Theoretical investigation of surface acoustic wave in the new, three-layered structure: ZnO/AlN/diamond

The new layered structure, ZnO/AlN/diamond, for surface acoustic wave (SAW) devices is investigated for gigahertz-band applications. This structure combines the advantages of both piezoelectric materials, with a high electromechanical coupling coefficient (K2) of ZnO and high acoustic velocity of AlN. Theoretical results show that Rayleigh mode SAWs with large phase velocities up to 12,200 m/s and large K2 from 1 to 3% were generated with this new structure.     

Growth of A1N piezoelectric film on diamond for high-frequency surface acoustic wave devices

Diamond films are very desirable for application to SAW devices because of their high acoustic wave velocity, which allows the extending of the frequency limit of operation at a given interdigital transducer line-width resolution. Use of high-quality AIN as the piezoelectric layer in conjunction with diamond is also desirable because of its high SAW velocity--the highest among all piezoelectric materials--together with its excellent electrical, mechanical, and chemical properties. The problems arising in the growth of A1N films on diamond have prevented, until now, the use of this combination of materials. In this paper we present recent results on the growth of highly oriented, low-stressed A1N films on diamond. SAW propagation on A1N/diamond has been theoretically investigated together with electromechanical coupling for both the Rayleigh and the Sezawa modes. The theoretical calculations show that high SAW velocities are achievable with good coupling efficiencies. Under proper conditions very large piezoelectric couplings are predicted--k2 = 2.2 and 4% for the Rayleigh and the Sezawa wave, respectively--comparable to those observed in strongly piezoelectric single crystals such as LiNbO3, but with SAW velocities approximately two-fold higher. Experiments performed on A1N/diamond/Si SAW test devices have shown good agreement between experimental results and theoretical predictions and demonstrate the feasibility of SAW devices based on this technology.     

Theoretical study on SAW characteristics of layered structuresincluding 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, LiNbO₃/diamond/Si and LiTaO₃/diamond/Si structures. It was also found that a SiO₂/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%     

Simulation of characteristics of a LiNbO3/diamond surface acoustic wave

High-frequency surface acoustic wave (SAW) devices based on diamond that have been produced to date utilize the SiO2/ZnO/diamond structure, which shows excellent characteristics of a phase velocity of over 10,000 m/s with a zero temperature coefficient; this structure has been successfully applied to high-frequency narrowband filters and resonators. To expand material systems to wideband applications, c-axis-oriented LiNbO3 on diamond was studied and a coupling coefficient up to 9.0% was estimated to be obtained. In this paper, the characteristics of LiNbO3/diamond with the assumption that the LiNbO3 film is a single crystal have been studied by theoretical calculations to find higher coupling coefficient conditions. Calculations are made for the phase velocity, the coupling coefficient, and the temperature coefficient of the Rayleigh wave and its higher mode Sezawa waves. As a result, LiNbO3/diamond is found to offer a very high electromechanical coupling coefficient of up to 16% in conjunction with a high phase velocity of 12,600 m/s and a small temperature coefficient of 25 ppm/deg. This characteristic is suitable for wide bandwidth applications in high-frequency SAW devices.     

Simulation of characteristics of a SiO2/c-axis-oriented LiNbO3/diamond surface acoustic wave

High-frequency surface acoustic wave (SAW) devices based on diamond that have been realized to date utilize c-axis-oriented ZnO as the piezoelectric thin film. This material, with SiO2 overlay, shows excellent characteristics of a high phase velocity of over 10,000 m/s and a zero temperature coefficient, and it has been successfully applied to high-frequency SAW filters and resonators. To expand on materials used on diamond, the theoretical calculation has been carried out for LiNbO3/diamond, and a high electromechanical coupling coefficient up to 9.0% is expected. In this work, the characteristics of SiO2/LiNbO3/diamond were studied by computer simulation, emphasizing a zero temperature coefficient with a high coupling coefficient. Calculations are carried out for the phase velocity, the electromechanical coupling coefficient, and the temperature coefficient of the Rayleigh wave and its higher mode Sezawa wave. As a result, SiO2/IDT/LiNbO3/diamond is found to offer a zero temperature coefficient with a very high coupling coefficient up to 10.1% in conjunction with a high phase velocity of 12,100 m/s.     

Surface acoustic wave properties of (100) AIN films on diamond with different IDT positions

(100)AlN films have better surface acoustic wave (SAW) properties than (002) AlN films. In this research, (100) AlN films were combined with diamonds as a new composite SAW substrate. The SAW properties of (100) AlN films on diamonds were analyzed with 4 composite structures: interdigital transducer (IDT)/(100)AlN/diamond, (100)AlN/IDT/diamond, IDT/(100)AlN/metal/diamond, and metal/IDT/(100) AlN/diamond, and they exhibited some excellent SAW properties. Our research results provide a predictable and theoretical basis for further application on high-velocity SAW devices.     

Platinum and palladium high-temperature transducers on langasite

There is a pressing need for the fabrication of surface acoustic wave (SAW) devices capable of operating in harsh environments, at elevated temperature and pressure, or under high-power conditions. These SAW devices operate as frequency-control elements, signal-processing filters, and pressure, temperature, and gas sensors. Applications include gas and oil wells, high-power duplexers in communication systems, and automobile and aerospace combustion engines. Under these high-temperature and power-operating conditions, which can reach several hundred degrees Centigrade, the typically fabricated aluminum (A1) thin film interdigital transducer (IDT) fails due to electro and stress migration. This work reports on high temperature SAW transducers that have been designed, fabricated, and tested on langasite (LGS) piezoelectric substrates. Platinum (Pt) and palladium (Pd) (melting points at 1769 degrees C and 1554.9 degrees C, respectively) have been used as thin metallic films for the SAW IDTs fabricated. Zirconium (Zr) was originally used as an adhesion layer on the fabricated SAW transducers to avoid migration into the Pt or Pd metallic films. The piezoelectric LGS crystal, used as the substrate upon which the SAW devices were fabricated, does not exhibit any phase transition up to its melting point at 1470 degrees C. A radio frequency (RF) test and characterization system capable of withstanding 1000 degrees C has been designed and constructed. The LGS SAW devices with Pt and Pd electrodes and the test system have been exposed to temperatures in the range of 250 degrees C to 750 degrees C over periods up to 6 weeks, with the SAW devices showing a reduced degradation better than 7 dB in the magnitude of transmission coefficient, /S21/, with respect to room temperature. These results qualify the Pt and Pd LGS SAW IDTs fabricated for the above listed modern applications in harsh environments.     

SAW COM-parameter extraction in AlN/diamond layered structures

Highly c-axis oriented aluminum nitride (AlN) thin piezoelectric films have been grown on polycrystalline diamond substrates by pulsed direct current (DC) magnetron reactive sputter-deposition. The films were deposited at a substrate temperature below 50/spl deg/C (room temperature) and had a typical full width half maximum (FWHM) value of the rocking curve of the AlN-002-peak of 2.1 degrees. A variety of one-port surface acoustic wave (SAW) resonators have been designed and fabricated on top of the AlN films. The measurements indicate that various SAW modes are excited. The SAW phase velocities of up to 11.800 m/s have been measured. These results are in agreement with calculated dispersion curves of the AlN/diamond structure. Finally, the coupling of modes parameters have been extracted from S/sub 11/ measurements using curve fitting for the first SAW mode, which indicate an effective coupling K/sup 2/ of 0.91% and a Q factor of about 600 at a frequency of 1050 MHz.     

Analysis of SAW properties of epitaxial ZnO films grown on R-Al2O3 substrates

ZnO thin films with a high piezoelectric coupling coefficient are widely used for high frequency and low loss surface acoustic wave (SAW) devices when the film is deposited on top of a high acoustic velocity substrate, such as diamond or sapphire. The performance of these devices is critically dependent on the quality of the ZnO films as well as of the interface between ZnO and the substrate. In this paper, we report the studies on piezoelectric properties of epitaxial (112¯0) ZnO thin films grown on R-plane sapphire substrates using metal organic chemical vapor deposition (MOCVD) technique. The c-axis of the ZnO film is in-plane. The ZnO/R-Al₂O₃ interface is atomically sharp. SAW delay lines, aligned parallel to the c-axis, were used to characterize the surface wave velocity, coupling coefficient, and temperature coefficient of frequency as functions of film thickness to wavelength ratio (h/λ). The acoustic wave properties of the material system were calculated using Adler's matrix method, and the devices were simulated using the quasi-static approximation based on Green's function analysis     

Analysis of SAW properties in ZnO/AlxGa1-xN/c-Al2O3 structures.

Piezoelectric thin films on high acoustic velocity nonpiezoelectric substrates, such as ZnO, AlN, or GaN deposited on diamond or sapphire substrates, are attractive for high frequency and low-loss surface acoustic wave devices. In this work, ZnO films are deposited on AlxGa1-xN/c-Al2O3 (0 < or = chi < or = 1) substrates using the radio frequency (RF) sputtering technique. In comparison with a single AlxGa1-xN layer deposited on c-Al2O3 with the same total film thickness, a ZnO/AlxGa1-xN/c-Al2O3 multilayer structure provides several advantages, including higher order wave modes with higher velocity and larger electromechanical coupling coefficient (K2). The surface acoustic wave (SAW) velocities and coupling coefficients of the ZnO/AlxGa1-xN/c-Al2O3 structure are tailored as a function of the Al mole percentage in AlxGa1-xN films, and as a function of the ZnO (h1) to AlxGa1-xN (h2) thickness ratio. It is found that a wide thickness-frequency product (hf) region in which coupling is close to its maximum value, K(2)max, can be obtained. The K(2)max of the second order wave mode (h1 = h2) is estimated to be 4.3% for ZnO/GaN/c-Al2O3, and 3.8% for ZnO/AlN/c-Al2O3. The bandwidth of second and third order wave modes, in which the coupling coefficient is within +/- 0.3% of K(2)max, is calculated to be 820 hf for ZnO/GaN/c-Al2O3, and 3620 hf for ZnO/AlN/c-Al2O3. Thus, the hf region in which the coupling coefficient is close to the maximum value broadens with increasing Al content, while K(2)max decreases slightly. When the thickness ratio of AlN to ZnO increases, the K(2)max and hf bandwidth of the second and third higher wave modes increases. The SAW test devices are fabricated and tested. The theoretical and experimental results of velocity dispersion in the ZnO/AlxGa1-xN/c-Al2O3 structures are found to be well matched.     

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 10 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.     

Surface acoustic wave excitation on SF6 plasma-treated AlGaN/GaN heterostructure

In this work, we introduce a new modified approach to the formation of interdigital transducer (IDT) structures on an AlGaN/GaN heterostructure. The approach is based on a shallow recess-gate plasma etching of the AlGaN barrier layer in combination with “in-situ” SF₆ surface plasma treatment applied selectively under the Schottky gate fingers of IDTs. It enables one to modify the two-dimensional electron gas (2DEG) density and the surface field distribution in the region of the IDTs, as is needed for the excitation of a surface acoustic wave (SAW). The measured transfer characteristics of the plasma-treated SAW structures revealed the excitation of SAW at zero bias voltage due to fully depleted 2DEG in the region of the IDTs. High external bias voltages are not necessary for SAW excitation. SIMS depth distribution profiles of F atoms were measured to discuss the impact of SF₆ plasma treatment on the performance of the AlGaN/GaN-based IDTs.     

The effects of ZnO films on surface acoustic wave properties of modified lead titanate ceramic substrates

Poly-crystal zinc oxide (ZnO) films with c-axis (002) orientation have been successfully grown on the strontium (Sr) modified lead titanate ceramic substrates with different Sr dopants by r.f. magnetron sputtering technique. Highly oriented ZnO films with c-axis normal to the substrates can be obtained under a total pressure of 10 mTorr containing 50% argon and 50% oxygen and r.f. power of 70 W for 3 hours. Crystalline structures of the films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency of surface acoustic wave (SAW) devices with ZnO/IDT/PT (IDT, inter-digital transducer; PT, PbTiO3 ceramics) structure were investigated. The devices with ZnO/IDT/PT structure shows that the ZnO film effectively raise the electromechanical coupling coefficient (kappa2) from 3.8% to 9.9% of the device with the concentrations of Sr dopants of 0.15. It also improves the temperature coefficient of frequency of SAW devices.     

Detection and high-precision positioning of liquid droplets using SAW systems

The capability to accurately handle liquids in small volumes is a key point for the development of lab-on-chip devices. In this paper, we investigate an application of surface acoustic waves (SAW) for positioning micro-droplets. A SAW device based on a 2 x 2 matrix of inter-digital transducers (IDTs) has been fabricated on a (YXI)/128 degrees LiNbO3 substrate, which implies displacement and detection in two dimensions of droplets atop a flat surface. Each IDT operates at a given frequency, allowing for an easy addressing of the active channel. Furthermore, very low cross-talk effects were observed as no frequency mixing arose in our device. Continuous as well as pulsed excitations of the IDTs have been studied, yielding, respectively, continuous and step-by-step droplet displacement modes. In addition, we also have used these two excitation types to control the velocity and the position of the droplets. We also have developed a theoretical analysis of the detection mode, which has been validated by experimental assessment.     

Analysis of general multi-channel planar waveguides

Using the planar waveguide concept in surface acoustic wave (SAW) technology is often advantageous when the modeling of transversely distributed phenomena is indispensable for an accurate design of SAW devices. This is especially true when complex multi-track structures such as transversely coupled resonator filters (TCRFs) are under consideration where, e.g., transverse velocity and stiffness profiles have to be incorporated in the device simulation. The interdigital transducers (IDTs) and the reflector gratings composing those devices behave as planar waveguides, supporting, in principle, all kinds of modes such as bound, semi-bound, and radiation modes. Therefore, to model these SAW propagation effects, we subdivide the SAW structures in transverse direction into several parallel waveguiding channels (N regions), and take, as the wave-describing quantity, a two-dimensional scalar potential function. By doing so, we obtain a complete set of orthonormal modes into which an arbitrary transverse excitation function can be expanded to study its propagation. The general mode spectrum includes a discrete spectrum of bound modes and continuous spectra of semi-bound and radiation modes. We calculate all types of modes by making use of the stack matrix technique. The present work, which arose from the requirement of creating an efficient mathematical tool for the simulation of TCRFs, provides the complete analysis of general SAW multi-channel structures.     

Precise measurement of SAW velocity using SAW delay line

A method of nondestructive evaluation (NDE) by measuring the acoustic properties of materials using a surface-acoustic-wave (SAW) delay line is presented. A SAW delay line with three interdigital transducers (IDTs) deposited on a piezoelectric substrate is used to measure the SAW velocity of the sample material, using a fluid couplant. The SAW velocity is obtained from the frequency dependence of the delay line, and movement in the z-direction is not required. Measurements have been made for an anisotropic material at frequencies from 35 to 55 MHz. The experimental results agree well with the theoretical results. Moreover, it has been found that the focused SAW excited from a Fresnel-phase-plate IDT is suitable for mapping the two-dimensional variation of SAW velocity on an anisotropic sample surface.     

Surface acoustic wave properties of freestanding diamond films

"Ideal" diamond has the highest acoustic velocity of any material known, and is of great interest as a substrate material for high frequency surface acoustic wave (SAW) device structures. However, little is known of the acoustic wave propagation properties of polycrystalline diamond grown by chemical vapour deposition (CVD) techniques, the commercially accessible form of this material. We report on propagation of laser-generated SAW on three forms of freestanding CVD diamond samples, "white" polycrystalline, "black" polycrystalline, and "highly oriented" diamond. Despite differing sample nature, SAW waves propagating along the smooth (nucleation) side of the diamond showed similar velocities in the range 10600-11900 ms(-1). These results are discussed in terms of the potential of each form of CVD diamond for SAW device fabrication.     

High velocity SAW using aluminum nitride film on unpolished nucleation side of free-standing CVD diamond

High performances surface acoustic wave (SAW) filters based on aluminium nitride (AlN)/diamond layered structure have been fabricated. The C-axis oriented aluminum nitride films with various thicknesses were sputtered on unpolished nucleation side of free-standing polycrystalline chemical vapor deposition (CVD) diamond obtained by silicon substrate etching. Experimental results show that high order modes as well as Rayleigh waves are excited. Experimental results are in good agreement with the theoretical dispersion curves determined by software simulation with Green's function formalism. We demonstrate that high phase velocity first mode wave (so-called Sezawa wave) with high electromechanical coupling coefficient are obtained on AlN/diamond structure. This structure also has a low temperature coefficient of frequency (TCF), and preliminary results suggest that a zero TCF could be expected.     

金刚石声表面波器件的研究与进展

高频无线通讯系统的迅速发展推动了对高频声表面波 (SAW)器件需求的不断增大。金刚石具有最高的声速和许多优于其他材料的特性 ,因此金刚石声表面波器件受到了越来越多的关注。本文介绍了金刚石声表面波器件的构成、制备方法 ,讨论了金刚石声表面波器件近年来国际上的研究进展 ,包括理论研究与实验进展的概况 ,并讨论了金刚石声表面波器件的未来发展趋势     

Synthesis of slanted and quasi-slanted SAW transducers

This paper is devoted to the synthesis of interdigital transducers (IDTs) with variable electrode periodicity in the direction orthogonal to the SAW propagation, including straight electrodes (slanted or fan-shaped IDTs) and stepped electrodes (quasi-slanted IDTs). A polarity weighting of electrodes is the only tool for realization of high-frequency selectivity in fan-shaped (slanted) transducers. Quasi-slanted transducers (QST) offer additional opportunities: optimization of apertures, periods, and relative shifts of acoustic channels. Moreover, a discrete apodization gives an extra flexibility for QSTs in realization of desired frequency responses without increasing SAW diffraction sensitivity. Original synthesis algorithms have been developed for optimization of slanted and quasi-slanted IDTs in SAW filters with a wide range of characteristics.