XIIIèmes Journées Maghrébines des Sciences des Matériaux- JMSM’2020
Oran, Algérie, 09-11 Mars 2020
Laboratoire de Physique des Couches Minces et Matériaux pour l’Electronique Université Oran 1 Ahmed Ben Bella
Effect of the c-axis tilting angle in piezoelectric ZnO crystal on the performances of electroacoustic
SAW sensors
Farouk Laidoudi
1, Fayçal Medjili
1, Hassene Nezzari
1, Mouloud Mebarki
2, Fouad Boubenider
21Research Center in Industrial Technologies CRTI, P.O.Box64, Algiers, Algeria
2Laboratory of Physics of Materials, University of Sciences and Technology, (USTHB), Algiers, Algeria f.laidoudi@crti.dz
Abstract— This paper aims to study the effect of c-axis tilting angle of piezoelectric ZnO/Si on the performances of electroacoustic SAW sensors, the dispersion curves of phase velocity, the electromechanical coupling factor K² and sensitivity to mass loading of Rayleigh and Sezawa modes are studied for different hZnO/λ and for different c-tilting angles (0, θ, 90°). The effect of the tilting angle θ on the performances of electroacoustic devices, is studied by finite element analysis.
Based on the obtained results, SAW device is fabricated onto a ZnO/SiO2/Si multilayered structure. The obtained results show best performances and high sensitivity to gas and will contribute in enhancing the sensitivity and performances of SAW electroacoustic devices.
Keywords— Surface acoustic waves, Electroacoustic devices, Finite element analysis, Piezoelectric materials, c-tilted ZnO.
1. Introduction
Surface acoustic waves (SAWs) propagating along a piezoelectric substrate or multilayered structures have been widely studied for application in NDT, signal processing and sensing field.
Rayleigh waves are surface acoustic waves (SAWs) that travel along the surface of an isotropic half-space and are elliptically polarized in the sagittal plane. If the half-space is covered by a thin layer material whose SAW velocity is lower than that of the substrate (slow on fast configuration), several surface acoustic modes can propagate: when the acoustic wave is polarized in the in-plane of the structure, the first mode is called Rayleigh-like wave while the second is called Sezawa mode. The higher order modes are labelled with an increasing number.
The Sezawa mode, as well as the higher order modes, have a cut off frequency and are characterized by high velocity and high electromechanical coupling. These characteristics make the Sezawa wave devices suitable for the fabrication of high frequency and high sensitive electroacoustic devices.
In this paper, Rayleigh and Sezawa surface acoustic modes propagating along c-axis tilted (0°,θ°,90°) ZnO on Si (100) substrate are numerically investigated using finite element method FEM. Being θ the tilt angle in the direction parallel to the propagation direction, the phase velocity, the
electromechanical coupling factor for all modes, the reflectivity and sensitivity to mass loading for Rayleigh and Sezawa modes, are studied for different ZnO thickness-to- wavelength ratio hZnO/λ and c-axis tilt angle values. The application of Rayleigh and Sezawa modes in gas sensors is studied and good results are obtained. The last part of this paper is dedicated to experimental realization of the SAW device based on the theoretical calculations.
2. Dispersion curves
Figure. 1 shows the phase velocity dispersion curves dispersion curves, obtained by finite element analysis, it can be seen that all SAW modes are dispersive (phase velocity changes with frequency), the Rayleigh mode R0 velocity is limited by the shear bulk acoustic velocity in Si at hZnO/λ=0 and the shear bulk velocity in ZnO at high hZnO/λ values.
Sezawa mode has a cut of frequency (hZnO/λ=0.15 in our case) so that a threshold layer thickness is required for the propagation of this mode.
0.0 0.2 0.4 0.6 0.8 1.0
2500 3000 3500 4000 4500 5000 5500 6000
Layer shear bulk velocity
Phase velocity (m/s)
hZnO/l
Substrate shear bulk velocity
R0 R1
Fig. 1. Phase velocity dispersion curves for different h/λ values of Rayleigh R0, Sezawa R1 in ZnO/Si
Another important parameters of electroacoustic devices are the reflectivity at the interdigital level, and the electromechanical coupling factor K² : K2 = 2· [(vf – vm)/vf],
that express the amount of electrical energy converted in mechanical one after the excitation of the wave by IDT’s.
K² is related to the percentage difference of the velocity between a free surface (vf) and the surface coated with an infinitesimally thin perfect conductor (vm); it is used as direct estimation of the surface-wave coupling to interdigital transducers.
The SAW modes electromechanical coupling factors were calculated for two electroacoustic coupling configurations: the substrate/film/transducers (SFT) configuration, which refers to the position of the inter- digital transducers IDTs on top of the ZnO guiding layer, and the substrate/transducers/film (STF) configuration, that refers to the IDTs positioned at the substrate/guiding layer interface, the reflectivity and K² curves for Rayleigh and Sezawa modes are shown in Fig. 2.
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.5 1.0 1.5 2.0 2.5
Electromechanical coupling K²
hZnO/l
STF_R0 STF_Sezawa SFT_R0 SFT_Sezawa
a)
0.0 0.2 0.4 0.6 0.8 1.0
0.01 0.02 0.03 0.04 0.05 0.06
STF_R0 SFT_R0 STF_Sezawa SFT_Sezawa
Reflectivity
h/l
b)
Fig. 2. K² and reflectivity dispersion curves for different h/λ values of Rayleigh R0, Sezawa R1 in ZnO/Si
The K² vs hZnO/λ values curves show a maximum values of 2.5%
at hZnO/λ =0.5 for Sezawa mode, 0.35% at hZnO/λ=0.9 for Rayleigh mode and 0.25% at hZnO/λ=0.9 for Love mode under the SFT configuration. For the STF configuration, the Rayleigh mode K² increases and reaches a maximum of 1.98% at hZnO/λ=0.5, while the Sezawa mode shows a peak value of 0.82% at hZnO/λ=0.3.
In electroacoustic devices, such as sensors and resonators, it’s better to generate the mode at its minimum value of reflectivity.
Sezawa mode is characterized by high frequency, that makes it highly affected than Rayleigh mode, it can achieve a maximum reflectivity of 0.056 and 0.049 for STF and SFT configurations
respectively at hZnO/λ=0.4, then it starts to decrease at high hZnO/λ values. For Rayleigh mode, the maximum of reflectivity is 0.022 for STF configuration at hZnO/λ=0.5 and 0.017 for SFT configuration at hZnO/λ=0.1
In gas sensors, the sensitivity to mass laoding allows to verify the ability of the devices to detect small amounts of particles loaded on its free surface. In liquid sensors, this property will help to eliminate the effect of the mass of the fluid in order to measure its characteristics such as viscosity, conductivity. etc. When a thin layer of thickness ham and mass density ρam is anchored onto the SAW device surface (mass layer m=ρ·h), the wave velocity decreases due to the mass loading effect. The gravimetric sensitivity in air is calculated, as the frequency change per unit added mass, by the relation: Sm = (fam - ffree)/ρam ham. In which: ffree and fam are respectively the frequency of the acoustic mode under free or added mass boundary conditions, the added mass is a thin layer of gold (Au: density ρam=19300 kg/m3, Young modulus E=78 GPa and Poisson ratio ν=0.44) with a small thickness (20 nm) on the top surface of the guiding layer. Figure. 3 shows sensitivity to mass loading of R0 and R1 modes for different h/λZnO. Figure. 3 shows the sensitivity to mass of both Rayleigh and Sezawa modes.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -200
-180 -160 -140 -120 -100 -80 -60 -40 -20 0
Sensitivity (kg/m²)
hZnO/l
Sezawa R0
Fig. 3. Effect of hZnO/λ values on the sensitivity to mass loading of Rayleigh and Sezawa modes.
Rayleigh mode shows higher sensitivity to mass loading than Sezawa mode, and this is explained by the energy concentration of the former in vicinity of the mass layer, which is not the case for the latter. For different hZnO/λ values, Rayleigh mode shows a maximum sensitivity of -191.42 m²/kg at hZnO/λ=0.05 when it was -122.56 m²/kg at hZnO/λ=0.15 for Sezawa mode, this sensitivity decreases by increasing hZnO/λ till it reaches a constant values of about -49 m²/kg and -17 m²/kg for Rayleigh and Sezawa modes respectively.
3. Effect of c-axis tilting angle in ZnO on the performances of SAW devices
The effect of c-axis tilting angle (0°,θ°,90°) is studied after the rotation of the c-axis in the sagittal plane of the ZnO crystal. Its effect of the phase velocity, the electromechanical coupling factor K², the reflectivity and the sensitivity to mass loading fo Rayleigh and Sezawa acoustic modes is shown on Figure. 4.
0 10 20 30 40 50 60 70 80 90 3500
4000 4500 5000 5500
0 10 20 30 40 50 60 70 80 90
3360 3370 3380 3390 3400 3410 3420 3430 3440
R0 L1
Rotating angle (0°,q°,90°)
Phase velocity (m/s)
R0 R1 L1
a)
0 10 20 30 40 50 60 70 80 90
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
R1_STF R1_SFT R0_STF R0_SFT
K² (%)
Rotating angle (0°,q°,90°)
b)
0 10 20 30 40 50 60 70 80 90
0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050
0.055 STF_Sezawa
SFT_Sezawa SFT_R0 STF_R0
Reflectivity
Rotating angle (0°,q,90°)
c)
0 10 20 30 40 50 60 70 80 90
-120 -110 -100 -90 -80 -70 -60
Sensitivity (kg/m²)
Rotating angle (0°,q,90°) Sezawa
R0
d)
Fig. 4. Effect of c-axis tilting angle on SAW Device characteristics for Rayleigh and Sezawa modes
Figure. 4 shows that SAW characteristics are highly affected by the tilting angle, both Rayleigh and Sezawa modes are generated with high K² at θ = 30-40° for the STF configuration. At this angle of rotation, the two modes show minimum reflectivity, which makes possible to rotate the ZnO crystal by this angle to obtain high performances for electroacoustic devices.
4. Experimental verification:
Thin film piezoelectric c-axis tilted 30° ZnO was reactively sputtered on the Si film using a Zn target (99.999%) in Ar/O2 atmosphere at 200°C, rf-power 200 W, and pressure 3.5 mTorr.The ZnO Si exhibited a good adhesion strength as confirmed by SEM image on Fig. 5
Fig. 5. Cross section of the deposited ZnO c-axis tilted 30° on Si
The interdigital transducers IDT’s were patterned on the free ZnO surface using photolithography process, the scattering parameter S21 of the device was measured by a network analyzer of type (Agilent N5230A), available at IFN-CNR in Rome. The response of the fabricated device and the S21 curves of Rayleigh and Sezawa modes are shown in Fig. 6.
Fig. 6. Image of the network analyzer and the S21curves vs frequency of the Rayleigh and Sezawa modes
From Fig. 6, the Rayleigh and Sezawa modes are generated with high K² at the corresponding theoretical frequency. In order to prevent the transmission of acoustic wave in the Si substrate and the generation of undesirable HBAR modes, a SiO2 thin layer (1 µm) is added between the ZnO and the Si layers, the final devices is SAW sensor with enhanced performances.
5. Conclusion:
The effect of c-axis titling angle (0, θ°, 90°) of the ZnO piezoelectric crystal on the surface acoustic wave SAW parameters is studied, the phase velocity, electromechanical coupling factor K², reflectivity and sensitivity to gravimetric measurements for Rayleigh and Sezawa modes are studied for different hZnO/λ and different titling angles. The study shows the possibility to develop high sensitive gas sensors with enhanced performances based on ZnO c-tilted 30°. SAW electroacoustic device based on ZnO/SiO2/Si is fabricated and tested by network analyzer. The S21 parameters for both Rayleigh and Sezawa modes, show a good accordance with the theoretical calculations. The obtained results are interested in the fabrication of high performance SAW sensors based on c-axis tilted piezoelectric crystal.
Acknowledgment:
The experimental tests were done in IFN-CNR of Rome, authors are grateful to Dr C. Caliendo and M. Hamudullah for their help and support.
