Surface Acoustic Wave (SAW) sensors comprising a zinc oxide guiding layer deposited on a 36°-YX lithium tantalate substrate were developed. They were found to have greater mass sensitivity than other LiTaO3 based SAW sensors, such as the -SiO2/LiTaO3 configuration. In this paper, the fabrication of the ZnO/LiTaO3 sensor is described and micro-characterisation of the deposited films is presented. Sensitivity of these devices to surface mass and dielectric perturbations is then presented, followed by an analysis of temperature stability

Ippolito, S

Kalantar-Zadeh, K

Powell, D

Wlodarski, W

RMIT Research Repository

A Layered SAW Device Based on ZnO/LiTaO3 for Liquid Media Sensing 
Applications 
David A. Powell*, Kourosh Kalantar-zadeh, Samuel Ippolito and Wojtek Wlodarski 
Sensor Technology Group, School of Electrical and Computer Engineering 
 RMIT University, Melbourne, AUSTRALIA. 
CRC for Microtechnology, Melbourne, AUSTRALIA. 
 
Abstract – Surface Acoustic Wave (SAW) sensors 
comprising a zinc oxide guiding layer deposited on a 
36°-YX lithium tantalate substrate were developed.  
They were found to have greater mass sensitivity than 
other LiTaO3 based SAW sensors, such as the 
SiO2/LiTaO3 configuration. In this paper, the 
fabrication of the ZnO/LiTaO3 sensor is described 
and micro-characterisation of the deposited films is 
presented.  Sensitivity of these devices to surface 
mass and dielectric perturbations is then presented, 
followed by an analysis of temperature stability. 
I. INTRODUCTION 
90° rotated ST-quartz and 36°-YX LitaO3 substrates 
are the most commonly utilised piezoelectric 
materials which allow the propagation of shear 
waves. As a result, they are suitable for use in liquid 
sensing applications.  By depositing a wave guiding 
layer onto such substrates, mass sensitivity can be 
increased as the acoustic waves are more confined 
onto the surface. 
 
Layered SAW sensors for liquid media applications 
using SiO2 on 90° rotated ST-quartz have been 
intensively studied (eg. Kovacs et al [1] and Du et al 
[2]).  SiO2 on 36° YX-LiTaO3 layered structure is 
another SAW structure that offers potential for liquid 
sensing applications [3, 4].  However, such devices 
have relatively lower sensitivity than devices based 
on ST cut quartz.  
 
Adding a piezoelectric guiding layer generally 
increases the mass sensitivity.  It has been shown that 
replacing the SiO2 layer with ZnO, which is a 
piezoelectric material, results in considerable 
improvements in mass sensitivity [5]. In this paper, a 
ZnO/36° YX-LiTaO3 structure is introduced for 
sensing applications. 
Shoji et al. [6, 7] showed that deposition of ZnO layer 
on LiNbO3 or LiTaO3 results in an improved 
electromechanical coupling coefficient (K2), whilst 
Lim and Shindo [8] studied the defects in ZnO 
deposited on LiTaO3 by electron cyclotron resonance-
assisted molecular-beam epitaxy (ECR-assisted 
MBE).  In this paper, it will be shown that the 
deposition of ZnO on LiTaO3 can be used in the 
fabrication of a liquid media sensor with improved 
mass sensitivity. 
II. SENSOR FABRICATION 
The sensor consisted of a two-port resonator with 38 
input and output Inter Digital Transducer (IDT) finger 
pairs, 160 reflectors on each side, 700µm aperture 
width and a periodicity of 40µm.  A 2-port resonator 
structure was chosen over a delay line as its higher 
phase slope increases oscillation stability.  The IDTs 
and reflectors were formed by patterning a 300nm Au 
layer.  The Au layer was deposited upon 300nm 
Ni/300nm Ti for improved adhesion to the substrate.  
Devices were fabricated with ZnO layer thickness of 
1.5µm and 2.6µm, deposited by an RF magnetron 
sputterer.  Sputtering gas was 40% O2 in Ar at 1×10-2 
Torr, substrate temperature was 260°C, which 
resulted in a deposition rate of ~0.4µm/hour. For 
comparison, transducers with 3µm SiO2 were also 
fabricated. 
III. MATERIAL ANALYSIS 
ZnO films were micro-characterised using Scanning 
Electron Microscopy (SEM) and X-Ray Diffraction 
(XRD).  Figure 1 shows the SEM micrograph of the 
film at the edge of an IDT finger.  It can be seen that 
grain structure is quite different on the metallised and 
non-metallised regions. 
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 Figure 1: Micrograph ZnO on LiTaO3 (left) and Au (right) 
 
It has previously been reported that ZnO deposited by 
ECR-assisted MBE grows on LiTaO3 with c-axis 
alignment parallel to the surface [6].  To confirm that 
the same relative orientation of thin film and substrate 
had been achieved with deposition conditions 
described in section II, a sample with no metal layer 
was subjected to XRD analysis.  The results are 
presented in Figure 2.  The strong (110) peak 
indicates that the c-axis lies in the plane of the 
substrate.  A smaller (102) peak is also present, but is 
dominated by the (110) peak.  Diffraction angles for 
crystal orientations have been taken from [6], [10], 
[11]. 
 
10 20 30 40 50 60 70 80 90
2Θ
In
te
n
s
ity
(012) LiTaO3
(110) ZnO
(102) ZnO
 
Figure 2: XRD pattern of ZnO on LiTaO3 
 
The orientation of ZnO on Au under similar 
sputtering conditions has previously been reported by 
the authors [2] as showing a strong (002) peak, and a 
grain formation similar to that on the right side of 
Figure 1.  This is indicative of strong surface-normal 
c-axis alignment. 
IV. MASS SENSITIVITY ANALYSIS 
To compare the mass sensitivity of the devices with 
SiO2 and ZnO layers, a 0.4µm layer of the polymer 
APC ( Poly(Bisphenol A carbonate) ) was spin coated 
onto the surface of each SAW device.  The frequency 
shift was measured on a network analyser, and taken 
as the change at the point of maximum phase slope.   
From [12], the change in velocity due to mass loading 
is known to be 
 






−=
∆
2
44
2
2
4 V
c
P
Vhv
V
V ρ ,   (1) 
 
where V is the SAW phase velocity, h is the thickness 
of the perturbing layer, ρ and c44 are density and 
shear stiffness of the film respectively, P is the power 
flow per unit width, and v2 is the particle velocity of 
the horizontally polarised shear wave.  This equation 
was derived from perturbation theory, using the weak 
coupling assumption.  This is invalid for a strong 
piezoelectric material such as LiTaO3.  However, it is 
assumed here that the qualitative results are still valid. 
 
Table I: Frequency Response to Polymer Layer 
Device Frequency Shift 
1.5µm ZnO 250kHz 
2.6µm ZnO 500kHz 
3µm SiO2 300kHz 
 
Table I shows that a 2.6um ZnO device is 
approximately 1.6 times as sensitive as a 3µm SiO2 
device.  For comparison, the results of Hermann et al. 
[3] indicate a sensitivity of 0.5×10-8m2s/kg/Hz for a 
36° YX-LiTaO3 device with periodicity of 40µm and 
a 3µm SiO2 layer.  It was also noted that further 
sensitivity improvement could be gained by 
increasing the thickness of the SiO2 layer, however 
further research needs to be undertaken to determine 
if this is also the case for a ZnO layer. 
V. LIQUID SENSITIVITY ANALYSIS 
SAW devices show frequency shift when exposed to 
different liquids.  This is usually due to a combination 
of viscosity, conductivity and permittivity effects 
upon the wave propagation. 
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 To assess the response of the SAW devices in liquid 
media, they were placed within a 6µl flow cell.  To 
obtain maximum sensitivity, and to minimise the 
effects of variable gasket pressure or leakage, the 
flow cell covered the entire structure, including IDTs 
and reflectors.  This has the disadvantage that input 
and output impedance will be affected, which has the 
potential to prevent oscillation [13]. 
 
A pump with a flow rate of 250µL/min supplied DI-
water to the surface of the sensor.  The sample was 
injected into a 500µL sample loop and switched into 
the flow path.  Figure 3 shows the response of the 
three devices when exposed to methanol. 
 
0
50000
100000
150000
200000
250000
300000
0 40 80 120 160 200 240 t (s)d
f (
Hz
)
1.5um ZnO
3um SiO2
2.6um ZnO
 
Figure 3: Responses to Methanol 
The response is due to the lower dielectric constant 
and viscosity of methanol compared to water (see 
Table II).  The perturbation relations [12] for velocity 
shift due to the electrical and mechanical properties 
of a liquid are respectively 
 
2
0
2
00
22
)'()/'(
)')('()/'(
2 TPr
T
PrrrK
V
V
εεεωσ
εεεεεεωσ
++
+−+
=
∆
 (2) 
 








+=
∆
22
''
4
2
2 ll
P
Vv
V
V ωηρρωη
ω
,  (3) 
where σ is the conductivity, εr is the relative 
permittivity, η is the viscosity and ρl is the density of 
the liquid.  Quantities denoted by ′ indicate the 
perturbing liquid (methanol), while others indicate the 
water.  It is most likely that the dominant effect is the 
change in dielectric constant, since this sensitivity is 
proportional to K2.  It has been shown that K2 is 
higher for the 1.5µm ZnO device than the 2.6µm ZnO 
device [7]. 
Table II: Properties of Methanol and Water 
 εr η (mPa s) 
Water 80 1 
Methanol 33 0.58 
 
This shows that with an appropriate choice of ZnO 
layer thickness, it is possible to increase the mass 
sensitivity, whilst reducing cross-sensitivity due to 
dielectric constant and viscosity change.   
VI. TEMPERATURE COEFFICIENT 
For dispersive SAW devices, including any with 
layered structure or reflectors, the magnitude of the 
temperature coefficient of delay (TCD) and 
temperature coefficient of frequency (TCF) are not 
identical.  Instead, the relationship is [8] 
TCD
V
v
TCF g−= ,    (4) 
where vg is the group velocity of the acoustic wave. 
 
To measure TCF, the sensors were placed within an 
environmental chamber, with the remainder of the 
oscillator system being kept outside.  Figure 4 shows 
a typical plot of fractional frequency deviation versus 
temperature for a device with 2.6µm ZnO layer.  It 
can be seen that it is highly linear over a broad range 
of frequencies, and gives a TCF of –40ppm/K.  It was 
observed that a change in cable lengths between the 
SAW and amplifier, or an adjustment of the phase 
shift network, which alters the oscillation frequency, 
could have a strong impact upon the TCF.  Variation 
was observed over repeated measurement.  Since the 
SAW devices were not hermetically sealed, it is 
possible that humidity variation caused some 
additional error. 
 
Table III: TCF of SAW devices 
Device TCF 
(ppm/K) 
Variation 
(ppm/K) 
1.5µm ZnO -22 ±6 
2.6µm ZnO -40 ±4 
3µm SiO2 -25 ±4 
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 -1500 
-1000 
-500 
0 
500 
1000 
1500 
-20 0 20 40 60T(°C) 
∆f
(p
pm
) 
 
Figure 4: Frequency deviation vs.temperature for 2.6µm 
ZnO device. 
It can be seen that the TCF of the devices with ZnO 
layer is inferior to that of the SiO2 devices.  This is 
due to the opposite signs of the TCD of SiO2 and 36° 
YX-LiTaO3 having a cancelling effect. 
VII. CONCLUSION AND FUTURE  WORK 
Mass sensitivity of the 36° YX-LiTaO3 devices with 
1.5µm ZnO, 2.6µm ZnO and 3µm SiO2 layers were 
compared.  It was shown that mass sensitivity of the 
2.6µm ZnO device is 1.6 times that of the 3µm SiO2 
device, however it suffers from a higher temperature 
sensitivity.   The response to a change in liquid on the 
surface was measured, and it was found that the 
1.5µm ZnO and 3µm SiO2 devices had a similar 
response, which was about 1.3 times that of the 
2.4µm ZnO device. 
 
For liquid sensing application, it is proposed to use an 
SiO2 or polymer protective layer to ensure that ZnO 
does not react with any acidic materials within the 
liquid.  Work will be carried out by the authors to 
model the behaviour of sensors based on 
ZnO/LiTaO3, and to apply them to chemical and 
biochemical sensing applications.   
VIII. REFERENCES 
[1] G. Kovacs, et al., “Love Waves for 
(Bio)chemical Sensing in Liquids”, in Proc. 
IEEE Ultrason. Symp., 1992, pp. 281-285. 
[2] J. Du, G.L. Harding, A. F. Collings, P.R. 
Dencher, “An Experimental Study of Love-wave 
Acoustic Sensors Operating in Liquids”, Sens. 
Actuators A, vol. 60, pp. 54-61, 1997. 
[3] F. Herrmann, M. Weihnacht, “Properties of 
Sensors Based on Shear-Horizontal Surface 
Acoustic Waves in LiTaO3/SiO2 and Quartz/SiO2 
Structures”, IEEE Trans. on Ultrason., 
Ferroelec., and Freq. Contr., vol. 48, no. 1, pp. 
268-273, 2001. 
[4] J. Freudenberg, M. von Schickfus, S. Hunklinger, 
“A SAW Immunosensor for Operation in Liquid 
Using a SiO2 Protective Layer”, Sens. Actuators 
B, vol. 76, pp. 147-151, 2001. 
[5] K. Kalantar-zadeh, A. Trinchi, W. Wlodarski and 
A. Holland, “A Novel Love-mode Device Based 
on a ZnO/ST-cut Quartz Crystal Structure for 
Sensing Applications”, Sens. Actuators A, vol. 
100, pp. 135-143, 2002. 
[6] K. Nakamura, T. Shoji and H.B. Kang,  “Growth 
of ZnO Films on (012)LitaO3 by ECR-MBE and 
Determination of Their Polarity” in Proc. Int. 
Symp. App. Ferroelec., pp. 467-470, 2000. 
[7] T. Shoji, K. Nakamura and D. Yamazaki, 
“Propagation Characteristics of the SH-SAW on 
(110)ZnO/(012)LiTaO3” in Proc. IEEE Ultrason. 
Symp., 2001, pp. 215-219. 
[8] S.-H. Lim and A. Shindo, “Defects and 
Interfaces in an Epitaxial ZnO/LiTaO3 
Heterostructure”, Journ. of App. Phys., vol. 88, 
no 9, pp. 5107-5112, 2000. 
[9] B. Jakoby and M. J. Vellekoop, “Reducing the 
Temperature Sensitivity of Love-Wave Sensors” 
in Proc. IEEE Ultrason. Symp., 1998, pp. 447-
450. 
[10] Y. Shibata, N. Kuze and M. Matsui, 
“Piezoelectric LiNbO3 and LiTaO3 Films for 
SAW Device Applications” in Proc. IEEE 
Ultrason. Symp., 1996, pp. 247-254. 
[11] X. T. Zhang et al, “Temperature Dependence of 
Excitonic Luminescence from Nanocrystalline 
ZnO films”, Journal of Luminescence, vol. 99, 
pp. 149-154, 2002. 
[12] J. Kondoh, K. Saito, S. Shiokawa and H. Suzuki, 
“Simultaneous Measurements of Liquid 
Properties Using Multichannel Shear Horizontal 
Surface Acoustic Wave Microsensor”, Jpn. J. 
Appl. Phys., vol. 35, Part 1, No 5B, pp. 3093-
3096, 1996. 
[13] B. Jakoby and M. J. Vellekoop, “Analysis and 
Optimization of Love Wave Liquid Sensors”, 
IEEE Trans. on Ultrason., Ferroelec., and Freq. 
Contr., vol. 45, no. 5, pp. 1293-1302, 1998. 
                                                   
*
 Corresponding author: david.powell@ieee.org 
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A layered SAW device based on ZnO/LiTaO3 for liquid media sensing applications

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