DSpace at VNU: 3-D finite element modeling of SAW sensing system for liquids

Rayleigh wave sensors are highly sensitive in detecting properties of gas in contact with their surface including mass change, viscosity, density, velocity and electrical conductivity.

ua qu

Abstract—Like emerging devices, surface acoustic wave

(SAW) sensors received considerable interest towards their

integration within lab-on-chip and microfluidic structures

Rayleigh wave sensors are highly sensitive in detecting

properties of gas in contact with their surface including mass

change, viscosity, density, velocity and electrical conductivity

In this paper, primarily, a novel structure of Rayleigh surface

acoustic wave (R-SAW) sensors for charactering a liquid is

presented and sensitivities of the different liquids deposited in

the well are compared The fundamentals of the sensing system

using R-SAW sensor on X-cut, Y-propagation Lithium Niobate

substrate are described The numerical simulation results of the

wave propagation indicated the important role of the fluid

density whereas the viscosity influence is very low

Keywords: Acoustic wave sensor, SAW sensor, R-SAW

sensor

I INTRODUCTION

N the recent years, acoustic wave devices are used in a

wide applications as resonators, filters, actuators and

especially sensors There are several types of acoustic wave

sensors depending on vibrations, wave modes and sensed

objects Based on the mechanism detecting the mechanical

and electrical perturbations, current sensing systems sense

density, viscosity products, conductivity and dielectric

constant of liquid [1] Therefore, they have the potential to

be used in fields such as military, automotive, industrial and

environmental, food industry, medical science, meteorology

and specially biosensor applications

In 1887, Lord Rayleigh discovered propagation modes,

properties of the surface acoustic waves which was named

Rayleigh surface acoustic wave [2] R-SAWs have

longitudinal and vertical shear components propagating in

the surface piezoelectric substrate This coupling strongly

affects the amplitude and velocity of the wave The feature

enables to directly sense mass and mechanical properties

The surface motion also allows the devices to be used as

microactuators So, for gas sensors, R-SAW devices are

usually utilized and very sensitivity to mechanical and

electrical perturbations occurring on the surface For

mechanical properties, they are sensitive to mass loading and

viscosity-elastic changes like stiffening and softening For

electrical properties, the devices can be suitable for any

property that interacts with the electrical field when it is

Manuscript received January 15 rd , 2012

Authors are with the Department of MicroElectroMechanical Systems

and Microsystems, Faculty of Electronics and Telecommunications,

University of Engineering and Technology, Vietnam National University,

Hanoi, Vietnam

T Chu Duc also is with Nano and Energy Center, Vietnam National

University, Hanoi, Vietnam (corresponding author to provide phone: +84 4

3754 9338; fax: +84 4 7547460; e-mail: trinhcd@ vnu.edu.vn)

coupled to the propagating acoustic wave This effect is known as the term electro-acoustic interactions However, for liquid phase applications, most acoustic sensors were proposed using the shear horizontal acoustic plate mode (SH-APM), the thickness-shear mode (TSM), the Lamb wave or the flexural plate wave (FPW) and Love wave instead of Rayleigh wave [3] Because when SAWs are in contact with the liquid, leaky SAWs, which are converted from SAW, are excited and consequently its energy radiates into liquid On the other hand, because of surface-normal waves, they cause an excessive attenuation of the surface waves in the liquid [4] It is difficult to realize a liquid-phase

by the Rayleigh wave devices when liquid is put on the piezoelectric However, one advantage of these devices is that Rayleigh waves can readily radiate a longitudinal wave

in contact between the piezoelectric substrate and liquid This mode conversation can be utilized for generation and detection of underwater longitudinal waves

This work proposes a novel structure for the liquid sensing applications based on Rayleigh waves The attenuation and delay of R-SAWs in the liquid sensing system is investigated In addition, the fundamental characteristics of R-SAW sensors are reported

II FUNDAMENTALS OF RAYLEIGH WAVES

As an alternating electrical signal is applied in the input Interdigital Transducer (IDT), SAW, a mechanic wave, can

be generated and travel across the piezoelectric surface It can be picked up and converted back into electrical signal at output IDT placed in the wave propagation path When

3-D Finite Element Modeling of SAW sensing system for liquids

Thu-Hang Bui, Dat Nguyen Tien, Tung Bui Duc, and Trinh Chu Duc

I

Figure 1 (a) The structure of a SAW sensing system for liquid (b) Ultrasonic radiation into water by SAW

(a)

(b)

July 11-14, 2012, Kaohsiung, Taiwan

Figure 2 Schematic illustration of two-channel R-SAW sensor and

liquid well position

Figure 3 Design parameters of Channel 2 and well size

Rayleigh wave propagates on the piezoelectric crystal

surface, it is influenced by perturbation from physical and

chemical changes at the surface or the adjacent medium as

shown in Figure 1a The existence fluid medium between the

two IDTs will cause certain amount of wave attenuation and

delay

It is assumed that there exists a liquid medium positioning

in the propagation path When SAWs propagate along the

boundary between the solid and liquid medium, leaky waves

appear and excite the longitudinal waves into liquid at

Rayleigh angle R Figure 1b shows ultrasonic radiation

mechanism Surface waves are assumed to travel in the X1

direction along the surface whose normal is in the X2

direction The particle displacement and potential are

considered to be independent of the X3 coordinate

Travelling wave solutions can be calculated by extending

the method of Campell and Jones in the solid–liquid

structure [5] Hence, travelling wave solutions can be put in

the following form:

(1)

where is the particle displacement, is the potential of

the fluid, k = / is the wave number, is the velocity of

the wave, is the decay constant of the wave

in the X2 direction and is the relation

coefficient f is the fluid density and c f is the elastic

constant of the fluid

The fluid viscosity is ignored in Eq (1) Following the Eq

(1), these surface waveforms depend on the liquid density

Hence, with different materials, the amplitude and phase of

particle displacement of leaky wave are changed

III SENSING SYSTEM

In this section, we focus on the R-SAW sensor

configuration on X-cut, Y-propagation Lithium Niobate

substrate and the measurement method

A Sensor configuration

The R-SAW liquid sensing system consists of two

channels: a reference channel (Channel 1) and a sensing

channel (Channel 2) Figure 2 shows the top view and

cross-section of the R-SAW sensor IDTs for input and output are

covered by aluminum Changes in attenuation and velocity

of surface acoustic waves depend on the square of the

electromechanical coupling coefficient of substrate, K 2, thus

Lithium Niobate is approximate substrate to choose [2]

Therefore, the wave propagation path is sheltered by X-cut

Y-propagation Lithium Niobate The well pierces through

the centre of the Channel 2 Sample liquids are poured onto

well

Channels are constructed so that the sensor can simultaneously measure the changeable electrical and mechanical properties of sample liquids The density products are obtained from the differential phase shift between the reference channel and the sensing channel and amplitudes of both channels are detected

B Materials and Design of System

Design parameters of Channel 2 are as follows in Figure 3 Each channel includes two-port and a delay line The substrate was made of piezoelectric Lithium Niobate material in class 3m symmetry The stiffness, piezoelectric constant and permittivity of material are shown fully in the appendix To build a 3D model of X-cut and Y-propagation Lithium Niobate substrate, a 3D domain of size 240 µm along X-axis, 120 µm along Y-axis and 30 µm along Z-axis

is created There are two IDTs added on the top of this piezoelectric substrate Each of these IDTs has two pairs of interdigitated fingers with each finger being 10 μm wide and

100 μm long A long spacing which is present between the

neighboring fingers is 10 μm The aperture of IDT (W) is 82

μm The SAW velocity is found from V = f0 where the transducers period  or the acoustic wavelength is 40 µm According to the literature, the velocity of acoustic wave propagation in an X-Y Lithium Niobate substrate is about

3485 m/s [12] The interaction length of the particle

displacement (L) between the input and output IDT was 80

μm Using this velocity number, the estimated minimum time for the acoustic wave to propagate the specified distance between the input and output IDTs in the model is

23 ns

(a)

(b) Figure 5 (a) Point positions and (b) Total displacement of point groups before and after well of the 3D SAW model

TABLE I

P HYSICAL P ROPERTIES OF LIQUID

Liquid Viscosity  (mm2 /s)

Density  (g/cm 3 )

W/G mix 1

W/G mix 2

5.6 11.50

1.126 1.161

Propylene

Bromine

Mercury

0.09 0.31 1.55

0.514 3.123 13.579

Figure 4 Total displacement of corresponding points in two

channels Figure 7 Total displacement of corresponding points in Channel 1 and Channel 2 with doubled well diameter

Figure 6 Total displacement of corresponding points in two

channels

For the surface acoustic wave delay path, a cylinder well

with a diameter of 20 µm and height 30 µm is placed in the

center of delay line between input and output IDTs The

used liquids are classified into two groups: Group 1

including deionized water, water/glycerol mixtures (W/G),

and Group 2 consisting of propylene, bromine and mercury

which physical properties at 20 0C are given in Table 1 [15]

Glycerol percent weight was from 50 to 70 to increase the

viscosity by a factor of up to 11, whereas the density stayed

close to that of water The density of bromine and mercury is

much higher than that of propylene

After building this first model, a second model of the

Channel 1 with the same dimensions is created An electrical

potential in the form of a sinusoidal wave was applied to two

of the fingers (alternate ones) of the input IDT, while the

other two fingers were grounded

The piezoelectric surfaces of the developed models were

meshed with maximum element size of 11.2 µm and the IDT

boundaries were meshed with a maximum element size of

6.4 μm These parameters provided a much denser mesh at

the top of the model which is essential to achieve a high

accuracy in simulating the SAW propagation

A sinusoidal voltage 10 V of frequency 100 MHz is

applied to the input IDT to generate the needed SAWs The

output voltages in both cases were acquired at the alternating

fingers of the output IDT

IV RESULTS AND DISCUSSION

A Corresponding point comparison in two channels

Figure 8 Total displacement of the well behind points with three

liquid types

Figure 9 Output voltage of Group 1 from the 3-D SAW model with and without deposited well from 0 to 130 nsec

(a) Water well, (b) W/G mix 1 well and (c) W/G mix 2 well

In the first stage of the simulation, two R-SAW channels

were run within 130 nsec and a time step is 0.09 nsec The

contour plot for the total particle displacement of two

channels was shown in Figure 4 The fluid well which was

deposited in the delay path of the device resulted in a

perturbation

Figure 5 illustrates the contour plot for the particle

displacement of two point groups placed before and after the

well Each group includes a well closely neighboring point X

and a point Y on the boundary of liquid–solid medium

Distance between two points of each group is 1 µm

The amplitude and phase variance of these explored pairs

of points are not shifted Consequently, instead of

investigating adjacent points on the piezoelectric surface, it

is possible to choose points in the boundary between the

liquid and solid medium

To obverse clearly displacement changes, two points with

the corresponding positions in the two channels are

compared and shown in Figure 6 A solid line shows

displacement of the point behind the well of Channel 2 and a

dashed line illustrates that of the corresponding point of

Channel 1 Attenuation of R-SAW was observed as a result

of propagating across the liquid medium As the distance

which waves were propagated is too short, it is difficult to

detect its phase shift Hence, the well diameter was increased

from 20 µm to 40 µm as following in Figure 7

However, the size of well can not be too large for

achieving large enough signals in the output of SAW device

due to the decay constant of Lithium Niobate is lower than

that of liquid

B Sensing different liquid in well

The total displacements of points placed in the liquid–

solid boundary in all three cases of two groups in Figure 8

were plotted The solid line illustrates that of water, the

dashed line is for that of water/glycerol mixtures 2 in the

Group 1 and the dash-dot line is for that of bromine in the

Group 2

The output voltages in all cases obtained at the alternating

fingers of the output IDT were plotted as functions of time

as shown in Figures 9 and 10 To determine the delay value between output voltages, one of the time delay estimation (TDE) methods which is commonly used is a cross covariance [17] It is normalized by the root of the auto-covariance and based on the estimation of the time lag at which the cross-correlation function (CCF) estimation reaches its maximum value [18] Therefore, as compared to the device without well, a time delay of 0.09 nsec was calculated as a result of water and that of 1.08 nsec was for the W/G mix 2 well whereas that of bromine was 2.16 nsec Comparing the output voltages and the total displacements

of the channels containing water, W/G mix 1 and W/G mix

2 well, it can be seen that Rayleigh wave was less influenced

by the viscosity of material although their densities are approximately the same As a result, the influence of viscosity can be dropped from the equation of travelling solution of waveform To the contrary, it was affected much

by the density Moreover, when the density material is low, the needed time for acoustic wave propagating the distance

(a)

(b)

(c)

Figure 10 Output voltage of Group 2 from the 3-D SAW model

with and without deposited well from 0 to 130 nsec

(a) Propylene well, (b) Bromine well and (c) Mercury well

between input and output IDTs is lower because of the low

phase shift

As the transverse waves are generated from SAW devices,

noises appear and lead to the inhomogeneous output signals

In future work, more efficient ways of increasing the mesh

density and the investigation time, restraining noise from

transverse waves as well as decreasing the error tolerance

levels may provide more accurate analysis of the

performance of the SAW sensors

V CONCLUSION This paper reports a design of the sensing system using

Rayleigh waves for liquid They are influenced much by the

liquid density whereas the viscosity can be neglected The

3D finite element analysis was performed to investigate the

performance of R-SAW sensor for liquid The response of

SAW devices under alternating-current excitation showed

the benefit of positioning the liquid well in the middle of the two-port SAW delay-line device for enhancing liquid sensing capacity This study provides a strong meaning for manufacturing and designing R-SAW sensor in practice

APPENDIX Material constants for Lithium Niobate piezoelectric class 3m symmetry in simulation:

where C11 = 20.3×1010 Nm-2, C33 = 24.5 ×1010 Nm-2, C44 =

6.0×1010 Nm-2, C12 = 5.3×1010 Nm-2, C13 = 7.5×1010 Nm-2, C14 = 0.9×1010 Nm-2, e15 = 3.7 Cm-2, e22 = 2.5 Cm-2, e31 = 0.2

Cm-2, e33 = 1.3 Cm-2, ε11 = 44, ε33 = 29, and ρ = 4600 Kgm-3

ACKNOWLEDGMENT The authors would like to acknowledge the support of Dr

Le Quang Hieu of the Faculty of Information Technology, UET, VNU

Special thanks are due to Dr Tran Duc Tan of the Faculty

of Electronics and Telecommunications, UET, VNU and Dr Bui Thanh Tung of the National Institute of Advanced Industrial Science and Technology (AIST), Japan for their suggestions and discussions

This work is partly supported by the VNU project QG.B.11.30 and National Foundation for Science & Technology Development (Nafosted)

REFERENCES

[1] J Kondoh, Y Okiyama, S Mikuni and Y Matsui, “Development of

SH-SAW sensing system for liquid”, IEEE, 1-4244-0647-1, 2007

[2] D S Ballantine, R M White, S J Martin, A J Ricco and E T

Zellers, “Acoustic wave sensors”, Academic Press, 1997

[3] M I Rocha-Gaso, C March-Iborra, A Montoya-Baides and A

Armau-Vices, “Surface generated acoustic wave biosensors for the

detection of pathogens: A review”, Sensors 2009, 9, 5740-5769

[4] S Shiokawa, Y Matsui and T Moriizumi, “Experimental study on

liquid streaming by SAW”, Jpn J Appl Phys.28, Suppl 28-1,

126-128, 1989

[5] J J Campell and W R Jones, “A method for estimating optimal

crystal cuts and propagation directions for excitation of piezoelectric substrate waves”, IEEE Trans on Sonics and Ultrasonics, vol SU-15,

No 4, 1968

[6] S Shiokawa, Y Matsui and T Ueda, “Liquid streaming and droplet

formation caused by leaky Rayleigh waves”, IEEE Ultrasonics

Symposium – 643, 1989

(b)

(a)

(c)

[7] N G Suraji, “Progation of plate acoustic waves in contact with fluid

medium”, Master dissertation, Faculty of the Graduate School,

Marquette University, 2009

[8] Y Jiashi, “An introduction to the theory of piezoelectricity”, Dept

Engineering Mechanics, University of Nebraska-Lincoln, USA,

eBook ISBN: 0-387-23546-9, http://www.springeronline.com, 2005

[9] S Shikawa and J Kondo, “Surface acoustic wave sensor for

liquid-phase application”, IEEE Ultrasonics Symposium–445, 0-7803-5722,

1999

[10] W Richard Smith, H M Gerard, J H Collins and T M Reeder,

“Analysis of Interdigital surface acoustic wave transducers by use of

an equivalent circuit model”, IEEE Tran on Microwave theory and

Techniques, vol MIT-17, No 11, 1969

[11] J G Gualtiei, J A Kosinski and A Ballato, “Piezoelectric materials

for acoustic wave applications”, IEEE Tran on Ultrasonics,

Ferroelectrics and Frequency control, vol 41, No 1, 1994

[12] K K Wong, “Lithium Niobate”, No 28, INSPEC, 2002

[13] G Bu, D Ciplys and M S Shur, “Leaky surface acoustic waves in

single-crystal AlN substrate”, International Journal of High Speed

Electronics and Systems, Vol 14, No 3837-846, 2004

[14] J.C Andle and J.F Vetelino, “Acoustic Wave Biosensors”,

Proceedings IEEE Ultrasonics Symposium, pp 451-460, 1995

[15] B E Poling, J M Prausnitz and J P O’connell, “The properties of

gases and liquids”, McGRAW-HILL, DOI: 10.1036/0070116822

[16] J W Gardner, et al, “Microsensors MEMS and Smart Devices”,

307-325, John Wiley & sons, Inc New York, 2002

[17] J Semmlow, “Signals and systems for bioengineers”, Elsevier Inc

ISBN: 978-0-12-384982-3, http://www.elsevierdirect.com, 2 nd

Edition, 2011

[18] K Shin and J K Hammond, “Fundamentals of signal processing for

sound and vibration engineers”, John Wiley & Sons Ltd, ISBN-13

978-0470-51188-6, 2008

[19] COMSOL Multiphysics Reference Guide, Ver 3.5a, 2008

[20] COMSOL Multiphysics Matlab interface Guide, Ver 3.5a, 2008