Investigation and integration of piezoresistive silicon nanowires for MEMS applications

INVESTIGATION AND INTEGRATION OF PIEZORESISTIVE SILICON NANOWIRES FOR MEMS APPLICATIONS LOU LIANG (B. Eng. University of Science and Technology of China) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2012 ACKNOWLEDGEMENTS Acknowledgements First and foremost, I would like to gratefully acknowledge my thesis supervisors, Prof. Chengkuo Lee and Prof. Dim-Lee Kwong. I would like to express my most sincere gratitude to Prof. Lee for his invaluable guidance and insightful direction throughout my Ph.D. candidature. I will never forget the time he sacrificed on me and the encouragements he gave me. I deeply appreciate Prof. Kwong for offering the precious opportunity to learn at Institute of Microelectronics and his continuous consideration on me. I would like to thank the IME staff, Dr. Woo-Tae Park, Dr. Hanhua Feng, Ms. Lim Lishiah and Mr. Hamid etc, for their continuous support and guidance during my attachment. Without their continuous help, I can not successfully fabricate all the devices by myself. I would also like to extend my gratitude to the past and current members of CICFAR: Dr. Xiang Wenfeng, Mr. Zhang Songsong, Ms. Liu Huicong, Mr. Li Bo, Mr. Wang Nan, Mr. Qian You, Ms. Huang Wen, Ms. Chen Ji, Mr. Yan Hongkang, Mr. Zhang Xiufeng, Mr. Wang Tao, Mr. Ren Yi, Mr. Wang Jiayi, Mrs. Ho Chiow Mooi, and Mr. Koo Chee Keong. I would like to thank them for being such helpful and supportive co-workers. I would also thank my friends: My roommate Dr. Huofeng; my undergraduate classmates Dr. Duan Lixin, Mr. Ji Senshan, Mr. Gong Boqing, Mr. Teng Fei and Mr. Gong Xing etc, for the happiness and support we share with each other. i ACKNOWLEDGEMENTS Lastly but not the least, I would like to express my deepest gratitude to my parents for being my company and support all the time. Their unconditional love is the most precious gift in my life. ii SUMMARY Summary The piezoresistive silicon nanowires (SiNWs) have been extensively studied over the past decades. In the meantime, many applications requires scaling down the sensors without losing high sensitivities. With huge potential in downsizing devices, the SiNWs are expected to play a critical role in the migration from Micro-electro-mechanical-systems (MEMS) technology to Nano-electro-mechanical-Systems (NEMS). The SiNWs show merits of relative ease of scaling down, high sensitivity and CMOS compatibility, etc. However, up to date, inconsistencies and debates on the SiNWs piezoresistance still exist, and reports on successful integration of SiNWs into MEMS are quite limited. In this study, we use the top-down approach to fabricate and integrate SiNWs into diaphragm and cantilever structures. The SiNWs performance under an extra large strain range and their fatigue behavior are investigated for the first time. Two NEMS devices, a pressure sensor and a flow sensor, using SiNWs as sensing elements are demonstrated, characterized and optimized. The pressure sensor is an improved and optimized version base on the work by our colleague, while the flow sensor developed by us is the smallest piezoresistive flow sensor reported so far. Our work is a successful pioneer demonstration of integrating SiNWs into working sensors, which pushes the frontier of SiNWs integration for practical applications, provides a good reference for future SiNWs-based sensor design and potentially opens up new realms of miniaturized static and dynamic sensing. iii DECLARATION DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. _________________ Lou Liang 27 December 2012 iv TABLE OF CONTENTS Table of Contents Acknowledgements . i Summary . iii List of Figures vii List of Tables . xiii List of Symbols . xiv Preface .xv Chapter 1. Introduction and Literature Review 1.1 General Introduction of Piezoresistance 1.1.1 Basics of Piezoresistance 1.1.2 SiNW Piezoresistance 1.2 Piezoresistive MEMS Devices and Their Fatigue .16 1.2.1 Piezoresistive MEMS Devices .17 1.2.2 Fatigue of MEMS Devices .21 1.3 SiNW Behavior and Devices .25 1.3.1 SiNW Measurement under Large Strain 26 1.3.2 SiNW Fatigue .27 1.3.3 SiNWs-Based Sensors 29 Chapter 2. Device Fabrication and Testing Setups 33 2.1 Fabrication Process .34 2.1.1 Schematic drawing of the devices 34 2.1.2 SiNW Fabrication .35 2.1.3 Pressure Sensor Fabrication .38 2.1.4 Flow Sensor Fabrication .40 2.2 Testing Set-up 42 2.2.1 Probe Based Testing .42 2.2.2 Bulge Testing 43 2.2.3 Flow Sensor Testing .44 Chapter 3. Characterization of Silicon Nanowires .46 3.1 Device Configuration and Simulation .46 3.2 Basic Characterization of SiNWs 48 3.2.1 SiNW Implantation .48 3.2.2 Gauge Factor 52 3.2.3 Temperature Effect .53 3.2.4 Noise 53 3.3 Static Testing .54 3.3.1 SiNW Under an Extra Large Compressive Strain 54 3.3.2 Effect of Point Loading Position 57 3.3.3 Sensitivity versus SiNW Lengths Under Displacement Testing .60 3.4 Dynamic Testing 61 3.4.1 Fracture Pattern 61 v TABLE OF CONTENTS 3.4.2 S-N Curve 63 3.4.3 Pressure Sensor Characterization During Dynamic Testing 65 3.5 Conclusion 69 Chapter 4. Optimization of an Silicon Nanowires-Based NEMS Pressure Sensor 71 4.1 Design and Simulation 72 4.2 SiNW Optimization 74 4.2.1 SiNW Length . 74 4.2.2 SiNW Orientation 76 4.2.3 Temperature Effect of The SiNW 77 4.3 Diaphragm Optimization 78 4.3.1 Single SiO2 Layer vs. Multi-Layered Diaphragm 78 4.3.2 Effect of SiNx Layer Thickness 80 4.3.3 Surface Profile vs. Applied Pressure 83 4.3.4 Sensitivity versus SiNx Layer Thickness . 85 4.4 Reverse Direction Characterization & Working Range in Compressive Strain Region . 87 4.4.1 Reverse Direction Bulge Test . 87 4.4.2 Working Range of Pressure Sensor under Compressive Strain 88 4.5 Conclusion 89 CHAPTER 5. Characterization of SiNWs-Based Cantilever Flow Sensor . 91 5.1 Simulation on MEMS Water Flow Sensors Using SiNWs . 92 5.1.1 Design, Modeling and Simulation . 92 5.1.2 Results and Discussion 95 5.2 Characterization of SiNWs-Based Cantilever Air Flow Sensor 101 5.2.1 Flow Sensor Design . 102 5.2.2 Testing Results . 109 5.3 Conclusion 120 Chapter 6. Conclusions and Future Work 122 6.1 Conclusions on Current Work . 122 6.2 Directions for Future Work . 124 6.2.1 Packaged Pressure Sensor 124 6.2.2 SiNWs-Based Accelerometer . 126 References . 132 Appendix: Publication 144 Journal . 144 Conference 145 vi LIST OF FIGURES List of Figures Figure 1.1: Test configurations from Smith. A and C is for the extraction of longitudinal piezoresistive coefficient, and B and D are used to obtain transverse coefficients. The dotted lines refer to the electrodes, indicating the voltage drop. The arrow indicates an application of a uniaxial tensile stress to the test sample by hanging a weight. Figure reproduced from Reference [1].6 Figure 1.2: Piezoresistive coefficients under room temperature in the (100) plane of (a) p-type silicon (b) n-type silicon of low doses. Figure reproduced from Reference [20]. Figure 1.3: Piezoresistive coefficients against doping concentration. Figure reproduced from Reference [20]. Figure 1.4: (a) direction SiNWs bridging a trench that is formed from a SOI wafer; (b) Zoom-in SEM picture to show the morphology of a bridged SiNW, which grows from one side of the trench and bounces back when coming to the other side; (c) Conduction change as a function of applied strain. Four types of relationship are presented and the overview of L is shown in the inset; (d) The longitudinal piezoresistance coefficients of p-type SiNWs as a function of diameter and resistivity. The bulky silicon coefficient is shown as well. Different colors corresponds to different nonlinearity types in (c). Figure reproduced from Reference [3]. 11 Figure 1.5: (a) Schematic drawing of the test setup; (b) SEM picture of the sample cantilever; (c) MEDICI device simulation on the holes concentration in the SiNWs as a function of VGS (left to right: to 7.5 V, step 2.5 V); (d) The SiNW gauge factor against Vgs in three regions. Figure reproduced from Reference [59]. 12 Figure 1.6: The piezo-pinch effect in SiNWs from calculation. (a) The conductance change as a function of the applied stress with three different doping and resistivity; (b) The piezoresistance coefficient versus diameter and resistivity. Figure reproduced from Reference [61]. 13 Figure 1.7: (a) The schematic drawing of the testing set-up; (b) The SEM picture of a fabricated SiNW; (c) The comparison between the apparent conductivity change and true change against the applied stress; (d) The conductivity change of a SiNW under an alternating stress between MPa and -13.3 MPa as a function of time. Figure reproduced from Reference [63] . 14 Figure 1.8: Top view illustration of the pressure sensor; (b) Side view illustration of the sensor with the diaphragm under deformation; (c) The SEM picture showing the pressure sensor with a square diaphragm and four embedded piezoresistors and their arrangement. Figure reproduced from Reference [77]. . 19 Figure 1.9: (a) Top view of a single-crystal silicon diaphragm pressure sensor; (b) cross section showing the structure. Figure reproduced from Reference [78]. vii LIST OF FIGURES . 19 Figure 1.10: MEMS pressure sensor evolution from 1950s to 1980s. Figure reproduced from Reference [79]. 19 Figure 1.11: (a) Pre-stressed cantilever flow sensor; (b) Single-axis cantilever beams; (c) Bio-inspired flow sensor with manually glued wire. Figure reproduced from Reference [87] and [88]. . 21 Figure 1.12: (a) Schematic drawing of a MEMS mechanical-amplifier actuator; (b) A rectangular torsion bar subjected to a pure torque T. There is longitudinal stress in the torsion bar during twisted motion; (c) FEM simulation to determine maximum stress on tensile samples; (d) Resonant frequency change against time during the fatigue testing (test cycle: 108 cycles at stress amplitude 4.4 GPa). Figure reproduced from Reference [91]. . 24 Figure 1.13: (a) SEM picture of the micro-actuator with assembled silicon fiber; (b) Percentage change of the longitudinal piezoresistance against applied strain. Figure reproduced from Reference [11] 26 Figure 1.14: (a) The experimental set-up using the AFM; (b) The S-N curve of the sample SiNWs; (c)~(e) The typical fatigue patterns of the SiNWs. Figure reproduced from Reference [19]. 28 Figure 1.15: (a) The schematic drawing of the displacement sensor with the suspended sub-micron silicon beam; (b) The SEM picture of the fabricated device; (c) The comparison of sensors with beams of different dimensions. Figure reproduced from Reference [99]. 29 Figure 1.16: SEM picture of the fabricated device; (b) The side cross section of the pressure sensor to the SiNW position; (c) Zoom-in picture to show the morphology of the released SiNW; (d) The sensitivity of the pressure sensor with different sensing elements. Figure reproduced from Reference [101]. 31 Figure 1.17: (a) Schematic drawing of the pressure sensor and the testing set-up; (b) The diaphragm surface profile under different pressure levels; (c) The SiNW resistance change versus pressure against different gate bias; (d) The extracted sensitivity as a function of gate bias. Figure reproduced from Reference [35]. 32 Figure 2.1: Illustrations of device designs: (a) The pressure sensor; (b) The flow sensor . 34 Figure 2.2: Illustrations of device fabrications. (a) the SOI wafer in (100) plane; (b) SiNWs formation and P-type implantation; (c) second P-type implantation on paddle regions and first passivation layer (400 nm of SiO2) deposition by PECVD; (d) via open, last implantation on via regions and the metallization. . 35 Figure 2.3: (a) SEM picture of a μ SiNW after metal deposition; (b) TEM picture of the SiNW . 37 Figure 2.4: (a) Mask of a μm SiNW; (b) Kelvin structure for contact resistance measurement. 38 Figure 2.5: Process flow to fabricate the pressure sensor. 39 Figure 2.6: (a) Optical picture of the pressure sensor diaphragm after DRIE upon viii CHAPTER that PECVD oxide deposition is conducted to protect the sidewalls of the flexure, the release holes and the proof mass as shown in Figure 6.4 (h). The bottom oxide is cleared afterwards and a further DRIE is conducted for final release (Figures 6.4 (h) and (i)). Then the metal pads are opened for electrical readout as shown in Figure 6.4 (j). Finally, SF6 or XeF2 gases are employed to release the proof mass as a suspended structure. Figure 6.5: SEM pictures of two unfinished SiNWs-based accelerometers. The SEM pictures of two fabricated accelerometers in progress are shown in Figure 6.5. Attributed to the SiNWs, the flexures are able to be designed with width of μm. Consequently, the proof masses are able to be further shrunk to save the total accelerometer area. In Figures 6.5 (a) and (b), the proof masses are 50×50 μm and 30×100 μm, respectively. In comparison with W. T. Park's design with proof masses of around 400×400 μm, my designs leads to a big reduction in the sensor size. Currently, the two accelerometers are still in progress, pending final release. I will keep working on the fabrication and characterization of the accelerometers as the future work of SiNWs-based sensors development. 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Microeng., Vol.22, No. 5, 055012, 2012. 3. Liang Lou, Woo-Tae Park, Member, IEEE, Songsong Zhang, Li Shiah Lim, Dim-Lee Kwong, Fellow, IEEE, and Chengkuo Lee, Member, IEEE, "Characterization of Silicon Nanowire Embedded in A MEMS Diaphragm Structure Within Large Compressive Strain Range", IEEE Electron Device Letters, Vol. 32, No. 12, pp. 1764-1766, 2011. 4. Liang Lou, Chengkuo Lee, Xiangguo Xu, Lichun Shao, Rama Krishna Kotlanka and Dim-Lee Kwong, "Design and characterization of MEMS flow sensors using silicon nanowires". Nanoscience and Nanotechnology Letters, Vol.3, No.2, pp.1–5, 2011. 5. Songsong Zhang, Liang Lou and Chengkuo Lee, "Characterization of silicon nanowire based cantilever air flow sensor", J. Micromech. Microeng, vol. 22, no. 9, 095008, 2012. 6. Songsong Zhang, Liang Lou and Chengkuo Lee, Piezoresistive silicon nanowire based nanoelectromechanical system cantilever air flow sensor, Appl. Phys. Lett., Vol. 100, No. 2, 023111, 2012. 7. You Qian, Liang Lou, Julius Ming-Lin Tsai and Chengkuo Lee, "A dual-silicon-nanowires based U-shape nanoelectromechanical switch with low pull-in voltage", Appl. Phys. Lett., Vol. 100, No. 11,113102, 2012. 8. Huihui Guo, Liang Lou, Xiangdong Chen and Chengkuo Lee, "PDMS-coated piezoresistive NEMS diaphragm for chloroform vapor detection", IEEE Electron Device Lett., vol. 33, no. 7, pp. 1078-1080, 2012. 144 PUBLICATION Conference 1. Liang Lou, Hongkang Yan, Chengkuo Lee, Dim-Lee Kwong, and Woo-Tae Park, "Characterization of Si Nanowires-Based Piezoresistive Pressure Sensor by Dynamic Cycling Test". 19th IEEE International Symposium on the Physical and Failure Analysis of Integrated Circuits (IPFA), Singapore, 2-6 July, 2012. (Oral presentation) 2. Liang Lou, Songsong Zhang, Woo-Tae Park, Lishiah Lim, Dim-Lee Kwong, and Chengkuo Lee, "Characterization of a Multi-layered MEMS Pressure Sensor Using Piezoresistive Silicon Nanowire within Large Measurable Strain Range", 7th Annual IEEE International Conference on Nano/Micro Engineered and Molecular Systems, Tokyo, Japan, 2012. (Oral presentation) 3. Liang Lou, Songsong Zhang, Lim Lishiah, Woo-Tae Park, Hanhua Feng, Dim-Lee Kwong, and Chengkuo Lee, Characteristics of NEMS piezoresistive silicon nanowires pressure sensors with various Diaphragm Layers, Eurosensors XXV, Athens, Greece, Sept. 4-7, 2011. (Oral presentation) 4. Liang Lou, Ming Lin Julius Tsai, Woo-Tae Park, Hanhua Feng, Dim-Lee Kwong, and Chengkuo Lee, "Low-voltage-driven NEMS torsion switch with pre-tilted angle", International Conf. on Materials for Advanced Technologies (ICMAT), Singapore, Jun. 26 – Jul. 1, 2011. 5．Liang Lou, Chengkuo Lee, Guo Xiang Xu, Rama Krishna Kotlanka, Lichun Shao and Dim-Lee Kwong, "Design and characterization of MEMS flow sensors using silicon nanowires", Proceeding of intern. Conf. on Technological Advances of Thin Films and Surface Coatings, Harbin. China, Jul.11-14, 2010. 6. Liang Lou, Rama Krishna Kotlanka, Lichun Shao, Woo-Tae Park, Daquan Yu, Lishiah Lim, Yongjun Wee, Vaidyanathan Kripesh, Hanhua Feng, Benjamin. S. Y. Chua, Chengkuo Lee and Dim-Lee Kwong, "Sensorized guidewires with MEMS tri-axial force sensor for minimally invasive surgical applications", IEEE Engineering in Medicine and Biology (EMB), Buenos Aires, Argentina, Sept. 1-4, 2010. 7. Liang Lou, Wenfeng Xiang, Fu-Li Hsiao, Marie Stephen Leo and Chengkuo Lee*, "Characterization of electrostatic driven NEMS switch using torsion spring", Asia-Pacific Conference On Transducers and Micro-Nano Technology (APCOT), Australia. Jul. 6-9 2010. 145 [...]... practical value of SiNWs and their integration with MEMS for applications In this chapter on literature review, the focus is the piezoresistive effect of SiNWs and their applications in real devices The particular piezoresistive phenomena in nanoscale are reviewed and the issues involved are presented and commented Then the general concept of piezoresistive sensors and the fatigue of MEMS are briefly... interesting and remarkable properties of SiNW have been reported and various potential sensor applications have been demonstrated [3-10] SiNWs that are fabricated using as-grown and top-down approaches both present good behaviors Generally, we can categorize the efforts as basic investigation of the SiNWs for its unique properties and integration of SiNWs with Micro-Electro-Mechanical-Systems (MEMS) for various... (MEMS) for various applications In this thesis, we target to investigate, identify and integrate suitable SiNWs with MEMS for practical applications To realize such objectives, our work can be categorized and summarized into the following two aspects, i.e characterization of piezoresistive SiNWs and development of SiNWs based MEMS sensors Firstly, we will exhibit the basic properties of the top-down fabricated... the SiNWs is explored and characterized This thesis is organized into five chapters as following: Chapter 1 surveys the literature comprising three parts, i.e., the general concept of piezoresistance and SiNW piezoresistivity, MEMS piezoresistive sensors and the basic concept of MEMS fatigue, and the SiNW fatigue and SiNW based MEMS sensors The review presents the current progress of SiNW study, trying... resistances of SiNWs with lengths of 1μm, 2μm, 5μm and 10μm against bias voltage from 0.2V to 0.5V 77 Figure 4.6: (a) The 3-D picture of the buckled diaphragm made of pure oxide (b)&(c) The top view of the buckled (b) up and (c) down diaphragm (d) The 3-D picture of the diaphragm with 2.5 μm SiNx layer on top of 0.5 μm SiO2 78 Figure 4.7: (a) and (d), (b) and (e), (c) and (f) show... interface circuitry and the ease of process integration MEMS pressure sensors are among the most successfully commercialized micro sensors and are widely used in various applications Other than the well-known automotive applications for pressure sensors including engine manifold monitoring, tyre pressure monitoring, and both oil and brake fluid pressures [20-24], pressure is also one of the most important... researchers for several decades During recent years, piezoresistive SiNWs are extensively explored for their interesting properties and integration potential with MEMS devices The first part of this section provides the very basic concepts of piezoresistance and the most commonly concerned properties including orientation, doping concentration and fabrication The second part reviews the current situation of. .. directions as shown in Figure 1.2 [46,47] For the p-type silicon, the highest piezoresistive coefficient lies along the [110] direction both for longitudinal and transverse directions; whereas for the n-type silicon, the highest coefficients exist along [100] direction This graph provides a good reference for the sensor design For example, the p-type silicon properties are often used in the pressure sensor... show the morphology of a bridged SiNW, which grows from one side of the trench and bounces back when coming to the other side; (c) Conduction change as a function of applied strain Four types of relationship are presented and the overview of L is shown in the inset; (d) The longitudinal piezoresistance coefficients of p-type SiNWs as a function of diameter and resistivity The bulky silicon coefficient... reported the enhanced piezoresistive effect of top-down fabricated SiNWs using the lift-off and electron beam lithography (EBL) technique Both crystalline and polycrystalline SiNWs were studied as a function of stress and temperature [58] Piezoresistive coefficient of 633% increase was observed in the direction of crystalline SiNWs The authors also reported that the 11 CHAPTER 1 piezoresistive effect . INVESTIGATION AND INTEGRATION OF PIEZORESISTIVE SILICON NANOWIRES FOR MEMS APPLICATIONS LOU LIANG (B. Eng. University of Science and Technology of China) A THESIS SUBMITTED FOR. can categorize the efforts as basic investigation of the SiNWs for its unique properties and integration of SiNWs with Micro-Electro-Mechanical-Systems (MEMS) for various applications. In this. piezoresistivity, MEMS piezoresistive sensors and the basic concept of MEMS fatigue, and the SiNW fatigue and SiNW based MEMS sensors. The review presents the current progress of SiNW study, trying