Natalya Naumenko

The experimental and theoretical characterization of the SAW propagation properties for zinc oxide films on silicon carbide

The surface acoustic wave (SAW) propagation properties of zinc oxide (ZnO) films on silicon carbide (SiC) have been theoretically and experimentally characterized in the film thickness-to-acoustic wavelength ratio range up to 0.12. The experimental characterization of the SAW propagation properties was performed with a linear array of interdigital transducer (IDT) structures. The measurements characterized the velocity and propagation loss of two surface modes, a generalized SAW (GSAW) mode with velocities between 6000 and 7000 m/s, and a high velocity Pseudo-SAW (HVPSAW) mode with velocities between 8500 and 12 500 m/s. The experimentally determined characteristics of the two waves have been compared with the results of calculations based on published data for SiC and ZnO. Simulation of wave characteristics was performed with various values of the elastic constant C/sub 13/, which is absent in the published set of material constants for SiC, within the interval permitted by the requirement of positive elastic energy in a hexagonal crystal. The best agreement between the measured and calculated propagation losses of the HVPSAW has been obtained for C/sub 13/ near zero. Although for the GSAW mode the calculated velocity dispersion has been found nearly insensitive to the value of C/sub 13/ and consistent with the experimental data, for the HVPSAW, some disagreement between measured and calculated velocities, which increased with ZnO film thickness, has been observed for any C/sub 13/ value. Theoretical analysis of HVPSAW has revealed the existence of a previously unknown high velocity SAW (HVSAW). The displacement components of this wave have been analyzed as functions of depth and confirmed its pure surface, one-partial character.

Optimal cuts of langasite, La/sub 3/Ga/sub 5/SiO/sub 14/ for SAW devices

The results of a theoretical and experimental investigation of the SAW propagation characteristics in an optimal region of langasite defined by the Euler angles /spl phi/ from -15/spl deg/ to +10/spl deg/, /spl theta/ from 120/spl deg/ to 165/spl deg/, and /spl psi/ from 20/spl deg/ to 45/spl deg/ are presented. Based on temperature coefficients of the elastic constants derived from experimental data, some optimal orientations of langasite characterized by high electromechanical coupling factor (k/sup 2/), zero power flow angle (PFA) and low or zero temperature coefficient of frequency (TCF) were found. The SAW velocity in the region of interest is highly anisotropic; this results in a significant amount of diffraction, which must be taken into account in the search for orientations useful for SAW devices. An orientation having simultaneously zero PFA, zero TCF, negligible diffraction, and relatively high piezoelectric coupling has been found and verified experimentally. The experimental results are in excellent agreement with the calculated SAW characteristics. The frequency response of a SAW device fabricated on the optimal cut of langasite is presented and demonstrates that high performance SAW filters can be realized on this optimal cut of langasite.

A universal technique for analysis of acoustic waves in periodic grating sandwiched between multi-layered structures and its application to different types of waves

A numerical technique, which combines FEM analysis of electrode area with matrix formalism applied to spectral domain analysis (SDA) of eigen modes in multi-layered substrate and multi-layered dielectric upper half-space, is described as an efficient tool of investigating the wave characteristics in periodic metal grating sandwiched between two multi-layered half-spaces, with arbitrary thickness of each layer. The universal character of the developed technique is illustrated by few examples of its application to different types of structure providing propagation of common SAW, boundary waves at interface, plate modes, normal modes in a thin film deposited over the grating etc. The transformation between the waves with continuously increasing film thicknesses can be observed.

Experimental and theoretical investigations of some useful langasite cuts for high-temperature SAW applications

Passive high-temperature sensors are a most promising area of use for SAW devices. Langasite (La₃Ga₅SiO₁₄; LGS) has been identified as promising piezoelectric material to meet high-temperature SAW challenges. Because it is necessary to know the material behavior for an accurate device design, the frequency-temperature behavior of Rayleigh SAW (R-SAW) and shear-horizontal SAW (SH-SAW) LGS cuts is investigated on delay line and resonator test structures up to 700°C by RF characterization. In the range of the 434-MHz ISM band, the (0°, 22°, 90°) SH-SAW cut shows thermal behavior similar to the (0°, 138.5°, 26.7°) R-SAW cut. Associated with the (0°, 22°, 31°) cut, in which SAWs present mixed types of polarization, the (0°, 22°, 90°) SH-SAW orientation might allow differential measurements on a single substrate. In the temperature range of 400 to 500°C, delay line test devices using the SH-SAW cut show a considerable drop of signal. Theoretical analysis indicates that this newly described behavior might be a result of anisotropy effects in this cut, occurring in case of any slight misorientation of electrode alignment.

Transformation of acoustic waves in periodic metal grating sandwiched between piezoelectric and dielectric

The mechanism of SAW transformation with variation of film thickness is investigated in a piezoelectric substrate with a metal grating overlaid by a dielectric film, via simulation and visualization of the acoustic fields. By way of example, two orientations of lithium niobate substrates are analyzed, YX-LN and 128°YX-LN, with a Cu grating and an isotropic silica glass overlay. The motions, which follow the wave propagation in the sagittal plane, are visualized within two periods of the grating, with added contour plots showing the shear horizontal displacements. The continuous transformation of the waves nature is investigated for each wave propagating in the analyzed material structures when the film thickness is increased from zero to a few wavelengths. The examples of the SAW transformation into boundary waves and into plate modes of different polarization have been found and investigated. The behavior of the SAW characteristics in the grating is correlated with transformation of the wave structure with increasing overlay thickness.

3E-1 Fast Numerical Technique for Simulation of SAW Dispersion in Periodic Gratings and its Application to Some SAW Materials

A method for approximation of harmonic admittance of an infinite periodic grating, which takes into account interaction between SAW (or LSAW) and BAW, is presented. A numerical technique based on this method is non-iterative, fast and accurate and enables extraction of dispersion parameters from numerical admittance calculated at real-valued spectral frequencies. Excellent agreement with admittance can be obtained in a wide range of temporal and spectral frequencies, including the interval where there is a strong interaction between SAW and BAW. Application of the method to different types of surface waves - Rayleigh-type SAW in LiNbO3, SH-type leaky waves in LiTaO3 and high-velocity LH-type leaky wave in Li2B4O7, is demonstrated. The extracted dispersion curves are in a good agreement with the dispersion found by the search of poles in complex spectral domain.

Hybrid surface-bulk mode in periodic gratings

We report on a SAW/BAW hybrid mode (SBH), which arises from surface skimming bulk wave due to the trapping of its energy by a counter-propagating surface wave (SAW) or leaky surface wave (LSAW), in periodic grating structures. The SBH is the result of an interaction between surface and bulk waves and exists within limited frequency region, outside which it is decomposed into counterpropagating SAW and BAW. Different examples of SBH mode illustrate interactions of BAW with pure SAW, leaky and high-velocity leaky waves. The behavior of the SBH mode was numerically investigated using the harmonic admittance function. Calculation of velocity dispersion revealed the existence of an additional branch of the dispersion curve, which refers to the SBH mode. A stopband occurs due to SAW/BAW interaction and manifests itself by a resonance of harmonic admittance at its lower edge, which can be considered as the cut-off frequency of bulk wave radiation.

Analysis of interaction between two SAW modes in Pt grating on langasite cut (0°, 138.5°, 26.6°)

The numerical technique based on a previously developed rational approximation of harmonic admittance of a periodic grating was applied to analysis of SAW behavior in platinum grating on langasite cut with Euler angles (0°, 138.5°, 26.6°). The approximation is able to take into account interaction between surface and bulk waves or between two SAW modes. SAW dispersion was calculated at different values of electrode thickness varying between 1% and 4% of wavelength. It was found that with increasing Pt thickness, SAW behavior in the grating is strongly affected by interaction between two SAW modes propagating in the same orientation. An additional stopband, which results from this interaction, occurs at certain detuning from synchronous reflection condition and can cause spurious resonances of the admittance function. Interaction between two SAW modes is also responsible for anomalously slow growth of reflectivity with increasing platinum thickness.

Plate modes in LiTaO 3 for application in wide band resonator filters with improved temperature characteristics

Rigorous analysis of plate modes propagating in LiTaO3 plates was performed using the numerical technique SDA-FEM-SDA. The characteristics of zero- and first-order plate modes were calculated in rotated YX cuts of LT as functions of cut angle and plate thickness. Orientations providing high electromechanical coupling (up to 19%), zero TCF at resonance and high velocities (up to 19,000 m/s) were found and investigated. The wave motions following the propagation of some modes are visualized. The possibility of enhancement of some modes by deposition of metal film on the plate bottom surface and the sensitivity of higher-order modes to variation of plate thickness are discussed.

Dispersion of lamb waves under a periodic metal grating in aluminum nitride plates

Dispersion of Lamb waves propagating in AlN plates with a periodic Al grating on the top surface and an Al electrode on the bottom surface is investigated using the numerical technique SDA-FEM-SDA, which combines finite element modeling (FEM) analysis of the electrode region with spectral-domain analysis (SDA) of the adjacent multi-layered half-spaces. Characteristics of zero-order and higher-order Lamb waves are presented as functions of plate thickness and spectral frequency, which varies in the first Brillouin zone. The structures of typical Lamb waves are examined via visualization of the instantaneous displacement fields in the AlN plate confined between the grating and the bottom electrode. The mechanism of building hybrid modes, which arise from intermode coupling between the counter-propagating Lamb waves of different symmetry and order, is illustrated by two examples of modes propagating with wavelengths λ = 3p and λ = 4p, where p is the pitch of the grating.