A surface acoustic wave device for micro fluidic sensing applications

VIETNAM NATIONAL UNIVERSITY HANOI UNIVERSITY OF ENGINEERING AND TECHNOLOGY TUNG BUI DUC A SURFACE ACOUSTIC WAVE DEVICE FOR MICRO-FLUIDIC SENSING APPLICATIONS MASTER’S THESIS OF ELECTRONICS – TELECOMMUNICATIONS TECHNOLOGY Hanoi - 2015 VIETNAM NATIONAL UNIVERSITY HANOI UNIVERSITY OF ENGINEERING AND TECHNOLOGY TUNG BUI DUC A SURFACE ACOUSTIC WAVE DEVICE FOR MICRO-FLUIDIC SENSING APPLICATIONS Branch: Electronics - Telecommunications Technology Major: Electronics Engineering Code: 60520203 MASTER’S THESIS OF ELECTRONICS – TELECOMMUNICATIONS TECHNOLOGY Supervisor: Assoc Prof Chu Duc Trinh Hanoi - 2015 AUTHORSHIP “I hereby declare that the work contained in this thesis is of my own and has not been previously submitted for a degree or diploma at this or any other higher education institution To the best of my knowledge and belief, the thesis contains no materials previously published or written by another person except where due reference or acknowledgement is made.” Signature:……………………………………………… TABLE OF CONTENTS AUTHORSHIP TABLE OF CONTENTS List of Figures List of Tables List of Abbreviations .7 Abtract .8 Chapter INTRODUCTION 1.1 Motivation .9 1.2 Contributions and thesis overview 11 Chapter 12 BACKGROUND AND RELATED WORKS 12 2.1 Acoustic wave devices 12 2.1.1 Thickness Shear Mode (TSM) resonator 13 2.1.2 Acoustic Plate Mode (APM) devices 14 2.1.3 Flexural Plate Wave (FPW) or Lamb wave device 15 2.1.4 Surface Acoustic Wave Devices 16 2.2 The Finite Element Method (FEM) 18 Chapter 19 SYSTEM CONFIGURATION .19 3.1 Mathematical model 19 3.1.1 Relation Between the Ink Pressure and the Piezoelectric Wave Equation 19 3.1.2 Angular Spectrum of Plane Wave Theory for FIDT Structure 20 3.1.3 Integrated Injector Systems 22 3.2 System configuration 23 3.2.1 FSAW configuration .23 3.2.2 Input parameter of liquid 24 Chapter 26 RESULT AND DISCUSSION 26 4.1 Droplet States 26 4.2 Working Mechanism of the FSAW Device 27 Chapter 34 CONCLUSION .34 5.1 Conclusion 34 5.2 Future Work 34 Publications 36 References .37 List of Figures Figure 1-1: Acoustic wave propagation direction in Cartesian coordinate system (a) Compressional or longitudinal (b) Shear vertical (c) Shear horizontal[6] Figure 1-2: SAW radiation into fluid domain in 2D (a) Cross-sectional view of the typical structure and (b) Cross-sectional view of the vertical structure 10 Figure 2-1: Schematic sketches of the four types acoustic sensors (a) The Thickness Shear Mode (TSM) resonator, (b) The Surface Acoustic Wave (SAW) device, (c) The Acoustic Plate Mode (APM) device, (d) The Flexural Plate Wave (FPW) or Lamb wave device [18] 13 Figure 2-2: The thickness shear mode resonator [1, 18] .14 Figure 2-3: In the shear-horizontal acoustic plate mode (SH-APM) sensor, the waves travel between the top and bottom surfaces of the plate, allowing sensing on either side [1] 15 Figure 2-4: Schematic of a flexural plate wave device The side view shows the different layers and membrane movement Interdigital electrodes are used for actuation [18] .16 Figure 2-5: Rayleigh waves move vertically in a direction normal to the surface plane of a surface acoustic wave (SAW) sensor [18] .17 Figure 2-6: The wave energy is confined to within one wavelength from the surface of a SAW sensor [18] 17 Figure 3-1: Geometry of the FSAW sensor with the well in the middle of the propagation path (a) Two straight segments (b) Three straight segments 19 Figure 3-2: Concentric FIDTs with the shape as (a) circular arc and (b) three straight segments 21 Figure 3-3: Novel position of the SAW sensor in the injector 22 Figure 3-4: Inlet velocity is excited by one pulse within the first 14 μs 25 Figure 4-1: Position of the air/ink interface and velocity field at (a) t = 13 μs and (b) t = 14 μs 26 Figure 4-2: Positions of ink droplet at various times (a) t = μs (b) t = μs (c) t = μs (d) t = μs (e) t = 11 μs (f) t = 13 μs (g) t = 14 μs, (h) t = 25 μs 27 Figure 4-3: Effect of the piezoelectric substrate on the liquid 28 Figure 4-4: Sensitivity of the IDT device and two-segment FIDT device .28 Figure 4-5: Total amplitude fields of IDTs with the conventional and concentric shapes on the surface (a) Conventional IDTs (b) FIDTs with circular arcs (c) FIDTs with two straight segments (d) FIDTs with three straight segments .29 Figure 4-6: Total displacement measured at a point after the nozzle .30 Figure 4-7: Mechanical attenuation of SAWs after propagating through the inkjet nozzle .30 Figure 4-8: Spectral content of the mechanical wave motion of the FSAW devices with (a) curve fingers, (b) two-straight-segment fingers, and (c) three-straight-segment fingers .31 Figure 4-9: Output potential at the receiver FIDT of the SAW sensors 32 Figure 4-10: Insertion loss of the output signal of the conventional and focused SAW devices with (a) conventional fingers, (b) curve fingers, and (c) 3-straight-segment fingers .33 Figure 5-1: The cross-section view of the F-SAW sensors with optimized fingers: (a) dissimilar straight fingers, (b) curve fingers, (c) 2-segment fingers and (d) 3-segment fingers .35 List of Tables Table 3-1: Design parameter of IDT .23 List of Abbreviations 3D Three – Dimensions CMOS Complementary Metal-Oxide Semiconductor FEM Finite Element Method FPW Flexural Plate Wave IDT InterDigital Transducers MEMS Micro-Electro-Mechanical Systems QCM Quartz Crystal Microbalance R – SAW Rayleigh Surface Acoustic Wave SAW Surface Acoustic Wave SH – APM Shear Horizontal – Acoustic Plate Mode SH – SAW Shear Horizontal – Surface Acoustic Wave SPF Single-Phase Laminar Flow TSM Thickness-Shear Mode Abtract Surface acoustic wave (SAW) devices have been widely used in a variety of applications, either in consumer electronics or in industrial, commercial, medical and military applications or equipment Nowadays, the telecommunication industry is the largest user of these devices but SAW based devices have many attractive features to be explored Because of their small size, high sensitivity to external physical parameters and the properties of the film deposited on the SAW substrate, these devices can react very fast to the changes in the environmental conditions SAW sensors have some advantages such as:  They can be placed on moving or rotating parts  They can be used in hazardous environments such as high voltage plants, contaminated areas, strong radioactive areas, high vacuum process chambers, extreme heat  Besides, because SAW sensors can operate at high frequencies (GHz range), they can be well protected from the low frequencies electromagnetic interference that occurs in the vicinity of industrial equipment such as high voltage line The common applications of acoustic wave sensors are Temperature, Pressure, Torque, Mass, Humidity, Vapor Chemical, and Bio sensors Specially, SAW devices used in bio – sensing applications have demonstrated a high sensitivity in the detection of fluid properties such as density, viscosity, streaming velocity in particular and liquid status in general This thesis presents a possible creation of the optimized liquid sensors for the inkjet nozzles The proposed focused surface acoustic wave (FSAW) device utilizing aluminum nitride (AlN) single crystal as the piezoelectric substrate is based on the pressure variation due to the continuous droplet ejector The design, specification, and numerical simulation results are described Comparisons between the output response of the conventional and concentric structures indicate a more efficient operation of the multiple-segment focused interdigital transducer (FIDT) structure According to the angular spectrum of the plane wave theory, the amplitude field of FIDTs is calculated through that of straight interdigital transducers The 3-D integrated model of the FSAW device has a number of advantages, such as the enhancement of the surface displacement amplitudes and an easier fabrication It is able to detect the breakup appearance of the liquid in the droplet formation process For the piezoelectric substrate AlN, it is compatible with the CMOS fabrication technology, leading to an inexpensive and reliable system Moreover, for the proposed FIDTs with multiple straight segments, the acoustic energy is more optimized and focused near the center of the inkjet nozzle The droplet generation process begins at an output voltage of roughly 0.074 V within 0.25 µs, and the background level of the attenuation of both the mechanical and electrical energy 24 The piezoelectric substrate and inkjet of the developed models were meshed adaptively to adjust the scaling of the fields manually and reduce the computation time These parameters provided a much denser mesh at the nozzle boundary of the model, which is essential to achieve a high accuracy in simulations A sinusoidal voltage of frequency 1430 MHz is applied to the input FIDTs to generate the needed SAWs An input voltage of 0.1 V is applied to the receiver FIDTs Moreover, this also avoids receiving very small changes at the receiver because the influence of the liquid pressure that is compared with the input signal needs to be significant The output voltages in all cases are acquired at the alternating fingers of the output IDT Due to the vibration coming from the driving signal and the droplet formation signal, a cross-talk effect including electrical, direct, and pressure-induced crosstalk occurs when the frequencies of these signals are close to each other In experiments, for piezoelectric actuators, passive devices are used to reduce the effective piezoelectric substrates Another way is to use thin foil, external electrodes for the ground and inner electrodes for voltage [24] It is possible to apply these methods for the piezoelectric sensors in experiments In addition, the cross-talk effect of the piezoelectric sensor is much smaller because its frequency is much more than that of the droplet formation signal Moreover, in simulations the use of few IDT fingers and low energy reduces the cross talk 3.2.2 Input parameter of liquid The inlet velocity in the z-direction increases from to the parabolic profile during the first μs 𝑣𝑖 (𝑥, 𝑦, 𝑡) = 4.5 ( √𝑥 +𝑦 +0.1[𝑚𝑚] 0.2[𝑚𝑚] ) (1 − √𝑥 +𝑦 +0.1[𝑚𝑚] 0.2[𝑚𝑚] ) 𝑣(𝑡)(𝑚𝑚⁄𝑠) (9) Here t=u(t-1.10-6)-u(t-13.10-6), as shown in Figure 3-4 and u(t) is the unit function Hence, the pulse frequency of the droplet formation process is about 20 KHz The velocity is then v(x, y) within 10 μs and finally falls down to zero within another μs Therefore, the ink velocity at the nozzle throat is sought in the following form: (10) 𝑅12 𝑣𝑛 (𝑥, 𝑦) = 𝑣𝑖 (𝑥, 𝑦) 𝑅2 25 Figure 3-4: Inlet velocity is excited by one pulse within the first 14 μs The surface tension of the ink generates a capillary pressure that is ignored due to its insignificant influence To cut off the droplet, the pressure at the entrance of the nozzle has to overcome steady and unsteady inertia and forces resulting from the surface tension of the ink [24] Therefore, pressure includes the positive and negative excitation pressure After the negative excitation, the ink deformation at the nozzle happens to separate the droplet from the liquid reservoir 26 RESULT AND DISCUSSION The proposed simulation methodology has been implemented using finite element method and COMSOL Multiphysics 4.2a 4.1 Droplet States Figure 4-1: Position of the air/ink interface and velocity field at (a) t = 13 μs and (b) t = 14 μs 27 Figure 4-2: Positions of ink droplet at various times (a) t = μs (b) t = μs (c) t = μs (d) t = μs (e) t = 11 μs (f) t = 13 μs (g) t = 14 μs, (h) t = 25 μs Figure 4-1 shows the ink surface and the velocity field at t = 13 μs when the velocity magnitude of ink is still focused at the nozzle After 14 μs, the breakup phenomenon of the droplet generation occurs Figure 4-2 shows the time evolution of the ink jetting from the nozzle To move to the outlet of the target, the jetted droplet from the inlet needs 200 μs During the first 13 μs, ink at the nozzle throat is extensively forced [Figure 4-2 (a)-(f)] After the second actuation pressure, the breakup point occurs, as shown in [Figure 4-2 (g) and (h)] In other words, the potential energy becomes strong enough to cut off the droplet Therefore, to detect the initial period of the droplet generation, the running time of the simulation only needs to be carried out within 25 μs to determine the correlation between the droplet generation and the output signal variation 4.2 Working Mechanism of the FSAW Device Pressure produced by the piezoelectric substrate insignificantly affects the liquid (Figure 4-3) Moreover, it also indicates that due to the uniform IDT fingers of the conventional, the liquid is influenced more at the region far from the local line The sensitivity S is defined as the relative change of the output signal per unit of the applied pressure and the input voltage [25] Figure 4-4 shows that that of the twosegment FIDT structure is better than that of the conventional IDT structure 28 Figure 4-3: Effect of the piezoelectric substrate on the liquid Figure 4-4: Sensitivity of the IDT device and two-segment FIDT device Figure 4-5 shows that the total displacement fields of FIDTs have a narrow concentric SAW beam When the number of straight segments of the proposed structure increases, its SAW beam resembles that of curve FIDTs Moreover, the total displacement magnitude of FIDTs with multiple segments is close to that of FIDTs with circular arcs in Figure 4-6 29 Figure 4-5: Total amplitude fields of IDTs with the conventional and concentric shapes on the surface (a) Conventional IDTs (b) FIDTs with circular arcs (c) FIDTs with two straight segments (d) FIDTs with three straight segments 30 Figure 4-6: Total displacement measured at a point after the nozzle In Figure 4-7 the attenuation of the mechanical waves is almost due to the leaky wave phenomenon and the ink pressure Simulation results for four structures also show that the mechanical energy of the FIDTs is lower In other words, the FSAW devices organize more efficiently than the conventional devices Figure 4-7: Mechanical attenuation of SAWs after propagating through the inkjet nozzle The mechanical waves of the FIDT structure are observed in frequency-time domain in Figure 4-8 The spectrum of the mechanical motion at the output fingers in all focused structure cases illustrates that the total mechanical energy mostly focuses at 5.5 μs and 31 it has other sub-harmonics Hence, the performance of the proposed multiple-segment FIDTs is similar to that of FIDTs with circular arcs Figure 4-8: Spectral content of the mechanical wave motion of the FSAW devices with (a) curve fingers, (b) two-straightsegment fingers, and (c) three-straight-segment fingers In Figure 4-9, the contour plot illustrates the output signals of the FSAW devices at times ranging from to 25 μs The output signal of the FIDT structures is larger than that of the conventional IDT structure When it is excited by the first actuation pressure, the maximum voltage value still achieves 0.128 V After 13 μs, its velocity is able to overcome the surface tension force and becomes strong enough to cut off the droplet The breakup point may occur at around 0.074 V in this duration of 0.25 μs window (ranging from 13.2 to 15.7 μs) Hence, the alteration of the electrical signal at different generated pressures positioned at the nozzle wall and throat depends on the ink state 32 Figure 4-9: Output potential at the receiver FIDT of the SAW sensors For each droplet formation period, the attenuation responses of conventional and concentric fingers are shown in Figure 4-10 When all attenuation results of the electrical energy reach the background level, the separation process begins The separated droplet process keeps on moving due to inertia although the excitation impact does not exist After generating the droplet, as inertia oscillates, the significant attenuation continues and reduces gradually until the liquid surface tension returns to its resting state As the power of the conventional structure is dissipated around the medium and more absorption happens at the edges, the energy loss is highest Consequently, it is proved that the proposed FSAW devices not only keep the advantageous properties of circular arcs, but like conventional IDTs, they are also quite sensitive to the actuation pressures of the inkjet nozzle 33 Figure 4-10: Insertion loss of the output signal of the conventional and focused SAW devices with (a) conventional fingers, (b) curve fingers, and (c) 3-straight-segment fingers 34 CONCLUSION 5.1 Conclusion This thesis presented a novel sensor for discovering the pressure variation at the nozzle The relation between the liquid pressure at the nozzle and the wave motion was found in the equation of motion for the piezoelectric medium Based on the voltage, output power, and attenuation response of the electrical and mechanical signal, it is able to detect the droplet formation For the proposed FIDTs with multiple straight segments, the SAW beam is similar to that of the FSAW device with circular arcs The greater the number of straight segments they get, the more their properties resemble circular arc FSAW devices In addition, it influences insignificantly the flow rate at the nozzle due to the narrow SAW beam focused mostly on small arcs of the inkjet nozzle Moreover, because of its straight shape, the proposed device is easier to fabricate For the proposed FIDTs with multiple straight segments, based on the saturation state of the attenuation response of the electrical signal, it is still able to monitor the injected droplet process, such as estimating the beginning of the droplet generation process The output signal may achieve up to 128 mV for the positive excitation pressure and down to approximately 74 mV for the negative excitation pressure The breakup point keeps the potential value of 74 mV within 0.25 μs 5.2 Future Work Aluminum Nitrite is a material that compatible with CMOS technology but this design still cannot be fabricated using this process because of the thickness of substrate In the future, other calculations and simulations will be implemented to decrease this thickness To satisfy the requirement of CMOS technology, the thickness of substrate must be µm or thinner In another direction, a new architecture of the sensor is going to be examined In the new architecture, the shape of input IDT will be dissimilar straight finger and curve finger, as shown in Figure 5-1 (a) and (b) 35 Figure 5-1: The cross-section view of the F-SAW sensors with optimized fingers: (a) dissimilar straight fingers, (b) curve fingers, (c) 2-segment fingers and (d) 3-segment fingers 36 Publications Bui, Thu Hang, Tung Bui Duc and Trinh Chu Duc "An Optimisation of IDTs for Surface Acoustic Wave Sensor." International Journal of Nanotechnology 12, no (2015): 485-495 Thu Hang, Bui, Duc Tung Bui and Duc Trinh Chu "Microfluidic Injector Simulation with Fsaw Sensor for 3-D Integration." IEEE Transactions on Instrumentation and Measurement 64, no (2015): 849-856 Hang, Bui Thu, Bui Duc Tung, Nguyen Tien Dat and Chu Duc Trinh "Attenuation Coefficient for Surface Acoustic Waves in Fluid Region." Vietnam Journal of Mechanics 34, no (2012): 225-236 Thu, H B., P M Sarro, T B Duc and T C Duc "Associated Idts in Surface Acoustic Wave Devices for Closed-Loop Control Inkjet System." In 2014 IEEE SENSORS, 1936-1939, 2014 Thu-Hang, Bui, Tien Dat Nguyen, Duc Tung Bui and Duc Trinh Chu "3-D Finite Element Modeling of Saw Sensing System for Liquids." In 2012 IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM), 782-787, 2012 Duc, Tung Bui, Nam Pham Hoai, Hang Bui Thu and Trinh Chu Duc "Effect of the Focused Surface Acoustic Wave Devices on the Microfluidic Channel." In The 3rd International Conference on Engineering Mechanics and Automation, 221-225, 2014 Tung, BD, B Thu-Hang, NT Dat and CD Trinh "R-Saw Analysis on SingleCrystal Aln Substrate for Liquid Sensors." 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QCM Quartz Crystal Microbalance R – SAW Rayleigh Surface Acoustic Wave SAW Surface Acoustic Wave SH – APM Shear Horizontal – Acoustic Plate Mode SH – SAW Shear Horizontal – Surface Acoustic Wave. .. such as Thickness Shear Mode (TSM) resonator, Surface Acoustic Wave (SAW) device, The Acoustic Plate Mode (APM) device and The Flexural Plate Wave (FPW) or Lamb wave device are established After... displacement while shear mode, called as Shear Horizontal – Surface Acoustic Wave (SH – SAW), is a shear horizontal wave on the surface [3-5] Figure 1-1: Acoustic wave propagation direction in Cartesian