Nghiên cứu, thiết kế và mô phỏng bộ cảm biến vi lỏng có cấu trúc SAW đứng trên vật liệu AIN

14 2.3 Propagation of Acoustic Waves in contact with a Liquid Medium .... Bui Thu Hang Page 9 Moreover, the acoustic wave propagation strongly depends on the properties of nanostructure

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UNIVERSIRY OF ENGINEERING AND TECHNOLOGY

- -

BUI THU HANG

MICROFLUIDIC SENSOR BASED ON ALN

VERTICAL SAW STRUCTURE:

INVESTIGATION, DESIGN AND SIMULATION

MASTER THESIS in ELECTRONICS AND TELECOMMUNICATIONS

TECHNOLOGY

Hanoi – 2013

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Bui Thu Hang Page 1

TABLE OF CONTENT

GLOSSARY 3

ACKOWNLEDGEMENTS 4

LISTS OF TABLES 5

LISTS OF FIGURES 6

Chapter 1 Introduction 8

1.1 Motivation and Objectives 8

1.2 Organization of Thesis 9

Chapter 2 Theoretical Analysis of the AlN-based Microfluidic Sensor 12

2.1 Introduction 12

2.2 Surface Acoustic Waves 13

2.2.1 Shear Horizontal Surface Acoustic Waves (SH-SAWs) 13

2.2.2 Rayleigh Surface Acoustic Waves (R-SAWs) 14

2.3 Propagation of Acoustic Waves in contact with a Liquid Medium 16

2.3.1 Boundary Conditions 19

2.3.2 Standing and Linear Motion Medium 19

2.3.3 Moving Liquid Medium 20

2.4 Equivalent Circuit Model of SAW Devices 21

2.4.1 Model Implementation 21

2.4.2 Frequency Response 22

2.4.3 Attenuation 22

2.5 Conclusion 23

Chapter 3 3-D Design of AlN-based Microfluidic Sensor 24

3.1 General Description 24

3.2 Design Principles 25

3.3 FEM Simulation for AlN-based Microfluidic Sensor 29

3.3.1 General Configuration 29

3.3.2 Lithium Niobate 30

3.3.3 Aluminium Nitride 33

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4.3 Sensing Liquid Status 45

4.3.1 Constant Velocity 45

4.3.2 Non-constant Velocity 49

4.4 Conclusion 53

Chapter 5 Conclusions and Future Work 54

5.1 Conclusions 54

5.2 Future work 54

Reference 56

Appendix: Material Parameters for Piezoelectric Substrate 59

A Lithium Niobate 59

B Aluminium Nitride 59

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LOSS Y

SH-SAW Shear-Horizontal Surface Acoustic Wave

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and Assoc Prof Rusu Vasile Catelin for useful suggestions in my dissertation Their guidance enabled me to complete my thesis work

I am also highly thankful to all teachers at Dept of Electronics and Telecommunications for supports and encouragement Many thanks to staff in department for their helps of thesis defence procedures

Finally, it is my profound gratitude to my family, especially my mom, my cousin Phan Quoc Vi for their moral supports and encouragement in my life

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L S S OF BLES

Table 3.1: Physical properties of the chosen liquids 33Table 3.2: Parameters of SAW device based on Aluminium Nitride Crystal 34Table 3.3: Design parameters of AlN-based SAW device 34Table 3.4: The design parameters for AlN-based microfluidic sensor with single channel 36Table 3.5: The design parameters for AlN-based microfluidic sensor with multi-channel 36

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Figure 2.2: (a) The typical SH-SAW structure (b) Illustration of shear horizontal

(SH) polarized displacement 14

Figure 2.3: (a) Schematic of the particle motion for a Rayleigh wave (b) Ultrasonic radiation into water by SAW when sensing channel placed on substrate 15

Figure 2.4: (a) The simple SAW structure for sensing liquid (b) Ultrasonic radiation into water when sensing channel is placed along the vertical axis of device 16

Figure 2.5: Principle construction of multilayer SAW sensor 17

Figure 2.6: Geometry of the problem for analysing propagation of Rayleigh waves 18

Figure 2.7: Mason equivalent circuit model 21

Figure 3.1: Schematic drawing of the integrated inkjet system 26

Figure 3.2: Top and cross-view of one-channel microfluidic sensor 27

Figure 3.3: Top and cross-view of two-channel microfluidic sensor 28

Figure 3.4: Top and cross-view of one-input two-channel microfluidic sensor 28

Figure 3.5: Top and cross-view of multi-output microfluidic sensor 29

Figure 3.6: Schematic illustration of two-channel R-SAW sensor and liquid well position 30

Figure 3.7: Design parameters of Channel 1 and well size 31

Figure 3.8: Meshed image of 3D SAW model with the well in the middle of the wave propagation path 32

Figure 3.9: General view for all devices in one die 35

Figure 4.1: Total displacement of corresponding points in Channel 1 and Channel 2 with different well diameters 39

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Figure 4.2: Total displacement of the well behind points with three liquid types 40

Figure 4.3: Output voltage of Group 1 from the 3-D SAW model with and without deposited well from 0 to 130 nsec 41

Figure 4.4: Output voltage of Group 2 from the 3-D SAW model with and without deposited well from 0 to 130 nsec 43

Figure 4.5: (a) Total displacement envelops of points placed behind the well 43

Figure 4.6: Electrical attenuation response (shown as insertion loss) for the SAW device 44

Figure 4.7: The time delay of system with the well having liquid density =1, 3, 6, and 12 g/cm3 44

Figure 4.8: Potential amplitude at center frequency on the IDT receiver for linear group 45

Figure 4.9: Ratio coefficient of displacement amplitudes before and after the well for linear group 46

Figure 4.10: (a) Delay time and (b) Velocity decay coefficient when liquid moves linearly 47

Figure 4.11: Attenuation corresponding to linear motion function 48

Figure 4.12: Effect of SAWs on linear fluid flow 49

Figure 4.13: Potential amplitude at center frequency on the IDT receiver for exponential motion group 50

Figure 4.14: Ratio coefficient of displacement amplitudes before and after the well for exponential motion group 50

Figure 4.15: Velocity decay for exponential motion group 51

Figure 4.16: Delay time when liquid moves nonlinearly 51

Figure 4.17: Attenuation corresponding to exponential motion function 52

Figure 4.18: Effect of SAWs on exponential fluid flow 52

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miniaturization based on microfluidic technology has been growing up quickly In addition, expected devices may have advantages such as: small size, facile usage and low cost, fast detection speed, high accuracy, less consumption power and high integration capability One of the present microfluidic technologies utilizes surface acoustic wave (SAW) [1][2]

It is well-known owing to applications such as actuators, antennas and driven droplet manipulation using SAW atomization and jetting technique [3][4][5] SAW devices are also widely utilized in sensors [6] Such devices convert electrical energy into mechanical energy and vice versa Specifically, when the transformation from electrical to mechanical energy occurs at the InterDigital Transducder (IDT) transmitter, acoustic waves travel through the surface SAW waves include Rayleigh waves, and sliding shear waves The amplitude of the Rayleigh-SAWs of around 10Å is very small and exponentially declines Because wave penetration into the substrate is inversely proportional to frequency, in order

to limit reflections and refractions at the bottom, the material size is large enough This mechanical vibration on the surface continues until opposite transform process

at the IDT receiver Waves that do not retransform electrical energy at the receiver are absorbed by wax, polyimide placed before and after the input and output IDT Sensing mechanism is electrical perturbation on the IDT receiver due to obstacles

on the propagation path or even if R-SAWs travel through the different media [7] Prominent advantages of SAW devices are micro derivation size for fluid, high sensitivity and fabrication ability on compatible material The structure trend is vertical sensing channel This suggests the requirement of the vertical SAW sensor

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Moreover, the acoustic wave propagation strongly depends on the properties of nanostructure sensing layers which in turn can be altered by the wave vibration itself Here, it is able to be piezoelectric thin film on the substrate or piezoelectric crystal such as quartz, Lithium Tantalate (LiTaO3), Lithium Niobate (LiNbO3), especially Aluminium Nitride (AlN) because of high frequency and being compatible with the CMOS technology Thus, it is necessary to understand both the wave propagation and the nanomaterial properties in order to uncover the sensing mechanism and improve the performance of acoustic sensors This dissertation focuses:

1 Understanding the components and the propagation characteristics of liquid SAW devices

2 Investigating the electrical and mechanical properties of SAW devices on common piezoelectric, LiNO3, and CMOS material, AlN, when there are impacts of fluid such as density, viscosity and motion in the sensing channel

3 Developing novel SAW sensors for microfluidics

4 Integration ability in ink sensing applications

1.2 Organization of Thesis

We adopt the development process in Figure 1.1 It is assumed that the SAW sensor works in an ideal environment The theoretical derivation and analysis are performed to qualitatively verify the proposed design In order to achieve the quantitative analysis, the finite element method (FEM) will be studied and implemented with the software Comsol Multiphysics According to these verification results, it demonstrates the fabrication capacity of the AlN-based microfluidic sensor in the future

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Figure 1.1: The flow chart for the development process of an AlN-based Microfluidic

Sensor prototype

The dissertation is organized as following:

Chapter two describes the acoustic waves in the piezoelectric and liquid medium It gives electrical properties of SAW devices through the equivalent Mason circuit Also, the analysis of leaky phenomenon induced by Rayleigh wave interaction with the liquid medium is presented

Chapter three discusses the design and realization of SAW microfluidic sensor using LiNbO3, AlN Modelling procedure is conducted by Finite Element Method (FEM) Optimization of sensor parameters in the simulation driving to enhanced amplitude fields and lower propagation loses; thereby increasing device sensitivity

is discussed Besides, several masks in the experiment are described

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Chapter four is the major simulation results for sensing density and status of liquid

in the well, the explanations and analyses of obtained results

Chapter five summarizes the main contributions and provides suggestions for possible future studies

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SAW devices have been studied in chemistry, biomedicine and telecommunication for many decades, especially sensors including automotive (torque and tire pressure sensors), medical applications (biosensors) and industrial, commercial applications (vapour, humidity, temperature and mass sensors) [8] Measurands on acoustic wave sensor are wave velocity perturbation, changes of confinement dimensions, degree of the traveling wave damping and input-output variation [9] An important distinction between types can be defined according to the natures of the acoustic wave and vibration modes Traveling waves can be bulk acoustic waves (BAWs) propagating on the interior of the substrate and SAWs on the surface

To the SAW sensor, a mechanical wave, generated by piezoelectric crystal using metal electrodes or called interdigital transducers (IDTs), travels along the surface [6][10] It includes a Rayleigh and a shear mode which propagate through the surface as shown in Figure 2.1 The Rayleigh mode, called Rayleigh wave, is a combination of longitudinal and shear vertical particle displacement while the shear mode, called Shear Horizontal – Surface Acoustic Wave (SH-SAW), is a shear horizontal wave on the surface [11][12]

Other surface waves are Love waves (LWs) where the acoustic waves are guided in the foreign layer and surface transverse waves (STW) where guiding waves are on a piezoelectric substrate under a shallow groove or on thin metal strip gratings [13]

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Figure 2.1: Acoustic wave propagation direction in a Cartesian coordinate system

(a) Compressional or longitudinal; (b) Shear vertical; (c) Shear horizontal

2.2 Surface Acoustic Waves

2.2.1 Shear Horizontal Surface Acoustic Waves (SH-SAWs)

A typical structure of SH-SAW device for liquid has a horizontal channel on the substrate (see Figure 2.2a) When the liquid is loaded on the propagating surface, the SH-SAWs can travel along the interface between the liquid and the substrate and are influenced by its properties as shown in Figure 2.2b As the penetration depth of the shear horizontal (SH) particle into liquid is very low, SH-mode SAW sensors were utilized for sensing liquid without significant radiation losses [14] For example, in 1977, Nakamura et al proposed a pseudo SH-SAW on the 36-degree rorated Y-cut X-propagating LiTaO3 (36YXLT) In 1999, Shiokawa et al presented

a liquid-phase sensor using a SH-SAW on the 36YXLT [15]

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Figure 2.2: (a) The typical SH-SAW structure (b) Illustration of shear horizontal

(SH) polarized displacement

2.2.2 Rayleigh Surface Acoustic Waves (R-SAWs)

As mentioned above, Rayleigh wave is composed of compressional or longitudinal displacement and shear vertical displacements while the compressional component

is confined at the surface down to a penetration depth of the order of the wavelength The particle motion in the piezoelectric where Rayleigh waves pass is sought in the form of an ellipse as shown in Figure 2.3a [16] Hence, Rayleigh with

a particle displacement perpendicular to the device surface can be radiated into the liquid medium and cause an excessive attenuation if the contact between liquid and piezoelectric is too large On the other hand, leaky SAWs that are converted from

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SAWs are excited at a Rayleigh angle R in the boundary and consequently their energy radiates into the liquid in Figure 2.3b It is difficult to realize a liquid-phase

by the Rayleigh wave devices when the liquid is placed on the piezoelectric

Figure 2.3: (a) Schematic of the particle motion for a Rayleigh wave (b) Ultrasonic radiation into water by SAW when sensing channel placed on substrate

We proposed a novel vertical structure for the liquid sensing applications based on Rayleigh waves in Figure 2.4a [14] The key is that the solid-liquid contact area, closing to surface, is smaller (as shown in Figure 2.4b) It is presented that there exists a liquid medium in the propagation path with the coordinate system, thereby the longitudinal component should have an inappreciable attenuation

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Figure 2.4: (a) The simple SAW structure for sensing liquid (b) Ultrasonic radiation into water when sensing channel is placed along the vertical axis of device

2.3 Propagation of Acoustic Waves in contact with a Liquid Medium

As the SAW motion spreads below the surface to a depth of about one wavelength [7], the effect of the second layer below the piezoelectric might be ignored if the thin film is thick So, it is assumed that the thin film is thick enough to prevent acoustic waves from reflecting on the bottom, reflection wave interference phenomenon may be cancelled The geometry of acoustic waves propagating in a piezoelectric substrate in contact with a fluid and solid medium is shown in Figure 2.5 To satisfy the stress-free boundary, compression and shear waves propagate together on the substrate It is assumed that the generalized surface acoustic wave

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propagates in the (X 1 , X 2 ) and has a displacement profile which varies with the

depth X 3 of the single-crystal (see Figure 2.6):

( ) [ ] ( ) (1),

where k is the wave number, is the velocity of the wave, b is the decay constant of the wave in the X 3 direction, X 1 , X 2 , X 3 is the unit vectors and (l 1 , l 2 ) is the set of

propagation direction along the surface The component X 3 is perpendicular to the

surface and W 1 , W 2 , W 3 represent the displacement amplitudes of the X 1 , X 2 and X 3

directions, respectively It is assumed that there exists a liquid medium positioned in the propagation path with the coordinate system

The Rayleigh wave is characterized by the absence of a transversal component

Thus Eq 1 omits X 2 and l 1 equals to the unit Hence, the travelling wave form is

independent of the X 2 coordinate Since the displacement component u 2 is removed,

l 2 is zero

Figure 2.5: Principle construction of multilayer SAW sensor

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Figure 2.6: Geometry of the problem for analysing propagation of Rayleigh waves

In the following, the fluid situation is considered For a conducting, viscous fluid, the elastic constant matrix is presented in the Appendix As the

non-direction of fluid flow is across the X 3 coordinate, generated the surface acoustic

wave travels in the direction X 1 and has a displacement profile that varies with

depth X 3 into the piezoelectric substrate as following:

( ) (2) ( ) ( ) (3) ( ) (4),

where k is the wave number, is the acoustic wave velocity in the liquid medium, b f

is the decay constant of the wave in the X 3 direction and (l 1 , l 2 ) is the set of

propagation direction along the liquid medium and W 2 is the weight of the potential

The component X 3 is perpendicular to the surface and W 1 , W 3 represent the

displacement amplitudes of the X 1 and X 3 directions, respectively ( ) is the fluid motion function which disturbs not only the acoustic wave velocity but also the

decay constant b f The fluid viscosity is ignored in Eq 1

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2.3.1 Boundary Conditions

At the liquid-solid interface, as the normal and propagation components of the particle displacement , and the electric potential are continuous and the boundary conditions are , and The effect of coupling the displacements and the potential can be illustrated by [7]:

∑ (5),

where and are the piezoelectric stress constants and the permittivity tensors (as shown in Appendix)

Substituting Eq 1, 2, 3 and 4 into Eq 5, in the case of LiNbO3 piezoelectric crystal,

it leads to relationship between the weight of potential and displacement as following:

2.3.2 Standing and Linear Motion Medium

According to Newton’s Law, the equation of motion in the non-piezoelectric is found:

( )

(8)

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√ (10) (11)

Following Eqs 10 and 11, these surface waveforms depend on the liquid density Hence, with different materials, the amplitude and phase of the particle displacement of the leaky wave are changed

When the fluid motion ( ) is linear, the second order differential equation of the normal displacement rejects the flow influence The solution of this equation is similar to the case of standing liquid medium

2.3.3 Moving Liquid Medium

If the liquid velocity is non-linear, the motion equations in this case have an existence of fluid velocity which is sought in the form:

( ̇ ) (12)

(13),

where ( ) ̇( ) is the velocity function of liquid streaming Following the Eqs

7, 12 and 13, both the surface wave form and potential are influenced In this case,

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it is hard to define the order of the fluid motion function as both the numerator and

denominator are function of liquid velocity in the well

2.4 Equivalent Circuit Model of SAW Devices

2.4.1 Model Implementation

Without the well, the SAW device can be modelled with the Impulse Response

method based on the Mason equivalent circuit (Figure 2.7) [18] It has been widely

applied on different types of SAW sensors to calculate qualitative results on the

receiver According to this figure, the source voltage and both the source and load

impedances are not part of this model

Figure 2.7: Mason equivalent circuit model

For each IDT, the model includes the radiation conductance Ga(f), the acoustic

susceptance Ba(f) and the total capacitance CT [19] The number of finger pairs or

IDTs (Np) and the wavelength are calculated in the following:

(

) (14) (15),

where is the acoustic velocity on the piezoelectric, f 0 is the center frequency and

BW is the bandwidth

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over V 1 and is sought in the form of the cardinal sine function:

( ) ( ) (

) (16)

where f is the frequency, k is the piezoelectric coupling coefficient, D is the delay between IDTs in the wavelengths, C s is the capacitance for a finger pair per length The variable X is defined by:

( ) (17),

With the well, the frequency response H(f) of the SAW device is represented by:

( ) ( ) ( ) (18),

where H 2 (f) is the frequency response which is produced by liquid impacts in the

well such as chemical and physical properties

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output potential calculation for different densities and liquid state, attenuation is also influenced

2.5 Conclusion

According to the mechanical and electrical theory analysis, the AlN-based SAW structure is able to detect different liquids in the well by density value whereas the viscosity influence does not low Also, liquid velocity is an impact of changes on the output IDTs Mechanical characteristic analysis may help determine the relation between flow motion and output signal

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The SAW interaction with the fluid leads to an internal acoustically induced streaming and results in a mode conversion to the longitudinal leaky SAW which decays exponentially with distance to the fluid Therefore, as the above discussions, the well with moderate size is positioned at the delay line

To verify the sensing capacity of the proposed SAW structure, LiNbO3 is chosen because of its high electromechanical coupling, low electric loss and low velocity [20] Nonetheless, as its temperature depends linearly on all material cuts and propagation direction and it is not compatible with microelectronic CMOS processes, LiNbO3 piezoelectric substrate is not designed for temperature sensors and devices where the temperature is not stable Hence, the other substituted material with commercial potential is Aluminum Nitride In addition, Aluminum Nitride is able to propagate SAWs more swiftly and has a higher frequency It possesses this type of mode along the surface with the high velocity, good

temperature stability and a large electromechanical coupling coefficient (K 2 = 47%) [10] Compared to the leaky SAWs generated by piezoelectric materials like LiNbO3, LiTaO3 and quartz, it has larger K 2 values and better temperature stability for certain orientation [21][22][23] Nevertheless, the well area is small enough to avoid the excessive attenuation phenomenon on the output IDTs One more reason,

in favor of these materials, is that both of them are able be single crystal So, in this section, both LiNbO3 and AlN are selected for SAW device simulation

For the SAW device performance, the FEM method is chosen for simulation because it is suitable for numerical method to derive the influence of the geometrical variations and material parameters The design and simulation

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parameters for FEM simulation are presented The fabrication parameters presented

in this chapter for the surface micromachining technique are proposed to create exact sizes and positions for the sensing channel Also, it gives a mask designed to realize the sensors

Section 3.2 gives the design principles for the models of the microfluidic sensor Section 3.3 briefly presents the 3-D FEM simulation

Sections 3.4 and section 3.5 describe next stages for the fabrication in the future including real size values and design masked

3.2 Design Principles

As above mention, the SAW structure is appropriate for the tiny sensing area The proposed design is that it is applied for the inkjet nozzle as shown in Figure 3.1 Some suggested sensor positions are described When streaming flows through nozzle, it is able to be detected by sensor placed there or at the target As operation mechanism of piezoelectric which is conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy, passive sensor is proposed at the nozzle

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Figure 3.1: Schematic drawing of the integrated inkjet system

(a) Active sensor at the nozzle (b) Passive sensor at the nozzle (c) Active sensor at the

target

From the proposed SAW device positions for the inkjet system, several specific designs are described as following:

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Figure 3.2: Top and cross-view of one-channel microfluidic sensor

In SAW devices, transmitting IDTs simultaneously create the surface acoustic wave

in the opposite direction which reflects from the ends of the substrate They produce unwanted signals as spurious signal in the time domain or ripples in the frequency domain which affect the performance [20] In experiments, absorbers such as wax, polyimide, are used to prevent waves from reflecting back to the input and output IDT Thanks to such absorbers, the condition of infinite medium still exists

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Figure 3.3: Top and cross-view of two-channel microfluidic sensor

Figure 3.4: Top and cross-view of one-input two-channel microfluidic sensor

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Figure 3.5: Top and cross-view of multi-output microfluidic sensor

3.3 FEM Simulation for AlN-based Microfluidic Sensor

In this section, we focus on the R-SAW sensor configuration on X-cut, propagation LiNbO3 and AlN substrate used in FEM simulation

Y-3.3.1 General Configuration

The R-SAW liquid sensing system consists of two channels: a sensing channel (Channel 1) and a reference channel (Channel 2) Figure 3.6 shows the top-view and cross-section of the R-SAW sensor IDTs for input and output are covered by

Aluminium The wave propagation path is sheltered by X-cut Y-propagation

LiNbO3, AlN The well pierces through the middle of Channel 1 Sample liquids are poured onto well

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