The 2012 IEEE/ASME International Conference on Advanced Intelligent Mechatronics July 11-14, 2012, Kaohsiung, Taiwan 3-D Finite Element Modeling of SAW sensing system for liquids Thu-Hang Bui, Dat Nguyen Tien, Tung Bui Duc, and Trinh Chu Duc 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 I 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 3754 9338; fax: +84 7547460; e-mail: trinhcd@ vnu.edu.vn) 978-1-4673-2576-9/12/$31.00 ©2012 IEEE 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 (a) (b) Figure (a) The structure of a SAW sensing system for liquid (b) Ultrasonic radiation into water by SAW 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 782 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, the fluid, k = / is the wave number, is the potential of is the velocity of is the decay constant of the wave the wave, in the X2 direction and is the relation coefficient f is the fluid density and cf 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 shows the top view and crosssection 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, K2, 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 Sample liquids are poured onto well Figure Schematic illustration of two-channel R-SAW sensor and liquid well position 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 are as follows in Figure 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 783 Figure Design parameters of Channel and well size 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 including deionized water, water/glycerol mixtures (W/G), and Group consisting of propylene, bromine and mercury which physical properties at 20 0C are given in Table [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 (a) TABLE I PHYSICAL PROPERTIES OF LIQUID Liquid Water W/G mix W/G mix Propylene Bromine Mercury Viscosity  (mm2/s) 1.00 5.6 11.50 0.09 0.31 1.55 Density  (g/cm3) 1.00 1.126 1.161 0.514 3.123 13.579 After building this first model, a second model of the Channel 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 (b) Figure (a) Point positions and (b) Total displacement of point groups before and after well of the 3D SAW model IV RESULTS AND DISCUSSION A Corresponding point comparison in two channels Figure Total displacement of corresponding points in two channels Figure Total displacement of corresponding points in two channels Figure Total displacement of corresponding points in Channel and Channel with doubled well diameter 784 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 The fluid well which was deposited in the delay path of the device resulted in a perturbation Figure 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 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 A solid line shows displacement of the point behind the well of Channel and a dashed line illustrates that of the corresponding point of Channel 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 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 (a) (b) B Sensing different liquid in well (c) Figure Output voltage of Group from the 3-D SAW model with and without deposited well from to 130 nsec (a) Water well, (b) W/G mix well and (c) W/G mix well Figure Total displacement of the well behind points with three liquid types The total displacements of points placed in the liquid– solid boundary in all three cases of two groups in Figure were plotted The solid line illustrates that of water, the dashed line is for that of water/glycerol mixtures in the Group and the dash-dot line is for that of bromine in the Group 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 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 autocovariance 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 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 and W/G mix 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 785 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 (a) Material constants for Lithium Niobate piezoelectric class 3m symmetry in simulation: (b) 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 (c) Figure 10 Output voltage of Group from the 3-D SAW model with and without deposited well from 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 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] [2] [3] [4] V CONCLUSION This paper reports a 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