VIETNAM NATIONAL UNIVERSITY, HANOI
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  2013
Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation

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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


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
IDT InterDigital Transducer
SAW Surface Acoustic Wave
R-SAW Rayleigh Surface Acoustic Wave
SH-SAW Shear-Horizontal Surface Acoustic Wave
LiNbO
3
Lithium Niobate
Mo Molybdenum
Al Aluminium
AlN Aluminium Nitride
Si Silicon
SOI Silicon On Insulator
Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation

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
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.

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
Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation

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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

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.
Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation

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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 (LiTaO


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 LiNbO
3
, 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.
Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation

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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.

Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation

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Chapter 2 -


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 LiTaO
3
(36YXLT). In 1999, Shiokawa et al. presented
a liquid-phase sensor using a SH-SAW on the 36YXLT [15].
Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation

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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
Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation

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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
Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation

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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):















, 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

3
into the piezoelectric substrate as following:




















(2)












(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


. The effect of coupling
the displacements and the potential can be illustrated by [7]:


























(5),





 



(6),
Also, in the case of AlN substrate, it is:





































(8)
Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation

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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:







 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,
Microfluidic Sensor based on AlN Vertical SAW structure: Investigation, Design and Simulation

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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 G
a
(f), the acoustic
susceptance B
a
(f) and the total capacitance C
T
[19]. The number of finger pairs or
IDTs (N
p
) and the wavelength are calculated in the following:

2

over V
1
and is sought in the form of the cardinal sine function:


























(18),
where H
2
(f) is the frequency response which is produced by liquid impacts in the
well such as chemical and physical properties.
2.4.3 Attenuation
The electrical attenuation response is regarded as the insertion loss of electrical
systems. For SAW devices, insertion loss is a function of frequency:













  





(19),
The minimum insertion loss occurs when f = f
0