Surface Acoustic Wave Characteristics Research Articles

A New Nondestructive Evaluation Method for Surface Acoustic Wave Velocity

A new nondestructive evaluation method for surface acoustic wave (SAW) velocity on a piezoelectric wafer was proposed. Split electrodes were formed on a glass wafer and a nonconductive liquid was interposed between the piezoelectric wafer and the glass wafer. The mode of the SAW excited on the piezoelectric wafer surface was leaky (LSAW) and its velocity was evaluated using the electrical reflection characteristics of the split electrodes. The usefulness of the new method was experimentally proven by showing the high reproducibility of the measured velocity and the agreement between the measured and calculated values indicating the SAW characteristics on a rotated-Y langasite wafer.

SAW/LSAW COM parameter extraction from computer experiments with harmonic admittance of a periodic array of electrodes

A novel numerical method for determining the surface acoustic wave and the leaky surface acoustic wave characteristics is proposed. The Green's function method is used to simulate an infinite periodic transducer driven by a periodic voltage. We show that the coupling of modes parameters and the dispersion relation can be extracted from the change in the admittance as the periodicity of the driving voltage is slightly shifted. The method first introduced here leads to significant savings in computing time.

Fabrication of oriented silicon-based polymer thin film and its application to surface acoustic wave sensor

Polysilanes are silicon-based polymers, which consist of silicon backbone and organic substituents. Among a kind of polysilanes, poly(di-methyl silane) [Si(CH3)2]n has the most fundamental structure, and is now commercially available. In this paper, poly(di-methyl silane) thin films are fabricated by means of vacuum evaporation technique. Orientation of the silicon backbone is well controlled by changing growth rate or substrate temperature during the evaporation. Using these polymer films and LiNbO3 substrates, layered surface acoustic wave (SAW) devices are fabricated, and the characteristics of SAW's propagating along the layered structures are numerically calculated. As a result, it is found that the SAW with large shear horizontal (SH) component is available, and these layered structures are suitable for liquid sensor, which detects the change in the physical constants of liquid loaded on the device surface.

Characteristics of Rayleigh and Leaky Surface Acoustic WavePropagating on a La3Ga5SiO14 Substrate

The propagation characteristics of Rayleigh waves on a La3Ga5SiO14 substrate are theoretically investigated for all cuts and propagation directions. The results of the phase velocity, electro-mechanical coupling constant (K2), power flow angle (PFA) and temperature coefficient of delay (TCD) are shown as contour maps. The maximum value of K2 of a Rayleigh wave is 0.55%, and about 0.5% with zero TCD. These values are about 2–3 times larger than those of quartz. It is found that some cuts exist, which do not have a Rayleigh wave. The characteristics of leaky surface acoustic wave (SAW) on Z-cut La3Ga5SiO14 are also shown.

Study on Surface Acoustic Wave Characteristics of SiO2/Interdigital-Transducer/ZnO/Diamond Structure and Fabrication of 2.5 GHz Narrow Band Filter

Surface acoustic wave (SAW) characteristics of the SiO2/interdigital-transducer(IDT)/ZnO/diamond structure were studied theoretically and experimentally. It was found that the 2nd mode in this structure can provide a zero temperature coefficient of frequency (TCF) as well as high velocity of 10,000 m/s and a large electromechanical coupling coefficient of 1.4%. Utilizing this wave, a 2.5 GHz narrow-band SAW filter was successfully fabricated with IDTs of 0.9 µm finger width, which resulted in superior characteristics for a retiming filter with a small insertion loss of 6.7 dB, Q value of 660 and small frequency deviation of 100 ppm in the temperature range from -40 to 85°C. The insertion loss is smaller than any other retiming filters which have ever been reported with quartz, a conventional SAW material, and also the frequency deviation with temperature is smaller than that in the case of ST-cut quartz.

Theoretical Study on Surface Acoustic Wave Characteristics of LiNbO 3 Films on Sapphire Substrates

Mathematical analyses of the surface acoustic wave (SAW) properties of LiNbO 3 thin films on sapphire substrates (LiNbO 3 /α-Al 2 O 3 structures ) were performed by changing the propagation mode, thickness of the films and propagation direction on the film surface. The calculated values of maximum velocity and electromechanical coupling coefficients obtained are 1.5 and 4 times higher respectively than those of LiNbO 3 bulk crystals, and the temperature coefficients of the structures decrease by about 10 ppm/K of the value of the bulk crystals. Consequently, LiNbO 3 films on sapphire substrates may have enhanced efficiency, thermal stability and operational frequency, making them very suitable for high performance SAW devices.

Propagation Characteristics of Leaky Surface Acoustic Wave on Proton-Exchanged 36° Y-X LiTaO 3

The propagation characteristics of leaky surface acoustic waves (LSAWs) on proton-exchanged 36°-rotated Y-cut LiTaO3 were investigated experimentally. The dependence of the insertion loss between transducer pairs on the proton-exchange depth and the number of finger pairs of the transducer were measured at a frequency of 210 MHz. The insertion loss of the LSAW for the free surface case was improved considerably by the proton exchange because bulk wave radiation from the transducer was suppressed. The change of the amplitude of the mechanical displacement perpendicular to the surface was also measured using a laser probe method.

Surface Acoustic Wave Characteristics of a ZnO/Quartz Substrate Structure Having a Large Electromechanical Coupling Factor and a Small Temperature Coefficient

A substrate having a suitable electromechanical coupling factor (k s) value and a good temperature coefficient of frequency (TCF) is required for an applications of resonator surface acoustic wave (SAW) filters and rudder-type SAW filters requiring a specific bandwidth, because their bandwidth largely depends on the coupling factor of the substrate. Although a quartz substrate for SAW devices provides a good TCF, the electromechanical coupling factor obtained is small. The author speculated that an excellent TCF and an appropriate electromechanical coupling factor could be realized by combination of a quartz substrate having a positive TCF and a ZnO film having a negative TCF. As a result of theoretical and experimental investigations performed using a 29°45' rotated Y and a 42°45' rotated Y 35°X propagating quartz substrate, an excellent TCF (≈0 ppm/°C) and an appropriate coupling factor (k s=0.1–0.11; k s2=0.01–0.0121) were realized with a zero degree power flow angle on the structure of IDT/ZnO/quartz or IDT/ZnO/shorted-plane/quartz for the first time.

Characteristics of hydrogenated aluminum nitride films prepared by radio frequency reactive sputtering and their application to surface acoustic wave devices

Hydrogenated aluminum nitride (AlN:H) films have been deposited on silicon wafers by the rf reactive magnetron sputtering method with H2 gas addition to Ar-N2 gas mixture. The changes of physical properties of AlN:H films with different H2 gas addition have been examined by x-ray diffraction, scanning electron microscopy, and x-ray photoelectron spectroscopy. The surface acoustic wave (SAW) filters with wavelength of 60 μm using these films have been fabricated and their SAW velocities and electromechanical coupling coefficients have also been measured by a network analyzer. It is found that as the amount of H2 addition increases, the surface of film becomes smooth, the stress is relieved, the concentration of nitrogen atom increases and that of oxygen atom decreases. We also found that AlN/AlN:H/AlN tri-layered films can prevent the blistering phenomena of AlN:H film. By measuring SAW characteristics of AlN/AlN:H/AlN tri-layered films, the SAW velocity is calculated to be 5280 m/s and electromechanical coupling coefficient is 1.35% which is the highest value reported so far.

Propagation characteristics of a leaky surface acoustic wave on proton-exchanged 36 ° Y–X LiTaO3

The propagation characteristics of a leaky surface acoustic wave on proton-exchanged 36 ° Y−X LiTaO3 were experimentally investigated to evaluate the propagation loss. To measure the insertion loss, a pair of interdigital transducers was fabricated on a 36 ° Y−X LiTaO3 substrate with the proton-exchanged layer, which was formed by immersion in phosphoric acid at 240 °C. The period of a pair of fingers (λ) was 20 μm. Each transducer was composed of eight finger pairs with an overlap length of 100λ. The distance of the free surface between two transducers was 250λ. The measured insertion loss at the center frequency decreased from 24.4 dB for the unexchanged sample to 16.8 dB for the 4-h proton-exchanged one, though the reflection coefficient of the transducer increased from 0.27 to 0.36. Therefore, the above result means that the propagation signal is improved at least 0.03 dB/λ by the proton-exchange process. This phenomenon is probably due to the field confinement in the vicinity of the surface, which is caused by the formation of the proton-exchanged layer with a slow phase velocity as compared with an unexchanged substrate.

Propagation Characteristics of Leaky Surface Acoustic Waves and Surface Acoustic Waves on LiNbO 3 Substrates with a Ferrroelectric Inversion Layer

The propagation characteristics of the leaky surface acoustic wave (LSAW) propagating on rotated Y-X lithium niobate (LiNbO3) substrates with a ferroelectric inversion layer are theoretically analyzed. The results show that the attenuation of the LSAW on 41° Y-X LiNbO3 is smaller than 10-5 dB/wavelength over a range of kh (k: wave number, h: inversion-layer thickness) from 2 to 2.5 for both free- and metallized-surface cases, and that the phase velocity of the LSAW is decreased and the concentration of the displacement to the surface is improved by the electric-field short-circuiting effect of the domain boundary. The propagation characteristics of the surface acoustic wave on 128° Y-X LiNbO3 substrates with an inversion layer are also reported.

Surface Acoustic Wave Characteristics on ZnO/\\mbYZ·LiNbO 3 Structure and Their Application to Elastic Convolver

In an effort to improve the efficiency of an elastic convolver, we discussed ways to increase the electromechanical coupling factor of the LiNbO3 (LN) substrate. We examined, both theoretically and experimentally, the surface acoustic wave (SAW) characteristics of ZnO films formed on the +Y· and -Y·planes of the YZ·LN substrates. As a result, it was clear that a larger electromechanical coupling factor, k s, can be obtained with a ZnO film on the +Y·plane than on the -Y·plane of YZ·LN substrates. The maximum value of the coupling factor obtained was 0.32, which was 2.04 times the square (k s2) of the value obtained without the ZnO film. By applying the LN substrate with the ZnO film on its +Y·plane to the elastic convolver, we obtained a processing time of 7.6 µs, a 3 dB bandwidth of 44 MHz and an efficiency of -58 dBm.

Characteristics of Surface Acoustic Wave on AlN Thin Films

A surface acoustic wave (SAW) on a (00·1) plane of hexagonal aluminum nitride (AlN) deposited on R-plane sapphire ( (1\\bar1·2)Al2O3) with an inclination of about 26 degrees to the substrate was analyzed. A Rayleigh wave and shear-horizontal-type (SH-type) wave were found. The phase velocity of these was 5400-5800 m/s and 6100-6600 m/s, respectively. To confirm this results, the propagating characteristics of both SAW modes were measured by input admittance of single interdigital transducers (IDT) and by transmission characteristics between two IDTs. The phase velocity of the Rayleigh wave and the SH-type wave was 5600 m/s and 6500 m/s, respectively, and the propagation loss was 0 dB/λ and 0.04 dB/λ, respectively. These measured characteristics approximately agree with the results of the simulation.

Theoretical study on SAW characteristics of layered structures including a diamond layer

Diamond has the highest surface acoustic wave (SAW) velocity among all materials and thus can provide much advantage for fabrication of high frequency SAW devices when it is combined with a piezoelectric thin film. Basic SAW properties of layered structures consisting of a piezoelectric material layer, a diamond layer and a substrate were examined by theoretical calculation. Rayleigh mode SAW's with large SAW velocities up to 12,000 m/s and large electro-mechanical coupling coefficients from 1 to 11% were found to propagate in ZnO/diamond/Si, LiNbO/sub 3//diamond/Si and LiTaO/sub 3//diamond/Si structures. It was also found that a SiO/sub 2//ZnO/diamond/Si structure can realize a zero temperature coefficient of frequency with a high phase velocity of 8,000-9,000 m/s and a large electro-mechanical coupling coefficient of up to 4%.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Characteristics of interdigital transducers for mechanical sensing and non-destructive testing

An interdigital transducer (IDT) is one of the most essential components in surface acoustic wave (SAW) technology. The performance of the IDT is related to the propagation characteristics of SAWs on piezoelectric or non-piezoelectric substrates. In typical measurements of strain and acceleration, the construction of feedback loop delay-line oscillators is discussed for the use of SAWs on both a piezoelectric ceramic substrate and a layer structure consisting of a thin piezoelectric plate and fused quartz. The IDT can also operate as a transducer for radiating a longitudinal wave into water. The transducer performance for a solid—liquid boundary is discussed in terms of the real and imaginary velocities of leaky SAWs, the electromechanical coupling constant and the transducer efficiency for acoustic beam radiation into water. Practical C-mode acoustic imaging results are given for two types of transducer configurations.

SAW gas sensor with proper tetrasulphonated phthalocyanine film

A surface acoustic wave (SAW) piezoelectric device with a dual-path SAW delay line oscillator configuration has been developed to detect nitrogen dioxide (NO 2) gas. One delay line is coated with a Langmuir-Blodgett (LB) film of copper tetrasulphonated phthalocyanine (CuTsPc), while the other is uncoated. NO 2 gas can be selectively abosorbed by the CuTsPc LB film and then alters the SAW characteristics, which in turn causes the oscillation frequency to change. The concentration of NO 2 can be detected by measuring the relative change in the frequency of the two oscillators. The CuTsPc LB film-coated SAW device has been demonstrated to be sensitive to low concentrations of NO 2 gas. There is a linear relationship between NO 2 concentration and frequency change under 12 ppm NO 2 concentration. The sensitivity of the device is about 128 Hz/ppm NO 2. The response time is 10 min, and the recovery is 40 min. It can be applied as a small, inexpensive and sensitive NO 2 gas sensor.

Preparation of aluminum nitride thin films by reactive sputtering and their applications to GHz-band surface acoustic wave devices

Single crystal aluminum nitride (AlN) thin films were prepared by a low-temperature reactive sputtering on basal plane sapphire [(001)Al2O3] at a substrate temperature of less than 315 °C. Surface acoustic wave (SAW) characteristics with an interdigital transducer /(001)AlN/(001)Al2O3 structure were investigated. The phase velocity and temperature coefficient of delay time are 5750–5765 m/s and 55–63 ppm/°C at KH=1.2–1.6, respectively. Resonator-type 1-GHz-band SAW filters with its structure were fabricated. The insertion loss and suppression were 23 dB and more than 20 dB, respectively.

Gigahertz range resonant devices for oscillator applications using shear horizontal acoustic waves

It is shown that surface transverse wave (STW) resonant devices are not only very well suited for stable oscillator applications but have some unique features offering greater design flexibility than their surface acoustic wave (SAW) counterparts. Various designs for single- and multimode resonators and resonator filters are presented, and their properties in respect to applications in stable fundamental-mode fixed-frequency and voltage-controlled oscillators in the range of 750 MHz to 2 GHz are discussed. Characteristics of SAW and STW two-port metal strip resonators using identical designs are compared. Data from frequency trimming on STW resonators, using heavy ion bombardment, are presented.

Piezoelectric Characteristics of ZnO Films Deposited Using an Electron Cyclotron Resonance Sputtering System

Piezoelectric properties of ZnO films were investigated for the first time by means of an electron cyclotron resonance (ECR) sputtering system. It is confirmed that this system is capable of depositing a ZnO film with a large specific resistance, a good piezoelectric property and good c-axis orientation on an interdigital transducer (IDT)/glass substrate at a low temperature (less than 200°C) and under low gas pressure (∼10-4 Torr). These ZnO films exhibited excellent surface acoustic wave (SAW) characteristics (insertion losses) and effective electromechanical coupling factors (keff) compared with ZnO films deposited by a conventional RF magnetron sputtering system. Furthermore this ECR sputtering system was capable of depositing a ZnO film capable of driving a Rayleigh SAW of 700 MHz, without heating the substrate (room temperature), while it was impossible to drive such a wave conventionally, using other sputtering systems.

Strain effect in single-crystal silicon-based multilayer surface acoustic wave structures

A method for calculating the characteristics of surface acoustic wave (SAW) propagation in a deformable piezoelectric multilayer medium is presented. The effect of longitudinal and lateral mechanical strain on the SAW phase velocity in a (ZnO or AIN)/SiO2/Si thin film structure for {001}, {111} and {110} silicon crystal planes within the temperature range 293–673 K is studied. The effects of thickness and internal mechanical stresses in the ZnO or A1N piezoelectric film and SiO2 dielectric film on the sensitivity of the SAW phase velocity to strains in the structure are investigated. The Si{110}-based SAW structure with the SAW wavevector oriented in the 〈110〉 direction is shown to possess maximum operating frequency sensitivity to both longitudinal and lateral strain. The parameters of SAW structure stable to mechanical loads are determined. ZnO and SiO2 layer deposition on silicon is shown to increase the SAW phase velocity sensitivity to longitudinal strain and to decrease its sensitivity to lateral strain in the structure.