VIETNAM NATIONAL UNIVERSITY, HANOI
UNIVERSIRY OF ENGINEERING AND TECHNOLOGY
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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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
3.4
Masks designed........................................................................................... 35
Chapter 4
Results and Discussion ................................................................... 38
4.1
General Description .................................................................................... 38
4.2
Density and viscosity .................................................................................. 38
4.2.1
Lithium Niobate Crystal ...................................................................... 38
4.2.2
Aluminium Nitride Crystal .................................................................. 43
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
LOSS
Y
IDT
InterDigital Transducer
SAW
Surface Acoustic Wave
R-SAW
Rayleigh Surface Acoustic Wave
SH-SAW
Shear-Horizontal Surface Acoustic Wave
LiNbO3
Lithium Niobate
Mo
Molybdenum
Al
Aluminium
AlN
Aluminium Nitride
Si
Silicon
SOI
Silicon On Insulator
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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
KOW LED EME
S
I would like to sincerely thank my advisor, Assoc. Prof. Chu Duc Trinh for their
encouragement, guidance, and invaluable supports throughout the course of this
study. He guided me in studying microfluidics and always gave me meaningful and
profound explanations. I would like to gratefully acknowledge Dr. Tran Duc Tan
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
L S S OF
BLES
Table 3.1: Physical properties of the chosen liquids. ................................................33
Table 3.2: Parameters of SAW device based on Aluminium Nitride Crystal. .........34
Table 3.3: Design parameters of AlN-based SAW device........................................34
Table 3.4: The design parameters for AlN-based microfluidic sensor with single
channel. .....................................................................................................................36
Table 3.5: The design parameters for AlN-based microfluidic sensor with multichannel ......................................................................................................................36
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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
L S S OF F
ES
Figure 1.1: The flow chart for the development process of an AlN-based
Microfluidic Sensor prototype. .................................................................................10
Figure 2.1: Acoustic wave propagation direction in a Cartesian coordinate system.
...................................................................................................................................13
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
Chapter 1
ntroduction
1.1 Motivation and Objectives
In recent years, microfluidic technology received a lot of attention because its
widespread applications in printing, biomedicine. Device integration and
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
Chapter 2
heoretical nalysis of the l -based
Microfluidic Sensor
2.1 Introduction
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
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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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
propagates in the (X1, X2) and has a displacement profile
which varies with the
depth X3 of the single-crystal (see Figure 2.6):
(
)
[
where k is the wave number,
]
(
)
(1),
is the velocity of the wave, b is the decay constant of
the wave in the X3 direction, X1, X2, X3 is the unit vectors and (l1, l2) is the set of
propagation direction along the surface. The component X3 is perpendicular to the
surface and W1, W2, W3 represent the displacement amplitudes of the X1, X2 and X3
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 X2 and l1 equals to the unit. Hence, the travelling wave form is
independent of the X2 coordinate. Since the displacement component u2 is removed,
l2 is zero.
Figure 2.5: Principle construction of multilayer SAW sensor.
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Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation
Figure 2.6: Geometry of the problem for analysing propagation of Rayleigh waves.
In the following, the fluid situation is considered. For a non-conducting, nonviscous fluid, the elastic constant matrix is presented in the Appendix. As the
direction of fluid flow is across the X3 coordinate, generated the surface acoustic
wave travels in the direction X1 and has a displacement profile
that varies with
depth X3 into the piezoelectric substrate as following:
(
where k is the wave number,
(
)
(
)
(2)
( )
)
(3)
(4),
is the acoustic wave velocity in the liquid medium, bf
is the decay constant of the wave in the X3 direction and (l1, l2) is the set of
propagation direction along the liquid medium and W2 is the weight of the potential
. The component X3 is perpendicular to the surface and W1, W3 represent the
displacement amplitudes of the X1 and X3 directions, respectively. ( ) is the fluid
motion function which disturbs not only the acoustic wave velocity
but also the
decay constant bf. The fluid viscosity is ignored in Eq. 1.
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