LECD 38 Development and performance study of LECD process 3.3.1.1 Development of automatic mask detection In this process, the cathode is attached with the voice coil motor. In order to achieve the deposition it is very necessary to place the cathode surface properly on mask. If the cathode is not properly come contact to the mask, then there will be a gap between them. Through this gap electrolyte will leak and the deposition will not be localized (Figure 3.5 a). On the other hand, if the cathode is pressed extra, then there will be no leak for the electrolyte. However, two problems will arise: • There will be bending inside the vertical plans of the mask and the deposition structure will be affected (Figure 3.5 b). • There is always a big chance of breakdown of the mask due to the extra pressure. Due to the thickness of 250µm of the mask, it cannot tolerate extra load. Cathode Mask Cathode Mask (a) Mask Mask (b) Figure 3.5: (a) Improper positioning of the cathode and mask (b) mask is bent due to pressure of the cathode In order to overcome these situations, an automatic mask detection algorithm is developed and implemented in the software for the modified setup. However, in the 39 Development and performance study of LECD process initially developed setup, this mask detection was done by a capacitive sensor and that process was not fully automated. In the modified setup, this mask detection work was conducted by the voice coil motor. First, the motor is started and the cathode is attached with the motor. In order to hold the cathode to its original place, motor pass a certain amount of current through the motor coil. This motor current can be measured by the motor controller and the motor controller can send the data to PC by RS232 serial communication. In the next step, when the motor starts moving towards the mask and it touches the mask then due to this the mask will try to displace the cathode position. The motor controller mechanism is such that if it detects slight displacement of the cathode, then it will pass extra current to the coil to move the cathode and to make up the offset movement. Immediately the PC can recognize that due to some obstruction, extra current is moving through motor coil. In this way, the mask position can be detected automatically. The details algorithm is given below in Figure 3.6. 3.3.1.2 Development of control feedback system In order to develop the control feedback system the PC was used as the main control unit. In addition, in order to control the voice coil motor one separate controller (Harmonica Elmo motion controller) was used. The PC and the motor controller were interfaced by RS 232 serial communication. In order to measure, the deposition current inside the tank one current measuring device was used (Keithley Pico ammeter). In the same way, the PC and the current measuring device were also interfaced by RS 232 40 Development and performance study of LECD process serial communication. In order to control the pulse power supply USB communication is used (figure 3.7). START Measure and store the motor coil current as a reference (Iref) Activate the movement from PC by RS232 serial communication Reference Current (Iref) Motor move towards mask with the minimum resolution of 0.1µm Measure of current value (I) of present position False I > Iref True Stop motor movement and store the current motor position to PC END Figure 3.6: Mask detecting software algorithm 41 Development and performance study of LECD process When the deposition takes place on cathode surface, the effective distance between the cathode and anode starts reducing. This effective distance represents the resistance between the two electrodes. This is why the current starts increasing with the decrease of resistance in a constant applied voltage. The increase of current value is easily observable in the ammeter. Finally, in order to maintain the effective distance constant as well as to keep the current value constant to its initial value, the cathode is lifted up with the help of a voice coil motor. In this way, a constant distance is maintained between the electrodes and to do this whole process a closed loop feedback system is developed. The details of the process description will be given in section 4.5.3. Figure 3.7: Flowchart for close loop control LECD Setup 42 Development and performance study of LECD process 3.3.2 Development of micro-EDM sub setup In the EDM sub-setup, a workpiece fixing table is used to fix the workpiece that is kept inside an EDM tank and the dielectric is supplied in the working area with the help of a nozzle. Finally, the deposited electrode that is attached to the voice coil motor can do the EDM without changing the tool position. The micro-EDM RC spark control was also controlled from the PC. Figure 3.8 (a) shows the EDM tank in the setup and Figure 3.8 (b) shows the EDM operation. Figure 3.8: (a) LECD and EDM setup (b) EDM operation is running (c) LECD operation is running 43 Development and performance study of LECD process 3.4 Performance study of the LECD process In this current LECD process in order to deposit the metal on the cathode surface, four different operating parameters are used. They are pulse voltage amplitude, frequency, duty ratio and the gap between the anode and cathode. The focus of the study is to verify the capability of the LECD setup in depositing metal and the quality and homogeneity of the deposited structure. In addition, the effect of the copper concentration and additive is also studied in the section. The metal deposition capability was studied by measuring the height of the deposition for a certain amount of time and the homogeneity of the deposited structure was studied by measuring the hardness by a micro indenter (micro hardness). 3.4.1 Experimental plan and conditions 3.4.1.1 Anode and cathode material and electrolyte composition The composition of the electrolyte used in the study is given in Table 3.1. The material used for anode and cathode is pure copper, so the properties of anode and cathode material is also given in Table 3.2. Table 3.1: Composition of the electrolyte Property Values Copper Sulfate ( CuSO4.5H2O ) 200 g/l Sulfuric Acid ( H2SO4 ) 1.0 M Thiourea ( (NH2)2CS ) 0.04 g/l 44 Development and performance study of LECD process Table 3.2: Properties of the LECD electrode material Property Values Material composition (Wt %) 99.9% Cu Density 8.96 gm/cm3 Melting point 1084.62°C Magnetic ordering Diamagnetic Electrical resistivity (20 °C) 16.78 nΩ.m Thermal conductivity (300 K) 401 W/m·K 3.4.1.2 Electrode preparation In order to achieve uniform deposition, proper electrode surface preparation is required. This surface preparation is required for the surface where the metal will be deposited through the mask and the exposed surface of the anode inside the solution as well. Figure 3.9 shows, if the cathode surface is not properly polished then the metal starts depositing initially on the scratch marks. Due to this reason, void space is created inside the deposited structure. In order to achieve a well-polished surface, the electrode surface was polished by successive grade of 600, 1200, 1500 and 2000 silicon carbide papers. A final fine polishing was done by 1.0 μm diamond polish on nylon cloth until a smooth and mirror surface is obtained. Figure 3.10 shows that how electrode is polished before deposition by attaching it with the spindle. The carbide paper is attached on the base of the machine and the electrode is attached to a low speed spindle. 45 Development and performance study of LECD process (a) (b) Figure 3.9: SEM image of deposition (a) without and (b) with polishing Spindle Electrode Clamp Carbide paper Base of the machine Figure 3.10: Electrode polishing method before deposition 3.4.1.3 Experimental conditions The experimental parameters for this study are given in Table 3.3. In all the experiments, the deposition time was 20 minutes. The height of the deposit is 46 Development and performance study of LECD process measured by a Keyence VHX digital microscope (VH-Z450). In order to check the repeatability of the deposition growth five samples are prepared. In addition, scanning electron microscope (SEM) (JSM-5500, JEOL Ltd.) was used to acquire better picture of the deposited structures. The hardness of the deposited structure is measured by a micro hardness tester. In this study, with Vickers indenter 1000mN stepwise load was applied in four steps. The sequence of the loads was 245.2mN, 490.3mN, 765.2mN and 980.7mN. The hardness value was calculated with the following equation 3.1. For every load, the number of hardness measured ten times. Figure 3.11: Vickers Pyramid Diamond Indenter Indentation 136 2 = 1.854 F HV = 2 d d2 F HV ( GPa ) = 0.01818 2 d 2Fsin (3.1) 47 Development and performance study of LECD process Here: F= Load in kgf d= Arithmetic mean of the two diagonals, d1 and d2 in mm HV = Vickers hardness Table 3.3: Experimental Conditions Voltage test Pulse Voltage 1.0, 1.2, 1.5, (V) 2.0, 3.0 Frequency (kHz) 100 Frequency test 1.5 70, 85, 100, 115, 130 Duty ratio (On time/ Total Duty ratio test 0.33 0.33 350µm 350µm 1.5 100 100 0.67, 0.75 Distance between the 350µm electrodes (µm) Deposition time test 1.5 0.25, 0.33, 0.5, Pulse time) Gap distance 0.33 350µm, 450µm,600µm 20 min 48 Development and performance study of LECD process 3.4.2 Effect of Plating Solution Concentration and Organic Additives (a) (b) (c) (d) Figure 3.12: SEM images of the deposited structure top and side view (a) and (b) with 0.04 g/l thiourea; (c) and (d) without thiourea The Cu2+ concentration was varied in the plating solution over the range 0.05–1.5 M to observe the effect on the deposition rate. It was observed that the concentration had no significant effect on the deposition rate but the deposited structures were irregular and highly porous when the concentration was less than 0.1 M. This is because the number of ions available for discharging is low when the concentration is low, creating a 49 Development and performance study of LECD process depletion layer just beneath the electrode. However, when the concentration is high, many ions are available for deposition, resulting in firm and consistent deposits. Similar observations were made with a different experimental setup and electrode materials as well (El-Giar 2000). We found that fine-grained and microcrystalline deposits could be achieved by adding organic substances such as thiourea to the acidic copper sulfate solution. Figure 3 shows a comparison between deposition with and without 0.04 g/l thiourea added to the CuSO4 solution. It is clear from Figure 3 (a) and (b) that the presence of thiourea improved the surface structure of the copper deposit. El-Giar (2000) studied the use of thiourea and found similar results with a different experimental setup and electrode. 3.4.3 Deposition height study Figure 3.13 shows the deposition height and its repeatability for different operating conditions. Results show that for different voltage amplitude value the deposition height increases with the increase of voltage. Similarly, for the increase of voltage duty ratio the deposition height increases. On the other hand, for frequency and gap distance between the anode and cathode, the deposition height increases with the decrease of the values. The details explanation of these results will be discussed in chapter 4. Results also show that if the deposition height is higher, then the repeatability of the condition is lesser that represents the length of the error bars in the Figures 3.13. 50 160 160 140 140 Depostiion Height (µm) Depostiion Height (µm) Development and performance study of LECD process 120 100 80 60 40 20 120 100 80 60 40 20 0 0 1.0V 1.2V 1.5V 1.6V 70kHz 1.8V 100kHz 115kHz 130kHz Applied voltage frequency (kHz) Applied voltage aplitide (V) (a) (b) 160 160 140 140 Depostiion Height (µm) Depostiion Height (µm) 85kHz 120 100 80 60 40 20 0 120 100 80 60 40 20 0 0.2 0.25 0.33 0.4 0.5 Duty ratio (ton/tperiod) (c) 350 µm 450 µm 600 µm Electrode gap (µm) (d) Figure 3.13: Deposition height for different deposition conditions (a) different applied voltage amplitude (b) frequency (c) duty ratio and (d) anode and cathode electrode gap 3.4.4 Deposition microstructure study The term “micro hardness” test usually refers to static indentations made with loads not exceeding 1000mN. The indenter is used in this study is the Vickers diamond pyramid. In order to verify the homogeneity of the microstructure of the deposition this micro hardness testing is used. In this study, stepwise load was applied. The key concept of 51 Development and performance study of LECD process this technique is that if the deposition structure is inhomogeneous then the hardness value of that structure for low load will be higher than the higher load. This is because, if a structure is inhomogeneous then porosity will be more inside the structure. When higher load is applied, then the indenter will penetrate more than desire, which reduce the hardness value. It is known that the hardness value is inversely proportional to the indenter penetration diagonal distance (from equation 3.1). Figure 3.14 shows that for inhomogeneous structure, in case of higher load the penetration diagonal distances are more due to its inhomogeneity. For this reason, the hardness value will be less in case of higher applied load. Figure 3.14: Inhomogeneous structure; (a) and (b) penetration of the indenter for lower load and higher load, (c) and (d) indenter mark on workpiece for lower load and higher load 52 Development and performance study of LECD process Figure 3.15 (a) shows that with the increase of load, the hardness value for 1.6 and 1.8V deposited structure decreases compared to other values of the voltage. These results indicated the inhomogeneity of the structures for the particular voltage values. Similar conclusion can be drawn from the figure 3.16 that the grain size is coarse for the microstructure for 1.6V and 1.8V, which is an indication of porosity beneath the surface. Whereas, for other voltage values fine grained and compact surface is 1.80 1.80 1.60 1.60 Hardness (GPa) Hardness (GPa) observed. 1.40 1.20 1.00 1.40 1.20 1.00 0.80 0.80 245.2mN 490.3mN 765.2mN 980.7mN 245.2mN Load (mN) 1.0V 1.2V 1.5V 1.6V 70kHz 1.8V 490.3mN 765.2mN Load (mN) 85kHz 1.80 1.80 1.60 1.60 1.40 1.20 1.20 1.00 0.80 0.80 0.2 490.3mN 765.2mN Load (mN) 0.25 0.33 0.4 (c) 130kHz 1.40 1.00 245.2mN 115kHz (b) Hardness (GPa) Hardness (GPa) (a) 100kHz 980.7mN 980.7mN 0.5 245.2mN 490.3mN 765.2mN Load (mN) 350 µm 450 µm 980.7mN 600 µm (d) Figure 3.15: Deposition hardness for different deposition conditions (a) different applied voltage amplitude (b) frequency (c) duty ratio and (d) anode and cathode electrode gap 53 Development and performance study of LECD process Similarly, from Figure 3.15 (b) and (c) it can be seen that frequency value 70kHz and 85kHz as well as for duty ratio 0.4 and 0.5 the hardness value decreases with the increase of load. Figure 3.17 and 3.18 also show that the grain size of microstructures is coarse. Figure 3.15 (d) and Figure 3.19 show that for 450µm and 600µm the deposition structures are homogeneous compared to 350µm gap distance. In the Figure 3.19 (c) some scratch marks are visible in the structure. These scratch marks might be due to improper polishing of the electrode surface. For 600µm, the deposition height is less and this is why the scratch mark is still visible. In Figure 3.16 to 3.19, the deposited microstructure showed their deposited height could be found in Figure 3.13. In this “micro hardness study”, the same samples were used that were obtained from the “deposition height study” (section 3.4.3). (a) (b) (c) (d) Figure 3.16: Deposition microstructure at voltage frequency 100kHz, duty 0.33, electrode gap 350µm and amplitude level of (a) 1.0V (b) 1.5V (c) 1.6V (d) 1.8V 54 Development and performance study of LECD process (a) (b) (c) (d) Figure 3.17: Deposition microstructure at voltage amplitude 1.5V, duty 0.33, electrode gap 350µm and frequency level of (a) 70kHz (b) 85kHz (c) 100kHz (d) 130kHz (a) (b) (c) (d) Figure 3.18: Deposition microstructure at voltage amplitude 1.5V, frequency 100kHz, electrode gap 350µm and duty ratio level of (a) 0.2 (b) 0.33 (c) 0.4 (d) 0.5 55 Development and performance study of LECD process (a) (b) (c) Figure 3.19: Deposition microstructure at voltage amplitude 1.5V, frequency 100kHz, duty ratio 0.33 and electrode gap of (a) 350µm (b) 450µm (c) 600µm 3.5 Concluding remarks In this chapter, efforts have been made to describe the development of a combined LECD and micro-EDM setup. Detailed discussions are also carried out on the experimental preparation, parameters details and performance study of LECD process and following conclusions can be drawn from this chapter: ™ A modified LECD experimental setup was designed from an initial design in order to deposit electrode for micro-EDM. The mask is fabricated by micro milling process, which is economical form industrial point of view. The key features that are implemented in the new modified setup are voice coil motor for detecting the mask and for feedback control and micrometer screw for controlling the gap between the anode and cathode. 56 Development and performance study of LECD process ™ The influence of the concentration of the plating solution and the organic additives on the microstructure of the deposited electrode is studied. It was found that with proper concentration of electrolyte and organic additives the microstructure could be improved. ™ Performance study of the LECD process showed that all the four process parameters like voltage, frequency, duty ratio and gap between the electrodes had significant effect on the deposition height and microstructure. Result showed that if the deposition height is higher, then repeatability of the process reduced. The homogeneity of the microstructure was estimated by the micro hardness testing. Finally, the optimum values of these parameters were found out for effective fabrication of complex-shaped deposited electrode. Inhomogeneous structures were found where the deposition rate was more. From the above study, results suggested that good quality deposition could be achieved in the condition of voltage amplitude of 1.0V to 1.5V, frequency 100 kHz to 130 kHz, duty ratio of 0.2 to 0.33 and electrode gap of 350µm to 450µm. However, in the next chapter precise operating conditions could be found, where conditions were determined by mathematical modeling and experimental results. In the next chapter, a detailed study for the modeling and simulation of deposition height and rate of microelectrodes fabricated by LECD process are presented. At the same time, in order to verify the simulated results, it is compared with experimental results. 57 Modeling for LECD micro electrode fabrication process Chatper 4 Modeling for fabrication of micro electrodes by LECD 4.1 Introduction Localized electrochemical deposition (LECD) is governed by the laws of electrolysis where by Faraday’s law, the amount of metal deposited is directly proportional to the duration of the process. Important issues in electrochemical deposition include electrode kinetics, mass transport, cell design and the structure of electrodeposits. In recent years, this established technique has found importance in the field of microfabrication (Müller 1998 ) (Oskam 1998) (Rai-Choudhury 1997) (Suda 1996 ) (Voigt 1999) and even nanofabrication (Hofmann 1998) (Arie 1998). The electronics industry widely uses electrochemical deposition for applications such as in copperprinted circuit boards, through-hole plating, multilayer read/write heads and thin film magnetic recording media (Romankiw 1988). Madden and Hunter introduced LECD as a realistic technique for inexpensive free form micro-fabrication method and it has a huge prospect to afford solutions to a variety of challenges for the micro-fabrication of three dimensional metal structures (Madden 1996). In the previous chapter, design and development of the modified LECD process was shown for further application. In this chapter the modeling of LECD is discussed which is developed with the help of Faraday's laws of electrolysis and Butler-Volmer equation. Moreover, the mathematical simulation results are verified by the experimental findings. 58 Modeling for LECD micro electrode fabrication process 4.2 Theory 4.2.1 Concept of new LECD Electrochemical deposition in a predetermined and controlled area is known as Localized Electrochemical Deposition (LECD). In current LECD setup cathode is placed above the anode and between the anode and cathode, a non-conductive mask is located to create the non-circular shape of deposition. Copper is used for anode as well as cathode material for its availability and low cost. A small constant gap is maintained between the anode and mask during deposition time. When both of the electrodes are conducted electrically, current will pass through the plating solution. The positively charged metal ions get (Cu2+) deposited as solid metal on the cathode through the nonconductive mask. 4.2.2 Mechanism of new LECD process Once a cathode and anode electrodes are immersed in a solution, an interface consists of two equal and opposite layers of charge, one on the metal (φm) and other in solution (φs) is created. This pair of charged layers, called the double layer, is equivalent to a parallel plate capacitor (figure 4.1). The variation of potential in the double layer with the distance from the electrode is linear. The capacitance of the double layer is a function of potential. On both the electrode surfaces, electro-chemical double layer forms a capacitor. This double layer is charged when a potential is applied between the two electrodes. The charging time constant (τc) for the double layer is the product of resistance (R) and capacitance (cDL). The charging current has to flow through the electrolyte, whose resistance is proportional to the length of the current path; that is, the distance between the electrodes (dgap). Therefore, resistance is the product of the 59 Modeling for LECD micro electrode fabrication process gap distance between the electrodes (dgap) and the specific electrolyte resistivity (ρ). Finally, the time constant: τ C = R × C = ρ .cDL .d gap (4.1) Figure 4.1: (a) HP model of double later: φm, excess charge density on metal, φs excess charge density in solution (b) HP double layer: a parallel plate capacitor (c) Electrochemical cell upon application of a voltage pulse. In this process pulse potential is applied to deposit the metal ions. The charging time (tc) of the double layer should be at least 4 times of the time constant, that is 98% of the pulse on time. If the duration of the pulse on time (ton) is longer than the charging time (tc), the double layer will be charged properly for metal deposition. On the other hand, if the charging time (tc) is longer than the pulse on time (ton), the double layer will not be charged sufficiently for metal deposition. Since the chemical reaction rate 60 Modeling for LECD micro electrode fabrication process is exponentially proportional to the potential drop in the double layer, metal deposition can be controlled by controlling the pulse duration. Figure 4.2: Applied pulse voltage in LECD and DL time constant effect (a) tc damping (b) tc < ton small damping (c, d) tc > ton , tc ton no ton strong damping The charging time of double layer is the time before the charged pulse potential (φC) reaches the value corresponding to the applied pulse potential (φ0) (figure 4.2 b). If the charging time is longer than the duration of the pulse on time (ton) the double layer is not completely charged and φC never reaches to φ0 (figure 4.2 c, 4.2 d). A similar phenomenon occurs after the end of the pulse. The double layer must be discharged and it takes some time before the potential drops to the value corresponding to zero value. Therefore, it takes some time before φC drops to zero. If this time is longer than the off time (toff) the double layer is not completely discharged and φC never decreases to zero. Therefore, the charged potential of a double layer at any time (t): 61 Modeling for LECD micro electrode fabrication process t − ⎛ τC ϕC = ϕ 0 ⎜ 1 − e ⎜ ⎝ ⎞ t ⎟ ≈ ϕ0 ⎟ τC ⎠ (4.2) When an electrode is made a part of an electrochemical cell through which current is flowing, its potential will differ from the equilibrium potential. If the equilibrium potential of the electrode is E and the potential of the same electrode as a result of external current flowing is E(I), then its difference is known as overpotential (η). η = E (I ) − E (4.3) The overpotential (η) is required to overcome hindrance of the overall electrode reaction, which is usually composed of the sequence of partial reactions. There are four possible partial reactions and thus four types of rate control: charge transfer, diffusion, chemical reaction and crystallization. Thus, four different kinds of overpotential are distinguished and the total over potential (η) can be considered to be composed of four components η = ηct + ηd + ηr + ηc (4.4) Here, ηct , ηd ,ηr andηc are charge transfer, diffusion, chemical reaction and crystallization overpotential. In order to complete the deposition model, and simplify the formulation of determining the deposition rate and height, the following assumptions are considered (Said 2003): • In the deposition reaction, only copper ions are deposited. 62 Modeling for LECD micro electrode fabrication process • In the electrolyte there is no concentration gradients, therefore the solution is well stirred. • The diffusivity of the reacting species is constant during the deposition and that the rate of change of shape of the deposit is slow compared with the establishment of the concentration field, and • Localized electrochemical deposition current efficiency is unity. Since there is no electrochemical reaction or metal deposition during the pulse off time and pulse off time voltage is comparatively less than pulse on time voltage. This is why, charged potential (φC) can be judged as overpotential (η). From the ButlerVolmer equation, during the pulse on-time, reaction current density (i) is: ⎡ ⎛ (1 − α ) nF ⎞ ⎛ −α nF ⎞ ⎤ i = i0 ⎢exp ⎜ η ⎟ − exp ⎜ η ⎟⎥ RT ⎝ RT ⎠ ⎥⎦ ⎢⎣ ⎝ ⎠ ⎡ ⎛ (1 − α ) nF ⎞ ⎛ −α nF ⎞ ⎤ = i0 ⎢exp ⎜ ϕC ⎟ − exp ⎜ ϕC ⎟ ⎥ RT ⎝ RT ⎠ ⎦⎥ ⎝ ⎠ ⎣⎢ (4.5) For large negative values of over potential, the Butler-Volmer equation can be simplified. As the first exponential term in the equation (corresponding to the anodic partial current) decreases while the second exponential term (corresponding to the cathodic partial current) increases and the second exponential term can be neglected. ⎛ (1 − α ) nF ⎞ ϕC ⎟ exp ⎜ RT ⎝ ⎠ ⎛ −α nF ⎞ ϕC ⎟ exp ⎜ ⎝ RT ⎠ (4.6) 63 Modeling for LECD micro electrode fabrication process Therefore, now the Butler-Volmer equation, during the pulse on-time, reaction current density (i) is: ⎛ (1 − α ) nF ⎞ ⎛ (1 − α ) nF t ⎞ i = i0 exp ⎜ ϕC ⎟ ≈ i0 exp ⎜ ϕ0 ⎟ RT RT τC ⎠ ⎝ ⎠ ⎝ (4.7) Here, i0 exchange current density, α leakage factor, F Faraday constant, R gas constant, T temperature, n the number of electrons taking part in the reduction, ϕ potential. Since the reaction rate is proportional to the reaction current density, i.e. ζ ( t ) ∞i . This can be represents as ζ (t ) = ζ (t ) = i nF (4.8) ⎛ (1 − α ) F ϕ 0 t ⎛ (1 − α ) F i0 t ⎞ i exp ⎜ ϕ 0 ⎟ = 0 exp ⎜ ⎜ RT ρ c d τ C ⎠ nF nF DL gap ⎝ RT ⎝ ⎞ ⎟⎟ ⎠ (4.9) The electrochemical reaction or deposition occurs only during on time of pulse. For this reason, the deposition rate can be calculated by integrating the reaction rate during pulse on time. Therefore, localized electrochemical deposition rate, Z: Ζ (ϕ0 , ton , t period , d gap ) = = = 1 ton ∫ ζ ( t ) dt t period 1 0 ton t period i0 ⎛ (1 − α ) F ϕ0t ⎞ ⎟⎟dt d DL gap ⎝ ⎠ ∫ nF exp ⎜⎜ RT ρ c 0 d gap f ϕ0t period i0 RT ρ cDL (1 − α ) zF 2 (4.10) ⎡ ⎛ (1 − α ) F ϕ0ton ⎞ ⎤ ⎢exp ⎜⎜ ⎟⎟ − 1⎥ ⎢⎣ ⎝ RT ρ cDL d gap ⎠ ⎥⎦ 64 Modeling for LECD micro electrode fabrication process In this current study, four experimental parameters are used to control the localized electrochemical deposition rate and the height of deposited electrode. These four experimental parameters are pulse potential amplitude (φ0), pulse frequency (f), pulse duty ratio (D) and effective gap distance between two electrodes (dgap). We know, Frequency f = 1 t period and duty ratio D = ton t D = on = ton × f ⇒ ton = ton + toff t period f Therefore, localized electrochemical deposition rate, Z: Ζ (ϕ0 , f , D, d gap ) = d gap f i0 RT ρ cDL ϕ0 (1 − α ) zF 2 ⎡ ⎛ (1 − α ) F ϕ0 D ⎞ ⎤ ⎢exp ⎜⎜ ⎟⎟ − 1⎥ ⎢⎣ ⎝ RT ρ cDL d gap f ⎠ ⎥⎦ (4.11) So the deposition height (Ht) at time t = 0, 1, 2, 3 … will be H0 = 0 ⎡ ⎛ (1 − α ) F ϕ0 D ⎢exp ⎜⎜ ⎢⎣ ⎝ RT ρ cDL (d gap − H 0 ) f ⎛ (1 − α ) F (d − H1 ) f i0 RT ρ cDL ⎡ ϕ0 D H 2 = H1 + Z 2 = H 0 + gap exp ⎜ 2 ⎢ ϕ0 (1 − α ) zF ⎢⎣ ⎜⎝ RT ρ cDL (d gap − H1 ) f …… …… …… …… …… …… …… …… …… …… H1 = H 0 + Z1 = H 0 + (d gap − H 0 ) f i0 RT ρ cDL ϕ0 (1 − α ) zF 2 ⎞ ⎤ ⎟⎟ − 1⎥ ⎠ ⎥⎦ ⎞ ⎤ ⎟⎟ − 1⎥ ⎠ ⎥⎦ (4.12) ⎡ ⎛ (1 − α ) F ϕ0 D ⎢exp ⎜⎜ ⎢⎣ ⎝ RT ρ cDL (d gap − H t −1 ) f ⎞ ⎤ ⎟⎟ − 1⎥ ⎠ ⎥⎦ ⎛ (1 − α ) F ⎞ ⎤ (d − H t ) f i0 RT ρ cDL ⎡ ϕ0 D exp H t +1 = H t + Z t +1 = H t + gap ⎢ ⎜ ⎟ − 1⎥ ϕ0 (1 − α ) zF 2 ⎢⎣ ⎜⎝ RT ρ cDL (d gap − H t ) f ⎟⎠ ⎥⎦ H t = H t −1 + Z t = H t −1 + (d gap − H t −1 ) f i0 RT ρ cDL ϕ0 (1 − α ) zF 2 65 Modeling for LECD micro electrode fabrication process The deposition rate can be obtained by equation (4.11) over deposition time. The change of the deposition height can be calculated from the deposition rate. Equation (4.12) shows that the deposition rate is not constant for every unit time. This is why, in order to calculate the deposition height, the deposition rate needs to be calculated in every unit time. 4.3 Simulation plan and Experimental setup 4.3.1 Simulation and experimental plan In LECD, metal is deposited on the base of cathode, which is placed over the mask, and an optimum gap is maintained between the mask and anode. When a pulse voltage is applied, metal is deposited on the base of the cathode through the mask. When the deposition starts, the height of the electrode starts increasing. In order to study the effect of pulse amplitude ( ϕ 0 ) f , D and d gap are kept constant on 100 kHz, 0.33 and 350 µm. Similarly, to study the effect of pulse frequency ( f ) ϕ 0 , D and d gap are kept constant on 1.5V, 0.33 and 350 µm. Likewise, to study the effect of pulse duty ratio ( D) ϕ 0 , f and d gap are kept constant on 1.5V, 100 kHz and 350 µm. Lastly, to study the effect of electrode gap distance ( d gap ) ϕ 0 , f and D are kept constant on 1.5V, 100 kHz and 0.33. The details LECD parameters tabulated in table 4.11. Finally, the deposited structure is used as an EDM electrode to fabricate micro holes in different discharge energy level without changing its setup. In the micro-EDM, R-C spark control circuit is used. In this study, Keyence VHX digital microscope (VH-Z450) was used to determine the dimensions and examine the microstructures. In addition, 66 Modeling for LECD micro electrode fabrication process scanning electron microscope (SEM) (JSM-5500, JEOL Ltd.) was used to acquire better picture of the microstructures. Table 4.1: LECD parameter for simulation and experiments Parameters Value Pulse amplitude, ϕ 0 1.2 V, 1.5 V, 1.6 V, 1.8 V, 2.0 V Pulse frequency, f 70 kHz, 85 kHz, 100 kHz, 115 kHz, 130 kHz Pulse duty ratio, D 0.20, 0.25, 0.33, 0.40, 0.50 Electrode gap distance, d gap 350 µm, 400 µm, 450 µm, 500 µm, 600 µm Exchange current density, i0 1.5 mA/cm2 Specific electrolyte resistivity, ρ 10 Ω.cm Leak factor, α 0.5 Stoichiometric number, n 2 Specific capacitance, cDL 10 µF/cm2 Temperature, T 298.15 K Copper mole volume 7.11 cm3/mol Faraday constant, F 96485 C/mol Gas constant, R 8.314 J/molK 4.4 Effect of different LECD parameters Figure 4.4 to 4.11 presents the simulation of deposition height and rate with respect to deposition time at different level of voltage amplitudes, frequencies, duty ratios and gap distance based on equation (4.11) and (4.12). In order to verify the mathematical 67 Modeling for LECD micro electrode fabrication process model experimental results are also shown in the figures. Results show that in case of higher deposition rate the experimental results are not properly matched with the simulation results. The simulation results show that the deposition height and rate is higher than that of experimental results. (a) (b) Figure 4.3: (a) Showing the gap between the electrode and mask (b) SEM image showing the extra deposited material through the gap At the begging of the deposition process, the anode electrode touches the mask. Although the anode electrode surface is properly polished, but still there is some horizontal misalignment. This is why, the electrolyte leaks through the gap (figure 4.3). This cause the metal deposited on the places the electrolyte reaches. Due to this reason, in the experimental results the initial rate is not matched with simulation 68 Modeling for LECD micro electrode fabrication process results. However, this phenomenon has also occurred in low deposition rate but the effect is comparatively less. After the above leakage, the gap seals within a very short time. This is why the effect is not that much. Once the deposited metal enters into the mask, the gap is sealed. After the above conditions, the results show that the deposition height of experimental data is higher that simulation data. On probable occurrence may be that when the deposition starts the top surface is not perfectly flat all the time. Some peaks and valleys can be visible on the top surface (figure 4.3). For measuring the height of the deposition, only the peaks are taken in to account. This is why the total height is become higher. However, the actual equivalent height is less than the measurement height. The details effects of different parameters are given below. 4.4.1 Effect of pulse voltage amplitude From figures 4.4 and 4.5 indicate that the deposition height and rate increases with the increase of deposition time at all voltage amplitude. Moreover, at a certain point the deposition height and rate increases almost suddenly and the system becomes uncontrollable. This point of transition is different for different voltages. For lower voltage, the transition point arrives at higher deposition time. On the other hand, for higher voltage the position is vice versa. For a voltage value lower than 1.5 volt, the deposition rate is comparatively lower than the higher voltages. The results indicate that below 1.5 volt the supplied energy is not sufficient to deposit the material at a higher rate. 69 Modeling for LECD micro electrode fabrication process 180 160 Deposition height (µm) 140 120 100 80 60 40 20 0 0 500 1000 1500 2000 2500 Deposition time (sec) 1.2V (sim) 1.2V (exp) 1.5V (sim) 1.5V (exp) 1.6V (sim) 1.6V (exp) 1.8V (sim) 1.8V (exp) 2.0V (sim) 2.0V (exp) Figure 4.4: Effect of pulse voltage amplitude on deposition height (simulation and experimental) On the other hand, for a voltage value higher than 1.5 volt the deposition rate is comparatively higher than the lower voltages. Due to the higher deposition rate after the transition point the deposition structures become tree and powder type (figure 4.12 c). One possible explanation is that at a certain point of high voltage deposition, the corresponding current value exceeds the limit value of the electrolyte, which causes the deposit to become powdery. It is probable that the area surrounding the anode becomes depleted of ions for discharging the anode. At the same time, a high volume of hydrogen gas is produced around the anode. 70 Modeling for LECD micro electrode fabrication process 0.3 Deposition rate (µm/sec) 0.25 0.2 0.15 0.1 0.05 0 0 500 1000 1500 2000 2500 Deposition time (sec) 1.2V (sim) 1.2V (exp) 1.5V (sim) 1.5V (exp) 1.6V (sim) 1.6V (exp) 1.8V (sim) 1.8V (exp) 2.0V (sim) 2.0V (exp) Figure 4.5: Effect of pulse voltage amplitude on deposition rate (simulation and experimental) 4.4.2 Effect of pulse voltage frequency It can be seen from the simulation and experimental results of figures 4.6 and 4.7, the deposition height and rate increases with the increase of deposition time at any value of frequency. For a frequency value higher than 100 kHz, the deposition rate is comparatively lower than the lower frequencies. In contrast, for frequency values lower than 100 kHz the deposition rate is comparatively higher than the lower frequencies. 71 Modeling for LECD micro electrode fabrication process 180 160 Deposition height (µm) 140 120 100 80 60 40 20 0 0 500 1000 1500 2000 2500 Deposition time (sec) 70kHz (sim) 70kHz (exp) 85kHz (sim) 85kHz (exp) 100kHz (sim) 100kHz (exp) 115kHz (sim) 115kHz (exp) 130kHz (sim) 130kHz (exp) Figure 4.6: Effect of pulse voltage frequency on deposition height (simulation and experimental) As it is known that, the frequency has an inverse relationship with pulse period. This is why, the pulse period or duration increases with the decrease of the pulse frequency. With the increase of pulse period time the amount of energy per pulse also increases. Due to the increase of deposition energy, the number of ions also increases for deposition, which causes the deposition rate increase. Alternately, when the frequency increases the condition will be vice versa. 72 Modeling for LECD micro electrode fabrication process 0.45 0.4 Deposition rate (µm/sec) 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0 500 1000 1500 2000 2500 Deposition time (sec) 70kHz (sim) 70kHz (exp) 85kHz (sim) 85kHz (exp) 100kHz (sim) 100kHz (exp) 115kHz (sim) 115kHz (exp) 130kHz (sim) 130kHz (exp) Figure 4.7: Effect of pulse voltage frequency on deposition rate (simulation and experimental) 4.4.3 Effect of pulse voltage duty ratio For any particular duty ratio, the deposition height and rate increases with the increase of deposition time and the results are shown in figures 4.8 and 4.9. Although, for 0.2 and 0.25 duty ratio the deposition rate is relatively lower that higher duty ratio. Conversely, for 0.4 and 0.5 duty ratio the deposition rate is relatively higher that lower duty ratio and after certain point it increases hastily. A reasonable explanation is that this study is conducted on a fixed pulse frequency where the pulse period is fixed. As the duty ratio is the ratio of pulse on time and pulse period. This is why, due to the decrease of duty ratio the pulse on time also decreases. 73 Modeling for LECD micro electrode fabrication process 180 160 Deposition height (µm) 140 120 100 80 60 40 20 0 0 500 1000 1500 2000 2500 Deposition time (sec) 0.20 (sim) 0.20 (exp) 0.25 (sim) 0.25 (exp) 0.33 (sim) 0.33 (exp) 0.40 (sim) 0.40 (exp) 0.50 (sim) 0.50 (exp) Figure 4.8: Effect of pulse voltage duty ratio on deposition height (simulation and experimental) If the pulse on time is much less than the double layer time constant, then there will be strong damping in the system. This incidence causes the insufficient charging and discharging of the double layer. As a final point the deposition rate decreases due to this occurrence. Inversely, for higher duty ratio the pulse on time is much higher than the double layer time constant, cause increase in deposition energy. Due to the increase of deposition energy, the number of ions also increases for deposition, which causes the deposition rate increase. 74 Modeling for LECD micro electrode fabrication process 0.6 Deposition rate (µm/sec) 0.5 0.4 0.3 0.2 0.1 0 0 500 1000 1500 2000 2500 Deposition time (sec) 0.20 (sim) 0.20 (exp) 0.25 (sim) 0.25 (exp) 0.33 (sim) 0.33 (exp) 0.40 (sim) 0.40 (exp) 0.50 (sim) 0.50 (exp) Figure 4.9: Effect of pulse voltage duty ratio on deposition rate (simulation and experimental) 4.4.4 Effect of electrode effective gap distance As can be seen from the simulation as well as experimental results, the deposition height and rate increases with the increase of deposition time at any electrode effective gap distance (figures 4.10 and 4.11). In addition, with the increase of gap distance the overall deposition rate decreases. These results can be adequately explained by the double layer time constant characteristics, which is a product of resistivity, capacitance and gap distance. If the gap distance increases, then the time constant also will increase. When the time constant will increase, it will take more time to charge the double layer. This will lead to a strong damping condition. Due to this insufficient charging and discharging of the double layer occurs, the deposition rate decreases. 75 Modeling for LECD micro electrode fabrication process Figures 4.12 (a) and (b) show the deposited electrode at applied voltage 1.6 V, frequency 100 kHz, duty ratio 0.33 and 350 µm gap distance between the electrodes. Figure 4.12 (c) and (d) show the deposited electrode structure become tree type, normally this kind of scenario came when the voltage is 1.8V or more than that or duty ratio of 0.5 or more than that. 180 160 Deposition height (µm) 140 120 100 80 60 40 20 0 0 500 1000 1500 2000 2500 Deposition time (sec) 350 µm (sim) 350 µm (exp) 400 µm (sim) 400 µm (exp) 450 µm (sim) 450 µm (exp) 500 µm (sim) 500 µm (exp) 600 µm (sim) 600 µm (exp) Figure 4.10: Effect of gap distance on deposition height (simulation and experimental) 76 Modeling for LECD micro electrode fabrication process 0.25 Deposition rate (µm/sec) 0.2 0.15 0.1 0.05 0 0 500 1000 1500 2000 2500 Deposition time (sec) 350 µm (sim) 350 µm (exp) 400 µm (sim) 400 µm (exp) 450 µm (sim) 450 µm (exp) 500 µm (sim) 500 µm (exp) 600 µm (sim) 600 µm (exp) Figure 4.11: Effect of gap distance on deposition rate (simulation and experimental) 4.5 Concluding remarks In this chapter, a set of mathematical relations are developed in order to model the deposition rate. Results show the clear indication of the deposition rate and structure during the time of deposition and its operating ranges as well. From this study, the following conclusions can be drawn: ™ An increase in pulse voltage amplitude and duty ratio and a decrease in pulse voltage frequency helped to increase the metal deposition rate. However, this higher deposition rate does not help the deposition process to be localized in a proper position. For this reason, in these conditions deposited structures become powdery and tree type. 77 Modeling for LECD micro electrode fabrication process ™ If the four deposition parameters are maintained within a proper range a smooth, fine-grained, low porosity and complex shape copper electrodes are deposited in LECD process. From the above study, results suggested that good quality deposition could be achieved in the condition of voltage amplitude of 1.5V to 1.6V, frequency 100kHz to 115kHz, duty ratio of 0.33 and electrode gap of 350µm In the next chapter, detailed description of the design and implementation of two controllers one is open loop and another is close loop in LECD process are given. (a) (c) (b) (d) Figure 4.12: (a) LECD electrode side view (b) LECD electrode top view (c) Tree structure of deposited electrode side view (d) top view (improper deposition) 78 Control for LECD micro electrode fabrication process Chatper 5 Control for LECD micro electrode fabrication process 5.1 Introduction In the previous section, effects of different LECD parameters were shown. This section will provide the design of a controller for LECD process in order to increase the height of the deposited structure, which was one of the main objectives of this project. To increase the height of the deposited structure, it is necessary to lift up the cathode from the mask. The reason is, if cathode is kept in the same initial place then after certain time the deposited structure will come out from the mask and the actual structure of the deposited electrode will be affected. This is why, to keep the deposited structure inside the mask, it is required to lift up the cathode from its original position. However, in order to lift up the cathode it is very necessary to know the deposition height at the particular time. This is why, two different controllers are designed one is open loop controller and another is close loop controller in order to control the lifting mechanism of the cathode. For the two controllers two input heights are needed to be given for control simulation. One is the initial growth height and another is the final desired height. In the next sub sections the determination of the initial growth height, designing of an open loop and close loop controller are given. 79 Control for LECD micro electrode fabrication process 5.2 Determine of the initial growth height In this study, the initial growth height (Hi) means the height of the deposition before applying the control algorithm in the system. This initial growth height is very important for the proper control of the LECD process. There are two reasons behind the initial growth height. One is to operate the control of the deposition structure in a region where the deposition rate is moderately higher. Another reason is to seal the leak of the electrolyte through the anode and mask. These two phenomena are described below: 250 Deposition height (µm) 200 150 100 50 0 0 500 1000 1500 2000 2500 Deposition time (sec) 1.2V (sim) 1.2V (exp) 1.5V (sim) 1.5V (exp) 1.6V (sim) 1.6V (exp) 1.8V (sim) 1.8V (exp) 2.0V (sim) 2.0V (exp) Figure 5.1: Operating zone for LECD control 5.2.1 Operating in the higher deposition region It can be seen from Figure 4.4 to 4.11, the deposition rate and height at the starting time is comparatively lower. This is why, if the control algorithm is applied in that 80 Control for LECD micro electrode fabrication process zone then to get the desire height, it will take comparatively longer time. Figure 5.1 shows the tentative operating range for the condition of Figure 4.4. Figure shows that in order to reach the operating zone the initial growth height is required. Gap Anode Electrode Mask Mask Electrolyte (a) Anode Electrode Mask Electrolyte Hi Anode Electrode Mask (b) Mask Electrolyte Mask (c) Figure 5.2: (a) Showing the gap between the electrode and mask (b) control is applied without initial growth height (c) control is applied after initial growth height 5.2.2 Seal the leak for the electrolyte As it is described in section 4.4, although the anode electrode surface was properly polished, but still there was some horizontal misalignment. This is why, the electrolyte leak through the gap (Figure 5.2 a). If control algorithm was applied without the initial growth then after lifting up the cathode, electrolyte will again start to leak (Figure 5.2 81 Control for LECD micro electrode fabrication process b). However, if control algorithm is applied after the initial growth then after lifting up the cathode, there is no more possibility for the electrolyte to leak (Figure 5.2 c). 5.2.3 Determination of limit of the initial growth by FLUENT analysis Fresh solution touching area (region A) Deposition Area of less fresh solution (region B) Cathode Velocity profile Hflow Mask Mask Flow of electrolyte Anode Figure 5.3: Concept of FLUENT simulation It can be seen from Figure 5.3, when the electrolyte is flowing through the narrow passage between the mask and the anode, a flow region is created inside the mask. In that flow region, fresh copper ion is always available. Outside the flow region, the copper ion is reached by diffusion process. If the deposition structure reaches and touches the flow region, then in region A the deposition growth will be higher than the 82 Control for LECD micro electrode fabrication process region B. In region B, the fresh copper ion is lesser because ions are reaching there through diffusion and in region A, fresh ions are coming by force convection. This is why, the initial growth should not cross the flow region and the initial growth height should be Hi < Hflow. (a) (c) (b) (d) Figure 5.4: (a) flow analysis (b) grid inside the mask area (c) velocity for vertical grid line (d) velocity for the horizontal grid line 83 180 160 140 120 100 80 60 40 20 0 650 µm 550 µm 0.005 450 µm 0.01 0.012 0.015 0.017 350 µm 0.02 Elelctrode gap (µm) Initial growth height, Hi (µm) Control for LECD micro electrode fabrication process ELectrolyte flow rate (m/s) 0-20 20-40 40-60 60-80 80-100 100-120 120-140 140-160 160-180 Figure 5.5: Surface plot for initial growth height for different flow rate and electrode gap Figure 5.4 shows the FLUENT simulation for the electrolyte flow 0.01 m/s and 250µm mask height and 100µm gap between the mask and anode. From the above condition, it can be concluded that the safer initial growth height can be 50µm or less than this. For different flow rate condition and different electrode gap, a surface plot was done in Figure 5.5. Figure 5.6 show that when the deposition structure enters into the force convection zone of the electrolyte then the growth was mainly in the center area. Once few layers are deposited in the center area, immediately the structure starts to become tree type. 84 Control for LECD micro electrode fabrication process (a) (b) Figure 5.6: Tree structure of deposition due to force convection of electrolyte. (a) top view (b) side view 5.3 Design of an open loop control system for LECD process An open-loop controller, also called a non-feedback controller, is a type of controller that computes its input into a system using only the current state and its model of the system. It is often used in simple processes because of its simplicity and low-cost, especially in systems where feedback is not critical. A characteristic of the open-loop controller is that it does not use feedback to determine if its input has achieved the desired goal. This means that the system does not observe the output of the processes that it is controlling. Consequently, a true open-loop system cannot engage in machine learning and cannot correct any errors that it could make. It also may not compensate for disturbances in the system. 85 Control for LECD micro electrode fabrication process Figure 5.7: Algorithm for open loop control 86 Control for LECD micro electrode fabrication process In this process deposition rate and height is a significant parameter, which can be determined from equation (4.11) and (4.12). In this current modified setup for lifting the cathode, voice coil motor is used. A motor control program is written in Borland C++ Builder 6.0 based on equation (4.11) and (4.12) in order to give an interrupt for every n sec. In this process two height values input is required, one is for the initial growth height (Hi) and another is for the final desired height (Ht). The controller feedback system will start when the deposition will reach Hi height. When the controller receives the interrupt then it will shift the motor by (Ht - Hi) distance. In this way, it is possible to increase the aspect ratio by open loop control. The detail algorithm of the open loop control system is shown in the Figure 5.7. 5.4 Design of a closed loop control system for LECD process Generally, to obtain a more accurate or more adaptive control, it is necessary to feed the output of the system back to the inputs of the controller. This type of system is called a closed loop system. A closed-loop system utilizes feedback to measure the actual system operating parameter being controlled such as temperature, pressure, flow, level, or speed. This feedback signal is sent back to the controller where it is compared with the desired system set point. In order to design such a controller, a system model has been derived from Faraday’s basic law of electrochemistry. 87 Control for LECD micro electrode fabrication process Cathode Hdep Mask Mask Ha dH Anode Figure 5.8: Relation of deposition height and electrode gap It can be seen from Figure 5.8 that when deposition starts the gap between the two electrodes reduces and the deposition height can be explained from the Faraday’s law. It can be written that dH = H a − H dep = H1 − M Cu i dt nFADCu ∫ M Cu dH = 0− i dt nFADCu sH ( s ) = − M Cu I (s) nFADCu (5.1) I (s) M Cu M Cu =− s = − K dep s ; K dep = H (s) nFADCu nFADCu 88 Control for LECD micro electrode fabrication process Here, Atomic weight M Cu = 63.54 gm mol Stoichiometric number n = 2 Faradays constant F = 96500 C mol Crosssectional area A = 249000μ m 2 Density DCu = 8.94 gm cm3 = 8.94 ×10−12 gm μ m3 Equation (5.1) shows the transfer function of the LECD plant. In this plant, the input parameter is distance that will be manipulated by a voice coil motor and the output parameter is the current that will be measure by a pico ammeter. The voice coil motor used in this unit has a separate controller. The controller unit of the motor is given below Figure 5.9 and 5.10: Figure 5.9: Wiring diagram of a voice coil motor 89 Control for LECD micro electrode fabrication process 1 ms + b Km R + Ls 1 s Kb Figure 5.10: Controller of the voice coil motor In order to derive the transfer function of the motor it was assumed that Km is equal to Kb and neglecting the coil inductance L. Therefore the transfer function of the voice coil motor is: G (s) = H (s) Km = Va ( s ) s ⎡⎣( ms + b )( Ls + R ) + K m K b ⎤⎦ Km (neglecting L, L = 3.2mH) = s ( Rms + Rb + K m 2 ) = (5.2) τ2 s (τ 1s + 1) Here, Here,τ 1 = Km Rm and τ 2 = 2 Rb + K m Rb + K m2 Motor constant K m = 7.5 Resistance R = 9.9Ω at 25o C Total mass m = 500 gm ( including electrode holder, coil and assembly ) Frictional coefficient b = 0.00001 N − sec μm 90 Control for LECD micro electrode fabrication process τ2 KP s (τ 1s + 1) − K dep s K =1 Figure 5.11: Controller of the LECD process Closed-Loop Systems shown in Figure 5.11 have the following key features: • A Reference or Set Point that establishes the desired operating point around which the system controls. In this system the set point is the reference current which is measured after the initial growth height. • The process variable Feedback signal that “tells” the controller at what point the system is actually operating. A current measuring device can be a feedback device. This current measuring device is connected to a PC by RS232 serial communication. In this way, PC can get a pant current value. • A Comparator which compares the system Reference with the system Feedback and generates an Error signal that represents the difference between the desired operating point and the actual system operating value. The 91 Control for LECD micro electrode fabrication process comparator will calculate the error from deducting the present current value from the reference current. • System Tuning Elements which modify the control operation by introducing mathematical constants that settle in the control to the system stabilization, and adjust system response time. In this process control system these tuning elements are proportional functions. With the help of the P controller, the system is stabilized and response time is adjusted. • Zero order hold is also used in this process. It literally holds the digital signal for the sample time, then moves to the next digital sample and holds that signal for the sample time as well, in order to reconstruct the analogue signal. Here, the sample holding time is fixed from the PC. • A Final Control Element or mechanism which responds to the system Error to bring the system into balance, is the voice coil motor. Therefore, the position control of the motor will be the amount of error. This voice coil motor is also connected to PC by RS232 serial communication. The detail algorithm of the closed loop control that is implemented in the process is given below in Figure 5.12. 92 Control for LECD micro electrode fabrication process Figure 5.12: Algorithm for close loop control 93 Control for LECD micro electrode fabrication process 5.4.1 Controller gain optimization -4 5.2 -4 x 10 5.2 x 10 Kp = 4200 Kp = 4600 5 4.8 Current density (amp/cm2) Current density (amp/cm2) 5 4.6 4.4 4.2 4 4.8 4.6 4.4 4.2 3.8 0 0.2 0.4 Time (sec) 0.6 4 0 0.2 0.4 Time (sec) (a) 0.6 (b) -4 5.2 x 10 Kp = 4430 Current density (amp/cm2) 5 4.8 4.6 4.4 4.2 4 3.8 0 0.2 0.4 Time (sec) 0.6 (c) Figure 5.13: LECD system response for different proportional controller constant (a) K P = 4200 (overshoot) (b) K P = 4600 (undershoot) (c) K P = 4430 (optimize value) 94 Control for LECD micro electrode fabrication process In order to optimize the controller gain ( K P ) value of LECD process, MATLAB simulations were performed using equation (5.2) and Figure 5.11. Figure 5.13 shows some of the simulation results. In order to run the simulation the initial value of the current was set 0.4mA. After the increase of the deposition height, the current value was increased to a value of 0.5mA. Therefore, the task of the controller will reduce the current by lifting up the voice coil motor. In order to stabilize the current value the optimum controller gain is required. Figure 5.13 (a) and (b) show that for the value of K P = 4200 and K P = 4600 result is overshooting and undershooting for final value. Figure 5.13 (c) shows that for the value K P = 4430 the value is reaching to its final value with in 0.6 sec. Based on this result the implemented gain value for LECD process was fixed at K P = 4430 . 5.5 Comparison of open and close loop implemented algorithm In order to compare the output from the LECD process after implementing the open loop control and close loop control, online deposition height monitoring is the best possible solution. However, in this study the comparison was conducted by on machine current monitoring and comparing the final height of the deposition, which is due to the unavailability of the on machine height monitoring equipment. These results can be still justifiable from Faraday’s law, where it is well known that the deposition height is proportional to the amount of current passing through the system. In the next sub-section, comparison of open loop and close loop control is given below on monitoring current density profile and achieved final deposition height. 95 Control for LECD micro electrode fabrication process 5.5.1 Comparison on monitoring current density profile Figure 5.14 shows the simulation and experimental results (without control) for LECD current density profile. The simulated curve can be drawn from the equation (4.8). Experimental results showed that at the beginning, the current density was very high then it started to decrease and finally after some time it again started increasing. The initial higher current density is due to the leakage of the electrolyte, which is described in section 4.4. In the next step, the control implementation time that means when the control will start the operation needs to be decided. This time can be calculated from Figure 4.4. In this study, the initial growth is selected to be 50µm and the target height was 500 µm. For this reason, the control algorithm was applied at 1000 sec. Current density (amp/cm2) 0.0012 0.0010 0.0008 0.0006 0.0004 0.0002 0.0000 0 Simulated value 500 1000 Deposition time (sec) 1500 Experimental value (without control) Figure 5.14: Current density profile from simulation and experimental result for the condition of 1.6V, 100 kHz, 0.33 duty and 350 µm electrode gap 96 Control for LECD micro electrode fabrication process Current density (amp/cm2) 0.0012 0.0010 0.0008 0.0006 0.0004 0.0002 0.0000 0 Open loop control 500 1000 1500 Deposition time (sec) Without control 2000 Close loop control Figure 5.15: Current density profile from open loop, close loop and without control for the condition of 1.6V, 100 kHz, 0.33 duty and 350 µm electrode gap In case of open loop control, instead of getting constant current after the control starting point, the current was increasing. This is because, from Figure 5.14 it can be seen that the slope of the two curves after 1000 sec is not same. The slope of the simulated curve is less than the experimental one. For this reason, the rate of increase of current density in open loop is higher than the rate of decrease of current density in open loop control. On the other hand, close loop control showed almost stable current density throughout the process. However, in close loop control also there was a bit increasing of current density. One possible reason is that during the experimental time, the concentration of the electrolyte solution will be reduced due to the deposition process and in the current setup, there was no provision of maintaining the concentration level during deposition time. However, in the close loop control modeling it was assumed that the concentration would be constant. 97 Control for LECD micro electrode fabrication process 5.5.2 Comparison of deposition height and its repeatability 600 Deposition height (µm) 500 400 300 200 100 0 Open loop Close loop Controller type Figure 5.16: Deposition height for the open loop controller and close loop controller Figure 5.16 shows the deposited height for open loop and close loop control. Although the achieved height for the open loop control and close loop control is almost same, but the repeatability of open loop control is less than the close loop control. It can be shown in the previous section that the control rate is slower for open loop control. For this reason, the structural inaccuracy was found for open loop control (Figure 5.15 a). Due to the higher deposition rate, the deposited structure quickly enters to the force convection zone of the electrolyte as it was shown in figure 5.3, then the process become more inaccurate. On the other hand, in close loop the repeatability and structure are comparatively better. However, in the top side of the structure the deposited structure is not uniform. This may due to the change of copper concentration 98 Control for LECD micro electrode fabrication process during deposition process. This phenomenon also described in section 3.4.2 that when the concentration becomes less than a certain limit then the number of ions available for discharging is low when the concentration is low, creating a depletion layer just beneath the electrode. There is no such effect on the deposition rate due to this occurrence but the deposited structures are irregular and highly porous and it will become tree type. Similar observations of this concentration effect on deposited structure were made by El-Giar (2000) with a different experimental setup and electrode materials. (a) (b) Figure 5.17: Deposited structure for (a) open loop control (b) close loop control 99 Control for LECD micro electrode fabrication process 5.6 Concluding remarks In this chapter, detailed study on the design and implementation of open loop and close loop controller for LECD process has given. The following main points can be concluded from this chapter: ™ One important findings of this study is the requirement of the initial growth for LECD. The limiting value is decided from FLUENT analysis, on various electrolyte flow rate and various electrode gap. ™ The performance repeatability for close loop control is better than the open loop control. Results show that in order to stabilize the deposition current close loop controller performs better than open loop. Finally, SEM showed that the deposited structure for close loop control is uniform and homogeneous. Therefore, it can be concluded that to control the LECD process close loop feedback controller is the best choice. However, in order to control it more accurately on line concentration control is required. In the next chapter, performance analysis of LECD electrode for machining micro hole on different work piece is give. At the same time, few comparative studies are also discussed between LECD electrode and pure copper circular electrode. 100 Performance analysis of LECD electrode in micro-EDM application Chatper 6 Performance analysis of LECD electrode in micro-EDM application 6.1 Introduction For machining of hard conductive materials and fabricating complex shape structures EDM can provide an efficient solution. In recent years, numerous developments and modifications in micro-EDM have been focused on the fabrication of micro-features. However, due to the recent developments and modification to accommodate the micromanufacturing requirements, a number of challenges have arisen. Among all the major challenges, tool handling and electrode and workpiece preparation are the significant ones in micro-fabrication (Pham, et al. 2004). Moreover, non-circular tool fabrication is very challenging with any conventional machining processes. However, with LECD process non-circular electrode can be fabricated, which is described in the previous chapter. With this process, tool-handling problem can be minimized and the production rate can be increased. This chapter will present performance analysis of the LECD electrode on austenitic stainless steel (SUS 304) workpiece through a comparative study on four different workpiece materials such as SUS 304, copper, brass and aluminum. This chapter will also present one comparative performance study of LECD electrode with a circular copper electrode of equal cross sectional area and another comparative process study die sinking EDM of LECD electrode with scanning EDM of a circular copper electrode in fabricating same holes or cavities. 101 Performance analysis of LECD electrode in micro-EDM application 6.2 Parameter influencing the micro-EDM process In a RC-type pulse generator the discharge energy ( E ds ) per pulse can be obtained from the gap voltage and the capacitance of the RC circuit (Masuzawa 2001) and the equation is: E ds = 1 CV 2 2 (6.1) Where, C = capacitance and V= gap voltage Figure 6.1: Schematic diagrams of the RC type pulse generator used in this study So in RC-type the performance of the micro-EDM process can be more precisely controlled by knowing the effect of only the gap voltage and the capacitance. For the fine finish, micro-EDM the discharge energy should be minimized which can be more easily done in RC-generator by using low values of voltage and capacitance. Figure 6.1 shows the Schematic diagrams of the RC-type pulse generator. The main components are the discharge control resistors (DCR), the discharge control capacitors (DCC), the peak hold circuit (PHC) and the current transducer (CT). 102 Performance analysis of LECD electrode in micro-EDM application 6.3 Experimental conditions and procedures 6.3.1 EDM electrode, workpiece dielectric The tool electrodes for these studies are fabricated by LECD process. The workpiece materials used in this comparative study was austenitic stainless steel (SUS 304), copper, brass and aluminum. The workpiece was fabricated and ground to 50µm thickness of dimension 60mm×12.5mm×0.05mm. The important properties of different workpiece materials are given in Table 6.1. The dielectric fluid used in this study is commercially available “Total EDM 3” oil having relatively high flash point, high auto-ignition temperature and high dielectric strength. The fluid is a clear mineral oil exhibiting good resistance to oxidation; contains very low aromatic contents, low volatility, and low pour point creating possibility of outside storage and low viscosity, which ensures that metal chips are evacuated easily. The important properties of the dielectric are listed in Tables 6.2. Table 6.1: Properties of the EDM workpiece material Workpiece material Stainless Steel (SUS 304) Copper Brass Aluminum Composition (%) Melting point Thermal conductivity (W·m−1·K−1) Electrical Resistivity (Ω.m) Thermal Expansion (µm·m−1·K−1) 1450°C 16.2 0.72×10-6 17.3 99% Cu 57% Cu, 40% Zn, 1.5% Mn 1084°C 380 17.2×10-9 17.0 865°C 88.3 0.09×10-6 19.0 99% Al 660°C 237 28.2×10-9 23.0 0.08% C, 2% Mn, 0.75% Si, 0.045% P, 0.03% S, 18-20% Cr, 10.5% Ni, 0.1% N 103 Performance analysis of LECD electrode in micro-EDM application Table 6.2: Properties of the EDM oil 3 dielectric fluid Property Values Volumetric mass at 15°C 813 Kg/m3 Viscosity at 20°C 7.0 mm2/s Flash point Pensky-Martens 134/259 °C/°F Auto-ignition temperature 470°F Aromatics content 0.01 Wt% Distillation range, IBP/FBP 277/322°C 6.3.2 Experimental Procedure In the micro-EDM, selection of electrode polarity is important. For this reason, the electrode polarity was firstly verified and selected experimentally. To find out the electrode polarity, material removal rate (MRR) and relative wear rate (RWR) are studied for both LECD electrode positive and negative polarity. To determine the optimum conditions for quality micro-hole in different workpiece materials with LECD electrode a series of experiments were conducted by varying major operating parameters. The machining conditions for this study are listed in Table 6.3. Therefore, to evaluate the dimensional accuracy of the micro-holes the average spark gaps (ASG) and average taper angel (ATA) were also measured. Equations 6.2 to 6.5 were used to calculate the MRR, RWR, ASG and ATA respectively. Figure 6.2 (a) and (b) shows the ASG and ATA measuring procedure. MRR = Amount of material removed from workpiece (volm ) Unit time (6.2) 104 Performance analysis of LECD electrode in micro-EDM application RWR = Amount of material removed from electrode (volm ) Amount of material removed from workpiece (volm ) (6.3) 1 ⎛ g + g + g3 + g 4 ⎞ ASG = × ⎜ 1 2 −a⎟ 2 ⎝ 4 ⎠ ATA,θ = tan −1 (d top (6.4) − d bottom ) 2× h (6.5) Table 6.3: Machining Parameters of RC Pulse generator micro-EDM for micro holes machining of LECD Electrode Parameters Values EDM Circuit R–C Supply Voltage (V) 60, 80, 100, 120, 140 Capacitance (pf) 100, 220, 470, 1000, 2200 Resistance (kΩ) Fixed to 1 kΩ Dielectric Coolant EDM Oil 3 dtop g1 g2 g4 g3 h a a θ=Taper angle a a (a) Workpiece dbottom (b) Figure 6.2: Measurement of (a) average spark gap (b) taper angle θ 105 Performance analysis of LECD electrode in micro-EDM application Here, a is actual dimension (mask), g1, g2, g3, g4 are the machined dimensions (hole), dtop and dbottom are the top and bottom diameter of the hole and h is the height of the workpiece. 6.4 LECD electrode fabrication for micro-EDM From the previous chapter, it can be seen that LECD is an extraordinary method for making complex cross sectional electrode easily. This process has advantages in terms of fabrication time and cost compared to any other micro fabrication methods. In order to fabricate a mask is placed between the anode and cathode, which is immersed in mixed electrolyte of a plating solution of acidic CuSO4.5H2O. The deposition of copper is localized on the cathode surface by applying pulse voltage. When the deposition took place on cathode surface, the effective distance between the cathode and anode starts reducing. This effective distance represents the resistance between the two electrodes. This is why the current starts increasing with the decrease of resistance in a constant applied voltage. The increasing of current value is easily observable in the ammeter. Finally, in order to maintain the effective distance constant as well as to keep the current value constant to its initial value, the cathode is lifted up with the help of a voice coil motor. In this way, a constant distance is maintained between the electrodes. In this fashion a smooth, fine-grained, and low porosity X shape LECD electrodes of 0.249 mm2 cross sectional area are deposited. Figure 6.3 shows the SEM images, EDX spectrum analysis and details dimensions of deposited electrode before EDM process. The details discussion of the EDX analysis is given in section 6.7 and Table 6.4. 106 Performance analysis of LECD electrode in micro-EDM application Figure 6.3: (a) LECD electrode side view (b) LECD electrode top view (c) dimensions of LECD electrodes (c) EDX spectrum analysis of the LECD electrode top surface before micro EDM 6.5 Effect of electrode polarity In EDM, the choice of electrode polarity is an important factor. The effect of polarity on the material removal rate, relative wear ratio is illustrated in figures 6.4 (a) and (b) respectively for varying voltage at a capacitance of 470 pf, resistance 1 kΩ, a LECD electrode as the tool electrode with both negative and positive polarity and stainless steel as the workpiece material. The results show that in machining stainless steel with LECD electrode, the use of negative electrode polarity is more enviable. This is 107 Performance analysis of LECD electrode in micro-EDM application because the material removal rate is 10-80% higher than for machining with a positive tool, and the relative wear ratio is lower (40-50%) than using a positive electrode (150275%). In EDM of stainless steel, better machining performance is obtained with the tools as cathode and the workpiece as anode. This set up is called straight polarity. Negative polarity produces a constant relative wear ratio, whereas positive polarity gives a generally increasing trend. At high voltage settings, the value of the relative wear ratio for both negative and positive polarity electrodes converge and are quite close. From these observations, it is evident that negative tools perform better than positive tools in terms of material removal rate and relative wear ratio of the workpiece. In this experiment LECD electrode is engaged as the cathode material and the stainless steel is employed as the anode. The material removal rate is dependent on anode potential drop. The metal is evaporated from the anode when the current was too high. This flow of atoms coming out from the anode impeded with the electrons coming to the anode. The electrons gained the additional energy of the anode fall, although some of the metal atoms are ionized and crashed into the anode. This cause more vaporization and material removal. The anode gives up its work function energy (heat of vaporization of the electrons) due to the receiving of the electrons and their energy due to the anode drop. When the temperature was higher than the melting point (1450° C) of the anode, the material is thrown off in droplets or vaporized from the anode. Due to these reason a high machining rate and very low tool wear is found in this condition (Lee 2001). 108 0.0016 300 0.0014 250 0.0012 0.0010 RWR (%) MRR (mm3/min) Performance analysis of LECD electrode in micro-EDM application 0.0008 0.0006 0.0004 200 150 100 50 0.0002 0 0.0000 60 80 100 120 Gap Voltage (V) Electrode -ve Electrode +ve 140 (a) 60 80 100 Gap Voltage (V) Electrode -ve 120 140 Elecode +ve (b) Figure 6.4: Effect of polarity on (a) MRR (b) RWR 6.6 Performance study of LECD electrode on high melting point material (a) (i) 60 V, 100 pf Energy 0.18 µJ (ii) 100 V, 470 pf Energy 2.35 µJ (iii) 140 V, 2200 pf Energy 21.56 µJ (i) 60 V, 100 pf Energy 0.18 µJ (ii) 100 V, 470 pf Energy 2.35 µJ (iii) 140 V, 2200 pf Energy 21.56 µJ (b) Figure 6.5: (a) Entrance and (b) Exit side SEM image of micro hole with LECD electrode at different energy level of discharge energy Figures 6.5 (a) and (b) show that, at almost all level of discharge energy, the microholes obtained were free of burrs and recast layers and improved circularity is 109 Performance analysis of LECD electrode in micro-EDM application achieved on austenitic stainless steel (SUS 304). By using very low value of capacitance the discharge energy can be minimized which can give good surface finish and edge linearity can be achieved. Therefore, as the energy per pulse is smaller, smaller craters are generated which means smaller amount of material is removed per cycle. 6.6.1 Effect of gap voltage The gap voltage plays an important role in micro-EDM application. Figures 6.6 (a) and 6.7 (a) and (c) show that with the increase of voltage the MRR, ASG and ATA increases for all values of capacitance. This is because; from equation (6.1), it is clear that when gap voltage increases the discharge energy also increases as well as spark gap. ATA depends on the ASG and MRR. This is why when the spark gap increases the taper angle also increases. However, in the lower voltage the taper angle is higher than higher voltage. This is because the material removal rate is low in lower voltage that means the machining time longer, which cause the taper angle larger. In RC type pulse generator relatively smaller craters is generated due to its lower energy per pulse and debris created by machining is flushed away from the machining zone by the dielectric. Figure 6.6 (c) shows that the relative wear ratio increases with the increase of gap voltage due to increase of discharge energy. In the case of dimensional accuracy, figure 6.7 (a) shows that dimensional accuracy decreases as the average spark gap and taper angle increases due to increase of gap voltage. In order to improve the dimensional accuracy appropriate gap voltage and capacitance value can be used to achieve around 2 µm average spark gap and 2 degree of taper angle. 110 Performance analysis of LECD electrode in micro-EDM application 0.0035 0.0035 0.0030 0.0030 MRR (mm3/min) MRR (mm3/min) 6.6.2 Effect of capacitance 0.0025 0.0020 0.0015 0.0010 0.0025 0.0020 0.0015 0.0010 0.0005 0.0005 0.0000 0.0000 60 80 c = 100 pf c = 220 pf 100 100 120 140 Gap Voltage (V) c = 470 pf c = 1000 pf c = 2200 pf 220 60 Volt 100 Volt 2200 140 Volt (b) 60 60 50 50 40 40 RWR (%) RWR (%) (a) 470 1000 Capacitance (pf) 30 20 30 20 10 10 0 0 60 c = 100 pf 80 c = 220 pf 100 Gap Voltage (V) c = 470 pf 120 140 c = 1000 pf c = 2200 pf (c) 100 220 470 1000 Capacitance (pf) 60 Volt 100 Volt 2200 140 Volt (d) Figure 6.6: Effect of gap voltage on (a) MRR (c) RWR; Effect of capacitance on (b) MRR (d) RWR The capacitor acts a very important role in micro-EDM application. In RC-type pulse generator, the capacitor controls the charging and discharging pulse frequency. Therefore, nano pulse can be generated in RC-type with very short pulse duration. For this by using very low value of capacitor, the pulse energy minimization can be easily fulfilled. This is why by changing the capacitor value good dimensional accuracy can 111 Performance analysis of LECD electrode in micro-EDM application be achieved. Figure 6.6 (b) shows that with the increase of capacitance the MRR increased as the discharge energy increases. Therefore, the larger capacitance results in 20 18 16 14 12 10 8 6 4 2 0 ASG (µm) ASG (µm) deeper craters, which increase the material removal. 60 c = 100 pf 80 c = 220 pf 100 120 Gap Voltage (V) c = 470 pf c = 1000 pf 20 18 16 14 12 10 8 6 4 2 0 100 140 60 Volt c = 2200 pf (a) 470 1000 Capacitance (pf) 100 Volt 140 Volt 2200 (b) 12 12 10 10 8 8 ATA (degree) ATA (degree) 220 6 4 6 4 2 2 0 0 60 c = 100 pf 80 c = 220 pf 100 120 Gap Voltage (V) c = 470 pf c = 1000 pf 140 c = 2200 pf (c) 100 220 60 Volt 470 1000 Capacitance (pf) 100 Volt 140 Volt 2200 (d) Figure 6.7: Effect of gap voltage on (a) ASG (c) ATA; Effect of capacitance on (b) ASG (d) ATA The relative wear ratio also increased with the increase of capacitance. However, it is found that from figure 6.6 (d) that at very high value of capacitance the relative wear ratio decreases. Actually, this is not investigative of lower electrode wear. In this study, the electrode wear was measured as a ratio of volume of electrode material 112 Performance analysis of LECD electrode in micro-EDM application eroded to the volume of material removed from the workpiece. In the case of lower capacitance RWR is high for lower voltage, because in this condition MRR is very low that causes the electrode erode more. For this reason, the ratio shows a decreased value although more material is removed from the electrode compared to that of lower capacitance. Therefore, at very high value of capacitor the ASG increases which cause more material remove with respect to electrode erosion (figure 6.7 (b)). Moreover, the dimensional accuracy is also reduces as ASG and ATA (figures 6.7 (b) and 6.7(d)) increase with respect to gap voltage. 6.7 Performance comparison of LECD electrode on various workpiece material 6.7.1 EDX spectrum analysis of the LECD electrode Figure 6.8: LECD electrode top surface after micro-EDM on (a) stainless steel (b) copper (c) brass (d) aluminum 113 Performance analysis of LECD electrode in micro-EDM application Figure 6.8 shows LECD electrode top surface after micro-EDM on different workpiece. In order to get a clear idea about the black spots and surface defects on the machined surface, EDX spectrum analysis has been done on the complete machined area of the LECD electrode. Figure 6.9 (a)–(d) represent the EDX spectrum analysis of the complete machined surface shown in Figure 6.8 (a)–(d) respectively. In this study, four different LECD electrodes are used for four different workpiece materials. Figure 6.9: EDX spectrum analysis of the LECD electrode top surface after microEDM on (a) stainless steel shown in Figure 5(a), (b) copper shown in Figure 5(b), (c) brass shown in Figure 5(c) and (d) aluminum shown in Figure 5(d) The elements present in the surface are clearly indicated by the peaks corresponding to their energy levels. Moreover, the major relative percentages of different materials found in the surface are also given in Table 6.4. It can be seen from the analysis that 114 Performance analysis of LECD electrode in micro-EDM application the percentage of carbon is higher on the LECE electrode surface obtained machined on stainless steel. However, electrode surfaces machined on copper, brass and aluminum carbon percentage is less. This may be due to the high thermal conductivity and low electrical resistivity of aluminum. These properties make possible the discharging process to be uniform with reduced short-circuiting and arcing. In Figure 6.8, the black spot on the LECD electrode surface indicates the higher percentages of carbon. Carbon is present due to the decomposition of the working oil and affixed to the surface following the evaporation and melting of the workpiece as well as electrode material. In general, the carbon is removed in the form of debris by melting/evaporation. However, sometime it gets re-deposited on the surface as sometimes the debris cannot be flushed away properly from the machined zone due to the low working gap. Other major percentages of materials like oxygen and sulfur remains almost same for all the workpiece materials. Table 6.4: The relative percentages of material from the EDX spectrum analysis of deposited structures shown in Figure 6.9 Before EDM Machining [Figure 5.3 (d)] After EDM on Stainless Steel [Figure 5.9 (a)] After EDM on Copper [Figure 5.9 (b)] After EDM on Brass [Figure 5.9 (c)] After EDM on Aluminum [Figure 5.9 (d)] Copper (Cu) Carbon (C) Sulfur (S) Oxygen (O) 81.37% 12.31% 0.04% 2.73% 55.94% 39.87% 0.08% 4.09% 65.49% 31.70% 0.07% 2.37% 67.17% 27.81% 0.13% 3.38% 75.63% 22.56% 0.08 1.49% 115 Performance analysis of LECD electrode in micro-EDM application 6.7.2 Effect on MMR Figure 6.10 (a), (c) and (e) shows the effect of gap voltage on MRR for stainless steel, copper, brass and aluminum. Similarly, Figure 6.10 (b), (d) and (f) shows the effect of capacitance on MRR for stainless steel, copper, brass and aluminum. It can be seen from these experimental results that with the increase of voltage as well as capacitance the MRR increases for all four types of materials. From equation (6.1), it can be seen that the discharge energy is a function of capacitance and gap voltage. This is why, when the capacitance and gap voltage increase it increase the discharge energy. Due to the increase of discharge energy, the MRR also increases. It can be seen form Figure 6.10 that the overall MRR of stainless steel is lower than other materials. However, the MMR of brass and aluminum is higher than copper and stainless steel. Melting point plays a significant role in this occurrence. In micro-EDM, the metal is removed by melting and evaporation. The melting point of stainless steel is much than the others. This is why the MRR is lower for stainless steel. On the other hand, for brass and aluminum the melting point is lower as well as the thermal conductivity is higher with respect to stainless steel. For this reason, the MRR of brass and aluminum is higher than other materials. Although in case of copper, the thermal conductivity is higher and the electrical resistivity is relatively lower than other materials, but due to the higher melting point, the MRR of copper is lower than brass and aluminum. 116 0.0035 0.003 0.003 0.0025 MRR (mm3/min) MRR (mm3/min) Performance analysis of LECD electrode in micro-EDM application 0.0025 0.002 0.0015 0.001 0.002 0.0015 0.001 0.0005 0.0005 0 0 60 Steel 80 Copper 100 120 Gap Voltage (V) Brass 100 140 Steel Aluminium 0.01 0.009 0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0 Brass Aluminium 0.006 0.005 0.004 0.003 0.002 0.001 0 60 Steel 80 Copper 100 120 Gap Voltage (V) Brass 100 140 Steel Aluminium (c) 470pf 220 470 1000 Capacitance (pf) Copper Brass 2200 Aluminium (d) 100 V 0.014 0.014 0.012 0.012 MRR (mm3/min) MRR (mm3/min) Copper 2200 (b) 60 V MRR (mm3/min) MRR (mm3/min) (a) 100pf 220 470 1000 Capacitance (pf) 0.01 0.008 0.006 0.004 0.01 0.008 0.006 0.004 0.002 0.002 0 0 60 Steel 80 Copper 100 120 Gap Voltage (V) Brass (e) 2200pf Aluminium 140 100 Steel 220 470 1000 Capacitance (pf) Copper Brass 2200 Aluminium (f) 140 V Figure 6.10: Effect on MRR with the variation of voltage at a fixed capacitor value of (a) 100pf (c) 470pf (e) 2200pf. Effect on MRR with the variation of capacitor at a fixed voltage value of (b) 60V (d) 100V (f) 140V 117 Performance analysis of LECD electrode in micro-EDM application 6.7.3 Effect on RWR Figure 6.11 (a), (c) and (e) shows the effect of gap voltage on MRR for stainless steel, copper, brass and aluminum. It can be seen from these experimental results that with the increase of gap voltage the RWR increases. The discharge energy increases due to increase of gap voltage. Due to the increase in gap voltage RWR increases. However, at lower capacitance value of 100pf the RWR decreases in the 80V to 100V region. One possible reason may be, in this region the MRR is higher respect to lower voltage value of 60V. If the MRR is higher, then the machining time will be lower. This is why when the machining time decreases the RWR also decreases. Similarly, Figure 6.11 (b), (d) and (f) show that the effect of capacitance on RWR for stainless steel, copper, brass and aluminum. It can be seen from these experimental results that with the increase of capacitance value initially the RWR increases and after 470pf the RWR decreases. In this study, the electrode wear was measured as a ratio of volume of electrode material eroded to the volume of material removed from the workpiece. In the case of lower capacitance RWR is high for lower voltage, because in this condition MRR is very low that causes the electrode erode more. For this reason, the ratio shows a decreased value although more material is removed from the electrode compared to that of lower capacitance. Results also show that for stainless steel machining the RWR is higher due to its lower MRR. On the other hand, for brass and aluminum the RWR is lower due to its higher MRR. 118 Performance analysis of LECD electrode in micro-EDM application 35 30 RWR (%) RWR (%) 25 20 15 10 5 0 60 80 100 120 Gap Voltage (V) Steel Copper Brass 45 40 35 30 25 20 15 10 5 0 100 140 Steel Aluminium 470 1000 Capacitance (pf) Copper Brass 2200 Aluminium (b) 60 V 60 60 50 50 40 40 RWR (%) RWR (%) (a) 100pf 30 20 30 20 10 10 0 0 60 80 100 120 Gap Voltage (V) Steel Copper Brass 100 140 220 Steel Aluminium (c) 470pf 470 1000 Capacitance (pf) Copper Brass 2200 Aluminium (d) 100 V 50 45 40 35 30 25 20 15 10 5 0 60 50 RWR (%) RWR (%) 220 40 30 20 10 0 60 80 100 120 Gap Voltage (V) Steel Copper Brass (e) 2200pf Aluminium 140 100 220 Steel 470 1000 Capacitance (pf) Copper Brass 2200 Aluminium (f) 140 V Figure 6.11: Effect on RWR with the variation of voltage at a fixed capacitor value of (a) 100pf (c) 470pf (e) 2200pf. Effect on RWR with the variation of capacitor at a fixed voltage value of (b) 60V (d) 100V (f) 140V 119 Performance analysis of LECD electrode in micro-EDM application 6.7.4 Effect on ASG Figure 6.12 (a), (c) and (e) shows the effect of gap voltage on ASG for stainless steel, copper, brass and aluminum. Likewise, Figure 6.12 (b), (d) and (f) show that the effect of capacitance on ASG for stainless steel, copper, brass and aluminum. Results show that with the increase of gap voltage and capacitance the ASG increases. This is due to the increase of discharge energy and these increases due to the increase of voltage and capacitance. The ASG for stainless steel is lower than other materials. Stainless steel is hard to machine material in micro-EDM due to its high melting point, low thermal conductivity and high electrical resistivity. For this, at a certain energy level the ASG is lower than other materials. In case of copper, although its melting point is high, but due to its high thermal conductivity in some cases it shows high ASG comparatively others. On the other hand, in case of aluminum and brass the melting point is low and in case of aluminum the thermal conductivity is high and the electrical resistivity is low. This is why for aluminum and brass the ASG is higher than other two materials. Figure 6.14, Figure 6.15 and Figure 6.16 show that at lower energy the crater size is also lower which results in smaller spark gap. Smaller spark gap is always needed to obtain good dimensional accuracy. 120 9 8 7 6 5 4 3 2 1 0 25 20 ASG (µm) ASG (µm) Performance analysis of LECD electrode in micro-EDM application 15 10 5 0 60 Steel 80 100 120 Gap Voltage (V) Copper Brass 100 140 220 Steel Aluminium (a) 100pf Brass Aluminium 30 14 25 12 10 ASG (µm) ASG (µm) Copper 2200 (b) 60 V 16 8 6 4 20 15 10 5 2 0 0 60 80 100 120 Gap Voltage (V) Steel Copper Brass 140 Aluminium 100 220 Steel Copper (c) 470pf 470 1000 Capacitance (pf) Brass 2200 Aluminium (d) 100 V 30 25 25 20 20 ASG (µm) ASG (µm) 470 1000 Capacitance (pf) 15 10 15 10 5 5 0 0 60 80 100 120 Gap Voltage (V) Steel Copper Brass (e) 2200pf Aluminium 140 100 Steel 220 470 1000 Capacitance (pf) Copper Brass 2200 Aluminium (f) 140 V Figure 6.12: Effect on ASG with the variation of voltage at a fixed capacitor value of (a) 100pf (c) 470pf (e) 2200pf. Effect on ASG with the variation of capacitor at a fixed voltage value of (b) 60V (d) 100V (f) 140V 121 Performance analysis of LECD electrode in micro-EDM application 7 14 6 12 5 10 ATA (degree) ATA (degree) 6.7.5 Effect on ATA 4 3 2 8 6 4 2 1 0 0 60 Steel 80 100 120 Gap Voltage (V) Copper Brass 100 140 Aluminium Steel 220 470 1000 Capacitance (pf) Copper Brass (a) 100pf 8 ATA (degree) ATA (degree) 10 6 4 2 0 80 Steel Copper 100 120 Gap Voltage (V) Brass 18 16 14 12 10 8 6 4 2 0 140 Aluminium 100 220 470 1000 Capacitance (pf) Steel Copper Brass 18 16 14 12 10 8 6 4 2 0 80 Steel Copper 100 120 Gap Voltage (V) Brass 2200 Aluminium (d) 100 V ATA (degree) ATA (degree) (c) 470pf 60 Aluminium (b) 60 V 12 60 2200 Aluminium 140 18 16 14 12 10 8 6 4 2 0 100 Steel 220 470 1000 Capacitance (pf) Copper (e) 2200pf Brass 2200 Aluminium (f) 140 V Figure 6.13: Effect on ATA with the variation of voltage at a fixed capacitor value of (a) 100pf (c) 470pf (e) 2200pf. Effect on ATA with the variation of capacitor at a fixed voltage value of (b) 60V (d) 100V (f) 140V 122 Performance analysis of LECD electrode in micro-EDM application Figure 6.13 (a), (c) and (e) shows the effect of gap voltage on ATA for stainless steel, copper, brass and aluminum. In the same way, Figure 6.13 (b), (d) and (f) shows the effect of capacitance on ATA for stainless steel, copper, brass and aluminum. The reason for this taperness of the micro-holes is due to the corner wear of the electrode, which reduces the dimensional accuracy. Results show that with the increase of gap voltage and capacitance the ATA increases. With the increase of voltage and capacitance the MRR and ASG increases. Due to the increase of ASG, the electrode entrance diameter will increase. However, due to increase in MRR the electrode will reach to the exit of the workpiece quickly. If the electrode reaches the exit of the workpiece quickly then at the exit the ASG will increase less than the entrance. This phenomenon causes the ATA increase due to the increase of discharge energy. Results also show that the ATA of stainless steel is lower than other materials for its high melting point, low thermal conductivity and high electrical resistivity. Figure 6.14: (a) Entrance and (b) Exit side SEM image of micro hole with LECD electrode at the discharge energy of 0.18µJ (voltage 60V and capacitance 100pf) 123 Performance analysis of LECD electrode in micro-EDM application Figure 6.15: (a) Entrance and (b) Exit side SEM image of micro hole with LECD electrode at the discharge energy of 2.35µJ (voltage 100V and capacitance 470pf) Figure 6.16: (a) Entrance and (b) Exit side SEM image of micro hole with LECD electrode at the discharge energy of 21.56µJ (voltage 140V and capacitance 2200pf) 124 Performance analysis of LECD electrode in micro-EDM application 6.8 Comparative study of LECD electrode with circular electrode Figure 6.17: Circular copper electrode of equal LECD electrode cross sectional area Figure 6.17 shows that the micro electrode fabricated by micro turning and its cross sectional area is equal of the LECD electrode. With this electrode micro holes are fabricated on austenitic stainless steel (SUS 304) workpiece with same experimental parameter which are shown in figures 6.18 (a) and (b). Figure 6.18: (a) Entrance and (b) Exit side SEM image of micro hole with circular copper electrode at different energy level of discharge energy 125 0.0018 0.0016 0.0014 0.0012 0.0010 0.0008 0.0006 0.0004 0.0002 0.0000 MRR (mm3/min) MRR (mm3/min) Performance analysis of LECD electrode in micro-EDM application 60 c = 100 pf 80 c = 220 pf 0.0018 0.0016 0.0014 0.0012 0.0010 0.0008 0.0006 0.0004 0.0002 0.0000 100 100 120 140 Gap Voltage (V) c = 470 pf c = 1000 pf c = 2200 pf 220 60 Volt 100 Volt (a) 2200 140 Volt (b) 40 40 35 35 30 30 25 25 RWR (%) RWR (%) 470 1000 Capacitance (pf) 20 15 20 15 10 10 5 5 0 0 60 c = 100 pf 80 c = 220 pf 100 Gap Voltage (V) c = 470 pf 120 140 c = 1000 pf c = 2200 pf (c) 100 60 Volt 220 470 1000 Capacitance (pf) 100 Volt 2200 140 Volt (d) Figure 6.19: Effect of gap voltage on (a) MRR (c) RWR; Effect of capacitance on (b) MRR (d) RWR The results show that the performance of LECD electrode with respect to circular electrode in EDM operation is more enviable. This is because the MRR in circular copper electrode is 20-45% lower than the LECD electrode (figures 6.6 (a), (b) and 6.19 (a), (b)). The reason is in case of LECD electrode the debris can easily flash out because of complex cross sectional are. On the other hand, for circular electrode it is very difficult to flash out the debris from the center area of the electrode. In case of RWR the maximum value for circular electrode is 35% and for LECD electrode is 50% (figures 6.6 (c), (d) and 6.19 (c), (d)). Although the RWR is higher for LECD 126 Performance analysis of LECD electrode in micro-EDM application electrode, for higher capacitor values the relative wear ratio decreases up to 16%. In addition, for the higher value of the capacitor the material removal rate increases up to 190%. This proves that in higher capacitor value the performance of LECD electrode is better with respect to circular electrode because of higher material removal rate and lower relative wear ratio. The ASG and ATA of LECD electrode is 35-75% (figures 6.7 (a), (b) and 6.20 (a), (b)) and 25-30% (figures 6.7 (c), (d) and 6.7 (c), (d)) lower that the circular copper electrode. These results show that in case of LECD electrode the dimensional accuracy is more than circular copper electrode. 30 25 25 20 20 ASG (µm) ASG (µm) 30 15 10 15 10 5 5 0 0 60 c = 100 pf 80 c = 220 pf 100 120 Gap Voltage (V) c = 470 pf c = 1000 pf 100 140 220 60 Volt c = 2200 pf 470 1000 Capacitance (pf) 100 Volt 140 Volt (a) (b) 14 14 12 12 10 ATA (degree) ATA (degree) 2200 8 6 4 10 8 6 4 2 2 0 0 60 c = 100 pf 80 c = 220 pf 100 120 Gap Voltage (V) c = 470 pf c = 1000 pf 140 c = 2200 pf (c) 100 60 Volt 220 470 1000 Capacitance (pf) 100 Volt 140 Volt 2200 (d) Figure 6.20: Effect of gap voltage on (a) MRR (c) RWR; Effect of capacitance on (b) MRR (d) RWR 127 Performance analysis of LECD electrode in micro-EDM application 6.9 Performance comparison of LECD electrode and circular electrode for complex structure fabrication In order to compare the ‘X’ shape micro hole fabricated on austenitic stainless steel (SUS 304) workpiece by LECD electrode with a circular EDM electrode, a circular copper micro electrode is fabricated by micro turning (figure 6.21 (a) (i)). Here the diameter of the micro electrode is equal to the diameter of the micro milling cutter, which has used to fabricate the mask of the LECD electrode. Finally, figure 6.21 (b) shows micro hole is fabricated by scanning EDM, which has equal cross sectional area and similar shape of the micro hole fabricated by LECD electrode. Figure 6.22 (a) and (b) show EDM parameter comparison study of die sinking EDM by LECD electrode and scanning EMD by circular electrode. Results show that the micro hole fabricated by LECD electrode is more desirable. This is because; the MRR of scanning EMD by circular electrode is much lower than the die sinking EDM by LECD electrode which is 60-70% lower value. In addition, the RWR is also very high in case of scanning EDM, which is almost 3 to 4 times of the die sinking EDM by LECD electrode. Figure 6.21 (a) (ii) shows that in order to fabricate the ‘X’ shape hole by scanning EDM, the circular electrode needs to travel a pathway. Due to the travel of the circular electrode, debris can be easily removed from the machining area. In addition, the cross sectional area of the circular electrode is also lesser than the LECD electrode. For this reason, the debris can be easily flushed away during machining time. However, in case of LECD electrode due to its complex cross sectional are it is difficult to remove the debris from the machining are. This drawback can be improved by multiple flashing in case of LECD electrode. It can be seen from the results that the machining time for 128 Performance analysis of LECD electrode in micro-EDM application scanning EDM decreases with the increase of voltage due to high discharge energy. These two phenomena decrease the RWR with the increase of voltage. Figure 6.21: (a) Circular copper micro shaft and its scanning direction (b) Entrance and exit of the micro hole fabricated by scanning EDM. 250 0.0016 200 0.0012 0.0010 RWR (%) MRR (mm3/min) 0.0014 0.0008 0.0006 0.0004 150 100 50 0.0002 0 0.0000 60 80 100 120 Gap Voltage (V) Die sinking EDM Scanning EDM 140 (a) 60 80 100 Gap Voltage (V) Die sinking EDM 120 140 Scanning EDM (b) Figure 6.22: Effect of gap voltage on (a) MRR (b) RWR for die sinking EDM and scanning EDM 129 Performance analysis of LECD electrode in micro-EDM application Figure 6.23 shows the deposited electrode with NUS shape and NUS shape micro hole is fabricated on austenitic stainless steel (SUS 304) workpiece. (a) (b) Figure 6.23: (a) NUS shape deposited electrode (b) NUS shape hole was machined by NUS shape electrode with EDM discharge energy of 2.35µJ 6.10 Concluding remarks A detailed performance analysis has been conducted in order to obtain better quality micro-holes by micro-EDM machining with LECD electrode on high melting point material such as stainless steel as well as on copper, aluminum and brass. From this study, we can draw the following conclusions: ™ Results show that micro-holes with good quality can be achieved by LECD electrodes. In this process, minimum spark gap and taper angle of the micro hole can be achieved by controlling the discharge energy, which is resolute by voltage and capacitance only. 130 Performance analysis of LECD electrode in micro-EDM application ™ The performance of LECD electrode is more desirable than conventional electrode in terms of MRR, RWR and dimensional accuracy. At higher energy level, LECD electrode shows better performance than pure copper circular electrode. This is because; at higher value of capacitor, the MRR increases whereas the RWR decreases. This is a sign of good performance in the microEDM of LECD electrode. ™ To fabricate the same complex shape hole by other process like, scanning EDM is not effective with respect to die sinking EDM by LECD electrode in terms of MRR and RWR. This is the way in which complex shaped micro hole can be fabricated with minimum time and cost. ™ Micro holes are fabricated on stainless steel, copper, brass and aluminum workpiece with the LECD electrode by die sinking EDM in different energy level. Results also demonstrate that besides the discharge energy, the melting point, thermal conductivity and electrical conductivity of the electrode material play an important role in determining surface finish, MRR and EWR during the micro-EDM process. This is why the surface quality of stainless steel is better, due to its mechanical and thermal properties. On the other hand, brass and aluminum give higher MRR and lower RWR with respect to stainless steel and copper. 131 Conclusions, contributions and recommendations Chatper 7 Conclusions, Contributions and Recommendations This chapter summarizes the major findings (section 7.1) and discusses research contributions of this work for electrode fabrication by localized electrochemical deposition (LECD) for on-machine micro-EDM (Electro Discharge Machining) application (section 7.2). Limitations of the present work and suggestions for possible areas for future studies are discussed in section 7.3. 7.1 Major findings The focus of the study was to bring the LECD process into industrial application area like non-circular electrode fabrication for micro-EDM, which can be used to fabricate micro components for MEMS and bio-MEMS. This current research work in LECD and micro-EDM mainly composed of three major parts; development of a combine LECD and micro-EDM setup; performance study of the LECD process; LECD process modeling; LECD process control and performance study of LECD electrode in microEDM application. Some of the important conclusions that can be drawn from different studies in this thesis are summarized below: ™ A LECD and EDM combined setup which is mounted on a multi-purpose machine has been developed. It has been found that this setup is capable of fabricating on-machine deposited electrodes. These electrodes can be used as 132 Conclusions, contributions and recommendations an EDM electrode without changing or removing its original orientation. Here the cathode is attached on the machine z-axis, which is in top of the anode electrode. In order to achieve the non-circular shape electrode a non-conductive mask is placed between the anode and cathode. A linear voice coil motor is used in order to give feedback movement to the cathode and to detect the mask position. ™ At the initial stage, the focus of the study was to observe the effect of plating solution concentration and organic additives. The results suggest that the Cu2+ concentration has no significant effect on the deposition rate. However, the deposited structures were irregular and highly porous when the concentration was less than 0.1 M. On the other hand, if the concentration is high then the deposition is firm and consistent. It is also found that fine-grained and microcrystalline deposits can be achieved by adding organic substances such as thiourea (0.04 g/l) to the acidic copper sulfate solution. ™ In order to verify the capability of the LECD setup two separate performance study was conducted. Result showed that if the deposition height is higher, then repeatability of the process reduces. Regarding micro-structural homogeneity, results show that for voltage higher than 1.6V, frequency less than 100 kHz and duty ratio higher than 0.33 the homogeneity of the structure reduces and the size of the grain increases. Due to this reason, in these cases, the deposition becomes tree or powdery type and the repeatability of the process reduces. 133 Conclusions, contributions and recommendations ™ From the mathematical simulation results, it is possible to determine the height as well as the rate of the deposition at different condition and different deposition time. Results show that the deposition rate increases tremendously with the increase in pulse voltage amplitude and duty ratio and a decrease in pulse voltage frequency. However, by maintaining the four operating parameters within a proper range a smooth, fine-grained, low porosity and complex shape copper electrodes are deposited in LECD process. From the study, results suggested that good quality deposition could be achieved in the condition of voltage amplitude of 1.5V to 1.6, frequency 100 kHz to 115 kHz, duty ratio of 0.33 and electrode gap of 350µm. ™ The operating value of the initial growth height has been established from the surface plot, on various electrolyte flow rate and various electrode gap. The requirement of the initial growth height was justified and the operating ranges were calculated from FLUENT analysis. Results show that in controlling the process, in process repeatability as well as in maintaining the deposition structural quality, close loop control performed better than open loop control. In this study, only P controller was used for close loop feedback controller and deposition current was used as a feedback signal. ™ In order to examine the machinability of LECD in micro EDM operation on different workpiece materials like stainless steel, copper, brass and aluminum workpiece with the LECD electrode by die sinking EDM in different energy level. Results indicated that the machining capability of LECD electrode even 134 Conclusions, contributions and recommendations for the high melting point material as well. Besides this, results also demonstrate that the melting point, thermal conductivity and electrical conductivity of the electrode material have a vital position in determining surface finish, MRR and RWR during the micro-EDM process. Results showed that the surface quality achieved for stainless steel is better than other materials due to its high melting point and low thermal conductivity. On the other hand, brass and aluminum gives higher MRR and lower RWR with respect to stainless steel and copper. ™ In fabricating complex shape holes, die sinking EDM of LECD electrode performs better than other processes like scanning EDM by a circular pure copper electrode in terms of MRR and RWR. The performance of LECD electrode is more desirable than conventional electrode in terms of MRR, RWR, ASG and ATA. At higher energy level, the MRR increases whereas the RWR decreases for LECD electrodes. This is why, at this condition LECD electrodes show a better performance than pure copper electrode. These two comparative studies indicate that complex shape hole fabrication by LECD process is more effective and it help to reduce fabrication time and cost. 7.2 Research Contributions ™ One key contribution of the study is the LECD and EDM combined experimental setup. This setup is capable of fabricating on line EDM electrodes in variety of complex shape by changing the mask structure. 135 Conclusions, contributions and recommendations ™ An operating range of LECD process has been established from the derived mathematical modeling and its simulations. This finding is significant because by maintaining the parameters within the range a smooth, finegrained, low porosity and complex shape copper EDM electrodes are deposited. Moreover, by maintaining parameters within the range high aspect ratio microelectrode is fabricated. ™ Another contribution is the complex shape micro-holes fabrication, which are fabricated by LECD electrode. This fabrication process will be a good solution in MEMS and bio-MEMS industries, where complex shape holes and cavities are required to fabricate. Moreover, this fabrication process is very effective for industrial application, where the production time and cost can be minimized. 7.3 Limitations and recommendations ™ Variety of metal deposition was not taken into account in this study, because the focus of the study was to develop the process to fabricate onmachine electrode. For this reason, the study is limited to copper deposition only. However, further studies are needed to visualize other material deposition quality and performances. ™ One limitation of the mathematical modeling is that these models do not describe the deposition density or porosity as well as structural quality. 136 Conclusions, contributions and recommendations Future research should attempt to focus on the deposition density or porosity modeling. ™ This study did not consider the mask fabrication process, because the mask is only fabricated by micro milling process. A direct extension of this work is to fabricate the mask in different fabrication processes that can be laser beam machining (LBM). In future studies, the mask design can be improved in order to circulate the electrolyte properly through the mask that may be by machining the lower portion of the mask taper (Figure 7.1). In that case, the taper angle and height need to be optimized accordingly. Figure 7.1: New mask design for future research ™ An intelligent power supply can be developed for future research. In that power supply, instead of depositing the metal with same operating 137 Conclusions, contributions and recommendations conditions, the operating conditions can be changed according to the deposition state and condition. For example, if the deposition starts with 1.6 volt and the controller feels that the rate is increasing then it will shift to some lower voltage and stabilize the deposition condition. At the same time, a concentration monitoring system needs to be involved in the setup for maintaining the concentration of the electrolyte constant throughout the experiment. 138 Bibliography Chatper 8 Bibliography Ananthakrishnan, A., Sarma, R., Ananthasuresh, G. K. "Systematic mask synthesis for surface micromachined microelectromechanical systems." Journal of Micromechanics and Microengineering 13 (2003): 927–941. Arie, T., Akita, S., Nakayama, Y. "Rapid communication: growth of tungsten carbide nano-needle and its application as a scanning tunneling microscope tip." Journal of Physics D: Applied Physics 31 (1998): L49–L51. Asad, A.B.M.A., Masaki, T., Rahman, M., Lim, H.S., Wong, Y.S.,. "Tool-based micro-machining. Journal of Materials Processing Technology." Journal of Materials Processing Technology 192-193 (2007): 204 – 211. Chae, J., Park, S. S., Freiheit, T. "Investigation of micro-cutting operations." International Journal of Machine Tools & Manufacture 46 (2006): 313–332. Chung, C. K., Chang, W. T. "Effect of pulse frequency on the morphology and nanoindentation property of electroplated nickel films." Microsystem Technologies 13 (2007): 537–541. Corbett, J., McKeown, P.A., Peggs, G.N., Whatmore,R. "Nanotechnology: international developments and emerging products." Annals of CIRP 49 (2000): 523– 546. Czerwinski, F. "The microstructure and internal stress of Fe–Ni nanocrystalline alloys electrodeposited without a stress-reliever." Electrochimica Acta 44, no. 4 (1998): 667675. 139 Bibliography Dornfeld, D., Min, S., Takeuchi, Y.,. "Recent Advances in Mechanical Micromachining." Annals of the CIRP 55, no. 2 (2006): 745 – 768. El-Giar, E. M., Cairo, U., Thomson, J. D. "Localized electrochemical plating of interconnectors for microelectronics." Proc. IEEE Conf. Communications, Power and Computing, WESCANEX 97, 1997: 327–332. El-Giar, E. M., Said, R. A., Bridges, G. E., Thomson, D.J. "Localized Electrochemical Deposition of Copper Microstructures." Journal of the Electrochemical Society 147 (2000): 586–591. Fang, F.Z., Liu, K., Kurfess, T.R., Lim, G.C.,. "Tool-based Micromachining and Applications in MEMS." MEMS/NEMS Handbook Techniques and Applications, Springer US 3 (2006): 678 – 740. Gleiter, H. "Nanocrystalline Materials." Prog Mater Sci 33 (1989): 223-315. Habib, M. A., Gan, S. W., Lim, H. S., Rahman, M. "Fabrication of EDM Electrodes by Localized Electrochemical Deposition." International Journal of Precision Engineering and Manufacturing 9, no. 2 (2008): 75-80. Habib, M. A., Gan, S. W., Rahman, M. "Fabrication of Complex Shape Electrodes by Localized Electrochemical Deposition." Journal of Materials Processing Technology 209, no. 9 (2009): 4453–4458. Harley, B. A., Hastings, A. Z., Yannas, I. V., Sannino, A. "Fabricating tubular scaffolds with a radial pore size gradient by a spinning technique." Biomaterials 27, no. 6 (2006): 866-874. Heb, C., Borgwarth, K., Ricken, C., Ebling, D. G., Heinze, J. "Scanning electrochemical microscopy: study of silver deposition on non-conducting substrates." Electrochimica Acta 42 (1997): 3065–3073. 140 Bibliography Hofmann, D., Schindler, W., Kirschner, J. "Electrodeposition of nanoscale magnetic structures." Applied Physics Letters 73 (1998): 3279–3281. Hoyer, B., Sorensen, G., Jensen, N., Nielson, D. B., Larseen, B. "Electrostatic Spraying: A Novel Technique for Preparation of Polymer Coatings on Electrodes." Anal. Chem. 68 , no. 21 (1996): 3840–3844. Hu, C. C., Wu, C. M. "Effects of deposition modes on the microstructure of copper deposits from an acidic sulfate bath." Surface and Coatings Technology 176 (2003): 75-83. Hunter, I. W., Lafontaine, S. R., Madden, J. D. "Three-dimensional microfabrication by localized electrochemical deposition and etching." US Patent Specification 5641391, 1997. Ibanez, A., Fatas, E. "Mechanical and structural properties of electrodeposited copper and their relation with the electrodeposition parameters." Surface & Coatings Technology 191 (2005): 7-16. Ishihara S., Hanabusa, M. "Laser-assisted chemical vapor deposition of titanium nitride films." Journal of Applied Physics 84, no. 1 (1998): 596-599. Jansson, A., Thornell G., Johansson, S. "High resolution 3d microstructures made by localized electrodeposition of nickel." Journal of Electrochemical Society 147 (2000): 1810–1827. Kim, B. H., B. J. Park, and C. N. Chu. "Fabrication of multiple electrodes by reverse EDM and their application in micro ECM." Journal of Micromechanics and Microengineering 16 (2006): 843–850. 141 Bibliography Kim, Y.T., Park, S.J., Lee, S.J. "Micro/Meso-Scale Shapes Machining by Micro EDM Process." International Journal of Precision Engineering and Manufacturing 6, no. 2 (2005): 5–11. Kuo, C.L., Huang, J.D., Liang, H.Y., Huang, J.D. "Fabrication of 3D Metal Microstructures Using a Hybrid Process of Micro-EDM and Laser Assembly." International Journal of Advanced Manufacturing Technology 21 (2003): 796-800. LaGraff, J. R., Gewirth, A. A. "Enhanced electrochemical deposition with an atomic force microscope." The Journal of Physical Chemistry 98, no. 44 (1994): 11246– 11250. Lang, W. "Reflexions on the future of microsystems." Sensor and Actuators 72 (1999): 1–15. Lee, S.H., Li, X.P. "Study of the effect of machining parameters on the machining characteristics in EDM of WC." Journal of Materials Processing Technology 115 (2001): 344–355. Li, T., Gianchandani, Y.B.,. "A Micromachining Process for Die-Scale Pattern Transfers in Ceramics and Its Application to Bulk Piezoelectric Actuators." Journal of Microelectromechanical Systems 15, no. 3 (2006): 605-612. Lim, H. J., Lim, Y. M., Kim, S. H. "Fabrication of Arbitrarily Shaped Microelectrodes by Electrochemical Etching." Japan Journal Applied Physics 42 (2003): 1479–1485. Lim, H. S., Y. S. Wong, M. Rahman, and M. K. E. Lee. "A study on the machining of high-aspect ratio micro-structures using micro-EDM." Journal of Materials Processing Technology 140 (2003): 318–325. 142 Bibliography Liu, X., Devor, R.E., Kapoor, S.G., Ehman K.F.,. "The mechanics of machining at the micro scale: assessment of the current state of the science." Journal of Manufacturing Science and Engineering 126 (2004): 666-678. Lu, Z., Yoneyama, T. "Micro-cutting in the micro-lathe turning system." International Journal of Machine Tools & Manufacture 39 (1999): 1171–1183. Madden, J. D., Hunter, I. W.,. "Three-dimensional microfabrication by localized electrochemicaldeposition." Journal of micro electrochemical systems 5, no. 1 (1996): 24-32. Madou, M.J. "Fundamentals of Microfabrication." CRC Press, Boca Raton, 1997. Mandich, N. V. "Practical considerations in bright and hard chromium electroplating Part VII." Metal finishing 98 (2000): 375-385. Masuzawa, T. "Micro-EDM." In: Proceedings of the Thirteenth International Symposium for Electromachining, 2001: 3-19. Masuzawa, T. "State of the Art of Micromachining." Annals of the CIRP 49, no. 2 (2000): 473–488. Meeusen, W., Clijnen, J., Reynaerts, D., Van, H. "Micro-Electro-Discharge Machining as Microsensor Fabrication Technology." IEEE Sensors Journal 3, no. 5 (2003): 632639. Michel, F., W. Ehrfeld, O. Koch, and H. P. Gruber. "EDM for micro fabricationtechnology and applications." Proc. Int. Seminar on Precision Engineering and Micro Technology, 2000: 127–139. Müller, A. D., Müller, F., Hietschold, M. "Localized electrochemical deposition of metals using micropipettes." Thin Solid Films 366, no. 1-2 (2000): 32-36. 143 Bibliography Müller, G., Müller, F., Hietschold, M. "Electrochemical pattern formation in a scanning near-field optical microscope." Applied Physcis A, 1998: S453–S456. Okuyama, H., Takada, H. "Micromachining with SR and FEL." Nuclear Instruments and Methods in Physical Research B 144 (1998): 58-65. Oskam, G., Long, J. G., Natarajan, A., Searson, P.C. "Review article: electrochemical deposition of metals onto silicon." Journal of Physics D: Applied Physics 31 (1998): 1927–1949. Pham, D. T., S. S. Dimov, S. Bigot, A. Ivanov, and K. Popov. "Micro-EDM recent developments and research issues." Journal of Materials Processing Technology 149 (2004): 50-57. Popov, K. I., Kostic, T. M., Nikolic, N. D., Stojilkovic, E. R., Pavlovic, M. G. "A new approach to metal electrodeposition at a periodically changing rate part. I. The reversing overpotential method." Journal of electroanalytical chemistry 464 (1999): 245-251. Pratt, S. D., Muthuswamy, S., Pennisi, R.W. "Electrical Discharge Machining Electrode and Rapid method for fabricating same." US Patent 5818005, 1998. Rahman, M., Kumar, A. S., Prakash, J. R .S.,. "Micro-milling of pure copper." Journal of Materials Processing Technology 116 (2001): 39–43. Rahman, M., Lim, H.S., Neo, K.S., Kumar, A.S., Wong Y.S., Li, X.P.,. "Tool-based nanofinishing and micromachining." Journal of Materials Processing Technology 185, no. 1-3 (2007): 2 – 16. Rai-Choudhury, P. "Handbook of Microlithography, Micromachining and Microfabrication vol 2." (Bellingham, WA: SPIE Optical Engineering Press) 1997. 144 Bibliography Rajurkar, K.P., Yu, Z.Y. "3D Micro-EDM Using CAD/CAM." Annals of the CIRP 49, no. 1 (2000): 127-130. Rausch, N., Burte, E. P. "Thin TiO2 Films Prepared by Low Pressure Chemical Vapor Deposition." Journal of the Electrochemical Society 140, no. 1 (1993): 145-149. Reynaerts, D., P. H. Heeren, and H.V. Brussel. "Microstructuring of silicon by electrodischarge machining, part I: theory." Sensors and Actuators 60 (1997): 212-218. Romankiw, L.T., Palumbo, T.A. "Electrodeposition Technology, Theory and Practice." (Pennington, NJ: Electrochemical Society) 1988. Said, R. A. "Adaptive Tip-Withdrawal Control for Reliable Microfabrication by Localized Electrodeposition." Journal of Microelectromechanical Systems 13, no. 5 (2004): 822-832. Said, R. A. "Localized electro-deposition (LED): the march toward process development." Nanotechnology 15 (2004): S649–S659. Said, R. A. "Microfabrication by localized electrochemical deposition: experimental investigation and theoretical modeling." Nanotechnology 14 (2003): 523–531. Said, R. A. "Shape Formation of Microstructures Fabricated by Localized Electrochemical Deposition." Journal of The Electrochemical Society 150, no. 8 (2003): C549-C557. Schnupp, R., Baumgartner, R., Kuhnhold, R., Ryssel, H. "Electrodeposition of photoresist: optimization of deposition conditions investigation of lithographic processes and chemical resistance." Sensors and Actuators 85 (2000): 310-315. Schoth, A., Förster, A.,Menz, W. "Micro wire EDM for high aspect ratio 3D microstructuring of ceramics and metals." Microsystem Technologies 11 (2005): 250253. 145 Bibliography Seol, S. K., Yi, J. M., Jin, X., Kim, C. C., Je, J. H., Tsai, W. L., Hsu, P. C., Hwu, Y., Chen, C. H., Chang, L. W., Margaritondod, G. "Coherent Microradiology Directly Observes a Critical Cathode-Anode Distance Effect in Localized Electrochemical Deposition." Electrochemical and Solid-State Letters 7, no. 9 (2004): C95-C97. Shizhi, L., Yulong S., Hongrui, P. "Ti-Si-N films prepared by plasma-enhanced chemical vapor deposition." Plasma Chemistry and Plasma Processing 12, no. 3 (1992): 287-297. Suda, M., Nakajima, K., Furuta, K., Mitsuoka, Y., Sakuhara, T., Ataka, T. "Electrochemical and optical processing of microstructures by scanning probe microscopy (SPM) ." Proc. IEEE Microelectromechanical Systems: An Investigation of Micro Structures, Sensors, Actuators, Machines and Systems, 1996 : 296–300. Takahata, K., Shibaike, N., Guckel, H. "A novel micro electro-discharge machining method using electrodesfabricated by the LIGA process." IEEE International Conference on Micro Electro Mechanical Systems, 1999: 238-243. Takahata, K., Shibaike, N., Guckel, H. "High-aspect-ratio WC-Co microstructure produced by the combination of LIGA and micro-EDM." Microsystem Technologies 6 (2000): 175-178. Tang, Y., Hong, J., Zhou, H., Lu, B. "A new technique for the fabrication of graphite EDM electrodes." Journal of Materials Processing Technology 166 (2005): 199–204. Voigt, O., Davepon, B., Staikov, G., Schultze, J.W. "Localized electrochemical deposition and dissolution of Cu on microstructured Ti surfaces." Electrochemcial Acta 44 (1999): 3731–3741. Weck, M., Fischer, S., Vos, M. "Fabrication of micro components using ultra precision machine tools." Nanotechnology 8 (1997): 145–148. 146 Bibliography Weng, F. T., Shyu, R.F., Hsu, C.S. "Fabrication of micro-electrodes by multi-EDM grinding process." Journal of Materials Processing Technology 140 (2003): 332–334. Weng, F.T. "Fabrication of microelectrodes for EDM machining by a combined etching process." Journal of Micromechanics and Microengineering 14 (2004): N1– N4. Yeo, S. H., Choo, J. H. "Effects of rotor electrode in the fabrication of high aspect ratio microstructures by localized electrochemical deposition." Journal of Micromechanics and microengineering 11 (2001): 435-442. Yeo, S. H., Choo, J. H., Sim, K. H. A. "On the effects of ultrasonic vibrations on localized electrochemical deposition." Journal of Micromechanics and microengineering 12 (2002): 271-279. Yeo, S.H., Choo, J. H., Yip, K. S. "Localized electrochemical deposition—the growth behaviour of nickel micro-columns." Proc. SPIE 4174 (2000): 30–39. Yong, L., Yunfei, Z., Guanga, Y., Liangqiang, P. "Localized electrochemical micromachining with gap control." Sensors and Actuators A 108 (2003): 144-148. Yu, Z.Y., Masuzawa, T., Fujino, M.,. "Micro-EDM for three-Dimensional cavities – Development of Uniform Wear Method." Annals of the CIRP 47, no. 1 (1998): 169172. Zha, X.F., Du, H.,. "Web-Enabled Database System Development for Design and Manufacturing of Micro-Electro-Mechanical Systems (MEMS)." MEMS/NEMS Handbook Techniques and Applications, Springer US 1 (2006): 73-109. Zhang, X., Jiang, X. N., Sun, C. "Micro-stereolithography of polymeric and ceramic microstructures." Sensors and Actuators A: Physical 77, no. 2 (1999): 149-156. 147 Bibliography Zhao, J., Li, Y., Zhang, J., Yu, C., Zhang, Y. "Analysis of the wear characteristics of an EDM electrode made by selective laser sintering." Journal of Materials Processing Technology 138 (2003): 475–478. Zhao, W., Yang, Y., Wang, Z., Zhang, Y.,. "A CAD/CAM system for micro-EDmilling of small 3D freeform cavity." Journal of Materials Processing Technology 149 (2004): 573-578. 148 List of publications List of publications Papers in peer review international journals: • M. A. Habib, T. Shaleh and M. Rahman, "Modeling for fabrication of micro electrodes by localized electrochemical deposition for micro-EDM", Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture. DOI: 10.1243/09544054JEM1928. • M. A. Habib and M. Rahman, "Performance analysis of EDM electrode fabricated by localized electrochemical deposition for micro-machining of stainless steel", International Journal of Advanced Manufacturing Technology, DOI: 10.1007/s00170-009-2479-8. • M. A. Habib, S. W. Gan and M. Rahman, "Fabrication of Complex Shape Electrodes by Localized Electrochemical Deposition", Journal of Materials Processing Technology, Vol. 209, No. 2, pp. 4453-4458, 2009. • M. A. Habib, S. W. Gan, H. S. Lim and M. Rahman, "Fabrication of EDM Electrodes by Localized Electrochemical Deposition", International Journal of Precision Engineering and Manufacturing, Vol. 9, No. 2, pp. 75-80, 2008. International conference proceedings: • M. A. Habib and M. Rahman, "An evaluation of EDM electrodes by localized electrochemical deposition", 7th International Conference on Manufacturing Research (ICMR 09), 8-10 September 2009, University of Warwick, Coventry, UK, Organized by - Warwick Manufacturing Group (WMG), International Digital Laboratory (IDL) and Warwick Innovative Manufacturing Research Centre (WIMRC). 149 List of publications • M. A. Habib, S. W. Gan and M. Rahman, "Fabrication of Complex Shape Electrodes by Localized Electrochemical Deposition", 8th Asia-Pacific Conference on Materials Processing (APCMP), 15-20 June 2008, Guilin Guangzhou, China, Organized by - Guangdong University of Technology, China; National University of Singapore, Singapore; Nanyang Technological University, Singapore and The University of New South Wales, Australia. • M. A. Habib, S. W. Gan, H. S. Lim and M. Rahman, "Fabrication of EDM Electrode by Localized Electrochemical Deposition", 2nd International Conference of Asian Society for Precision Engineering and Nanotechnology (ASPEN), 6-9 November 2007, Gwangju, Korea, Organized by - Korean Society for Precision Engineering (KSPE). • Mohammad Ahsan Habib, Md.Rafiqul Islam and M. Arif Hasan Mamun, "Power Generation from Tidal Energy in the Coastal Area of Bangladesh", 5th International Mechanical Engineering Conference and 10th Annual Paper Meet, 30 September-02 October 2005, Dhaka, Bangladesh, Organized by Mechanical Engineering Division, The Institution of Engineers, Bangladesh. 150 Appendix A: Solidworks design of LECD setup Appendix A: Solidworks design of LECD setup Cathode Holder Distance d µm Mask Anode Holder Wedge Figure A.1: Schematic diagram of modified LECD setup designed in solidworks 151 Appendix A: Solidworks design of LECD setup Figure A.2: Solid works design for Outside Tank Figure A.3: Solid works design for Inside Tank 152 Appendix A: Solidworks design of LECD setup Figure A.4: Solid works design for Mask Figure A.5: Solid works design for Hole of Mask 153 [...]... well as circular electrode 1.4 Need for on- machine fabrication of micro- EDM non-circular electrode Depending on the type of electrode and electrode/ workpiece movement, many variations of micro- EDM technologies are used for manufacturing micro- features These include micro- wire EDM, micro- EDM die-sinking, micro- EDM drilling, and micro- EDM milling (Masuzawa 2000) (Pham, et al 2004) Micro- EDM die-sinking... so on) Fabrication processes involved like photolithography and non- conventional machining Micro EDM, an efficient solution for the fabrication of these micro parts Tool handling and fabrication of non circular tool are challenges in Micro- EDM On- machine electrode fabrication by LECD can be a good solution to overcome these challenges Figure 1.1: Background and purpose of this study 4 Introduction... that LECD electrode is capable of machining non-circular 3D structure on wide range of materials This study is expected to make a significant contribution in MEMS and bio-MEMS micro component fabrication, especially in the area of fabrication of on- machine noncircular microelectrodes for micro- EDM process Moreover, from fabrication time and economic point of view this study will be a good guide for mass... Precise micro fabrication is required for the fabrication of micro components such as micro scale fuel cells, micro scale pumps, micro fluidic systems (Weck 1997) (Liu 2004), as well as for the fabrication of micromold cavities for mass-production However, the fabrication of micro- mold cavities require very precise machining of 3D structures on hard to machine workpiece materials (Asad 2007) Over the years,... provides an extended literature review of different electrode fabrication process such as reverse EDM, rapid prototyping, etching, machining and present challenges in these processes The second section focuses on the review of micro- EDM non-circular electrode fabrication process and the role of localized electrochemical deposition (LECD) process in fabricating non-circular 15 ... Among these, tool handling and electrode and workpiece preparation are most important challenges in microfabrication For this reason, the focus of this thesis is to solve these two major issues The tool handling can be solved by on- machine electrode fabrication process and non- 5 Introduction circular electrode can be fabricated by LECD process, which is most likely only process that can fabricate non-circular... evaluated by micro hardness testing 3 Theoretical modeling and experimental investigation are conducted on the effect of different LECD parameters for fabricating variety of microstructures In order to estimate the rate of deposition and the condition of the deposited structure, a set of mathematical relations is developed with the help of Faraday's laws of electrolysis and ButlerVolmer equation Mathematical... dissertation A comprehensive literature review is given in the Chapter 2, which categorized into three sections First section describes different micro- EDM electrode fabrication process In the second section, fabrication process related to non-circular electrode fabrication and the role of LECD process in these circumstances Finally, extensive literature review on LECD process is discussed such as process. .. and their effect, process modeling and process control Chapter 3 describes the development of LECD and micro- EDM combined setup In addition, it describes the performance evaluation of LECD process by deposition growth study and microstructure homogeneity study Chapter 4 describes the modeling and simulation of the growth of the LECD structure, which is developed with the help of Faraday's laws of electrolysis... brief overview of the microfabrication process by micro- EDM and its challenging areas Among all challenging areas, more attention will be given to micro- EDM on machine non-circular tool fabrication process by localized electrochemical deposition (LECD) In addition, the significance of this research work will be elaborated in this chapter followed by the objectives and the scope of this work and finally