A surface acoustic wave device for micro fluidic sensing applications

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] .... 14 Fig

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

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

TABLE OF CONTENTS

AUTHORSHIP 1

TABLE OF CONTENTS 2

List of Figures 4

List of Tables 6

List of Abbreviations 7

Abtract 8

Chapter 1 9

INTRODUCTION 9

1.1 Motivation 9

1.2 Contributions and thesis overview 11

Chapter 2 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 3 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 4 26

RESULT AND DISCUSSION 26

4.1 Droplet States 26

4.2 Working Mechanism of the FSAW Device 27

Chapter 5 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] 9 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 = 1 μs (b) t = 3 μs (c) t = 5

μs (d) t = 9 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

INTRODUCTION

1.1 Motivation

Acoustic wave devices have been applied for sensing chemical and physical features in gas and liquid phases In the Surface Acoustic Wave sensors, a mechanical wave is generated on a piezoelectric crystal by metal electrodes or called Interdigital transducers (IDTs) [1, 2] It is a mix of a Rayleigh and a shear mode which propagate through the surface as shown in Figure 1-1 Rayleigh mode, called as Rayleigh wave, is

a combination of longitudinal and shear vertical particle 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 coordinate system (a) Compressional or longitudinal (b) Shear

vertical (c) Shear horizontal[6]

While SH – SAW has been developed for liquid sensing, the present of the normal displacement component is a reason why Rayleigh – SAW devices are poorly suited for liquid sensing applications [2] In contact of the solid-liquid medium, the shear vertical,

a component of Rayleigh, and shear horizontal wave couple into the liquid, generate compression waves and lead to dissipated power and extreme attenuation of SAWs If the contact is too large, this component is able to disappear and drive to change Rayleigh wave characteristic On the other hand, surface particles does not move in ellipses in planes normal to the surface and parallel to the direction of propagation [7, 8] Therefore, for liquid sensing applications, the longitudinal component needs to attenuate inappreciably In other words, decay constant along X3-axis is small in the fluid domain

A new vertical structure was presented to reduce this impact in my paper [9] Its key feature is that the contact area of solid-liquid which closes to surface is less (as shown

in Figure 1-2)

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

In bio-medical application, the methods for measuring and detecting the state of the droplet movement need to be simplified Expected sensors may account for the break-off time properly to assess the accuracy of generated ink-drops In order to achieve this, there are several sensing methods, such as membranes, cantilevers, cameras, and pressure sensors, which were proposed to be able to sense the droplet generation process [10-13] While some approaches are directly based on the vibration excited by the flow rate, others work at the bending level of the material Moreover, the principles of pressure sensors such as piezo-resistive, capacitive, and resonant sensing are based on the pressure variation at the orifice or gas reservoir However, the operation mechanism

of several existing sensors is able to obstruct the flow rate at the nozzle In our previous work, a surface acoustic wave (SAW) device was proposed for detecting the pressure state at the nozzle [10] For SAW devices with straight Interdigital transducers (IDTs), when SAWs uniformly spread on the whole piezoelectric substrate, the dissipated SAW energy may affect most points on the propagation path [1, 8, 14] Therefore, the SAW streaming and velocity fields throughout the delay path influence the whole nozzle because of the uniform fingers It may have more loss for environment and unwanted

noise such as reflected waves from the edges For small fixed sensing areas like the nozzle, the specialized IDT structures need to provide SAW beams with high intensity and large beam-width compression ratio In other words, for the determined sensing positions like the nozzle, the power generated by the focused IDTs (FIDTs) is mostly concentric on the local propagation path, and it decreases the energy loss to the medium [15-17] According to the conventional curve FIDT structure, the SAW beam may have

a close effect on the narrower arc Hence, as the reflection phenomenon of SAWs at edges and the power dissipation are limited, the performance of the concentric IDTs is better than the conventional structure However, it is not easy to fabricate various FIDTs with circular arcs Therefore, substituting curve fingers, FIDTs with multiple straight segments are presented

1.2 Contributions and thesis overview

The rest of this thesis is organized as follows

Chapter 2 provides the background of Surface Acoustic Wave sensor and research method At first, four type of SAW device 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 that, the Finite Element Method (FEM) is reviewed briefly

In Chapter 3, the simulation parameters of the 3-D integrated inkjet system are presented Chapter 4 shows comparisons between conventional and concentric structures including straight, curve, and multiple segment IDTs, and simulation results corresponding to each droplet state at the nozzle Finally, the conclusions and directions for future work are addressed in the final chapter of the thesis

BACKGROUND AND RELATED WORKS

2.1 Acoustic wave devices

Acoustic wave devices have been in commercial use for more than 60 years They offer many applications:

 In electronics, telecommunications industry, acoustic wave filters used in mobile cell phones and base stations These are typically surface acoustic wave (SAW) devices which act as band pass filters in both the radio frequency and intermediate frequency sections of the transceiver electronics They are also used

as resonators, delay lines, convolves or wireless identification systems (ID tags)

 In sensor devices:

 Automotive applications (torque, tire pressure sensors),

 Medical applications (chemical sensors),

 Industrial and commercial applications (vapour, humidity, temperature, mass sensors)

Acoustic wave sensors are so called because their detection mechanism uses a mechanic, or acoustic waves As the acoustic wave propagates through or on the surface

of the material, any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave Changes in velocity can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated

to the corresponding physical or chemical quantity being measured

All acoustic wave devices and sensors use a piezoelectric material to generate and detect acoustic waves Devices have been constructed in a number of configurations for sensor applications Those devices most commonly used in sensor applications include:

 The Thickness Shear Mode (TSM) resonator,

 The Surface Acoustic Wave (SAW) device,

 The Acoustic Plate Mode (APM) device,

 The Flexural Plate Wave (FPW) or Lamb wave device

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]

2.1.1 Thickness Shear Mode (TSM) resonator

The Thickness Shear Mode (TSM) Resonator widely referred to as a quartz crystal microbalance (QCM), is the best-known, oldest and simplest acoustic wave device As shown in Figure 2-2, the TSM typically consists of a thin disk of AT-cut quartz with parallel circular electrodes patterned on both sides The application of a voltage between these electrodes results in a shear deformation of the crystal

This device is known as a resonator because the crystal resonates as electromechanical standing waves are created The displacement is maximized at the crystal faces, making the device sensitive to surface interactions The TSM resonator was originally used to measure metal deposition rates in vacuum systems where it was commonly used in an oscillator circuit The oscillation frequency tracks the crystal resonance and indicates mass accumulation on the device surface In the late 1960s, the TSM resonator was shown to operate as a vapour sensor

The TSM features simplicity of manufacture, ability to withstand harsh environments, temperature stability and good sensitivity to additional mass deposited on the crystal surface Because of its shear wave propagation component, the TSM resonator is also capable of detecting and measuring liquids, making it a good candidate for a biosensor Unfortunately, these devices have the lowest mass sensitivity of the sensors examined here Typical TSM resonators operate between 5 and 30 MHz Making very thin devices that operate at higher frequencies can increase the mass sensitivity, but thinning the sensors beyond the normal range results in fragile devices that are difficult to

manufacture and handle Recent work has been done to form high-frequency TSM resonators using piezoelectric films and bulk silicon micromachining techniques

Figure 2-2: The thickness shear mode resonator [1, 18]

2.1.2 Acoustic Plate Mode (APM) devices

These devices utilize a shear-horizontal (SH) acoustic plate mode (APM), which has been developed for sensing in liquids SH modes have particle displacement predominantly parallel to the device surface and normal to the direction of the propagation The absence of a surface-normal component of displacement allows each

SH plate mode to propagate in contact with a liquid without coupling excessive amounts

of acoustic energy into the liquid By comparison, when surface acoustic waves are propagated at a solid-liquid interface, the surface-normal displacement radiates compressional waves into the liquid and severely attenuates the wave

These devices use a thin piezoelectric substrate, or plate, functioning as an acoustic waveguide that confines the energy between the upper and lower surfaces of the plate (Figure 2-3) This is in contrast to the SAW, for which nearly all the acoustic energy is concentrated within one wavelength of the surface As a result, both surfaces undergo displacement, so detection can occur on either side This is an important advantage, as one side contains the Interdigital transducers that must be isolated from conducting fluids or gases, while the other side can be used as the sensor

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]

Although being more sensitive to mass loading than the TSM resonator, SH-APM sensors are less sensitive than surface wave sensors There are two reasons: the first is that the sensitivity to mass loading and other perturbations depends on the thickness of the substrate, with sensitivity increasing as the device is thinned The minimum thickness is constrained by manufacturing processes Second, the energy of the wave is not maximized at the surface, which reduces sensitivity

2.1.3 Flexural Plate Wave (FPW) or Lamb wave device

A sensor concept similar to SAW sensors but employing Lamb waves was first presented by Stuart W.Wenzel, Richard M.White in 1988 [19] In a flexural plate wave (FPW) or Lamb wave device (Figure 2-4), an acoustic wave is excited in a thinned membrane with a thickness small compared to the propagation wavelength As with the other acoustic sensors mentioned above, the FPW device can sense quantities that cause its phase velocity to change

A unique feature of FPW is that it can be dimensioned so that its phase velocity is lower than that of most liquids, which lie in the range from 900 to about 1500 m/s When the FPW device contacts or is immersed in such a liquid, a slow mode of propagation exists

in which there is no radiation from the plate Thus, the FPW device functions well in a liquid environment and is therefore a good candidate for bio-sensing and chemical sensing in liquid [20]

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]

2.1.4 Surface Acoustic Wave Devices

The stress-free boundary imposed by the surface of a crystal gives rise to a unique acoustic mode whose propagation is confined to the surface and therefore is known as a surface acoustic wave (SAW) In 1887, Lord Rayleigh discovered the surface acoustic wave mode of propagation and in his classic paper predicted the properties of these waves [20] Named for their discoverer, Rayleigh waves have a longitudinal and a vertical shear component that can couple with a medium in contact with the device’s surface (Figure 2-5) The surface deformation is thus elliptic Such coupling strongly affects the amplitude and velocity of the wave This feature enables SAW sensors to directly sense mass and mechanical properties The surface motion also allows the devices to be used as micro-actuators The wave has a velocity that can be ~5 orders of magnitude less than the corresponding electromagnetic wave, making Rayleigh surface waves among the slowest to propagate in solids The wave amplitudes are typically ~10

Å and the wavelengths range from 1 to 100 microns in sensors applications

Figure 2-5: Rayleigh waves move vertically in a direction normal to the surface plane of a surface acoustic wave (SAW)

sensor [18]

Figure 2-6: The wave energy is confined to within one wavelength from the surface of a SAW sensor [18]

Figure 2-6 details the deformation field caused by a SAW propagating along the Z-axis and the associated distribution of potential energy Because Rayleigh waves have virtually all their acoustic energy confined within one wavelength under the surface, SAW sensors have the highest sensitivity of the acoustic sensors reviewed

2.2 The Finite Element Method (FEM)

The finite element method (FEM) is a numerical solution technique applicable to a broad range of physical problems, the variables of which are related by means of algebraic, differential or integral equations The technique is now an integral part of the design and analysis process and is used for many industrial and academic applications With the growth of computing power and commercially available programs, its ease of use, cost effectiveness and reliability is increasing being acknowledged and the technique is now

a standard tool for analysis

The FEM process includes modeling of the geometry of the model, meshing or discretizing the geometry created into elements so as to approximate the solution within

an element easily using simple functions, defining material properties which is easy to input but may require extensive testing or review of literature as in the case of this project is to obtain certain hard to determine material properties such as the Young’s modulus of red blood cells Boundary, initial and loading conditions must also be specified which require experience, knowledge and engineering judgment Finally, the solution is obtained by solving the discretized system, simultaneous equations for the field variables at the nodes of the mesh The reader is directed to refer to references for

a more detailed explanation of the FEM methodology and its practical uses