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Properties of higher-order surface acoustic wave modes in Al(1-x)Sc(x)N / sapphire structures
(2021)
In this work, surface acoustic wave (SAW) modes and their dependence on propagation directions in epitaxial Al0.68Sc0.32N(0001) films on Al2O3(0001) substrates were studied using numerical and experimental methods. In order to find optimal propagation directions for higher-order SAW modes, phase velocity dispersion branches of Al0.68Sc0.32N on Al2O3 with Pt mass loading were computed for the propagation directions <11-20> and <1-100> with respect to the substrate. Experimental investigations of phase velocities and electromechanical coupling were performed for comparison with the numerical results. Simulations carried out with the finite element method (FEM) and with a Green function approach allowed identification of each wave type, including Rayleigh, Sezawa and shear horizontal wave modes. For the propagation direction <1-100>, significantly increased wave guidance of the Sezawa mode compared to other directions was observed, resulting in enhanced electromechanical coupling (k2eff = 1.6 %) and phase velocity (vphase = 6 km/s). We demonstrated, that selecting wave propagation in <1-100> with high mass density electrodes results in increased electromechanical coupling without significant reduction in phase velocities for the Sezawa wave mode. An improved combination of metallization, Sc concentration x, and SAW propagation direction is suggested which exhibits both high electromechanical coupling (k2eff > 6 %) and high velocity (vphase = 5.5 km/s) for the Sezawa mode.
Partial substitution of Al atoms with Sc in wurtzite AlN crystals increases the piezoelectric constants. This leads to an increased electromechanical coupling, which is required for high bandwidths in piezo-acoustic filters. The crystal bonds in Ah-xScxN (AlScN) are softened as function of Sc atomic percentage x, leading to reduction of phase velocity in the film. Combining high Sc content AlScN films with high velocity substrates favors higher order guided surface acoustic wave (SAW) modes [1]. This study investigates higher order SAW modes in epitaxial AlScN on sapphire (Al2O3). Their dispersion for Pt metallized epitaxial AlScN films on Al2O3was computed for two different propagation directions. Computed phase velocity dispersion branches were experimentally verified by the characterization of fabricated SAW resonators. The results indicated four wave modes for the propagation direction (0°, 0°, 0°), featuring 3D polarized displacement fields. The sensitivity of the wave modes to the elastic constants of AlScN was investigated. It was shown that due to the 3D polarization of the waves, all elastic constants have an influence on the phase velocity and can be measured by suitable weighting functions in material constant extraction procedures.
Laser ultrasound was used to determine dispersion curves of surface acoustic waves on a Si (001) surface covered by AlScN films with a scandium content between 0 and 41%. By including off-symmetry directions for wavevectors, all five independent elastic constants of the film were extracted from the measurements. Results for their dependence on the Sc content are presented and compared to corresponding data in the literature, obtained by alternative experimental methods or by ab-initio calculations.
In einer SAW-Vorrichtung, welche einen SAW-Chip umfasst, der einen SAW-Wandler aufweist, welcher innerhalb einer ersten Signalleitung angeordnet ist, werden Parasitärsignale infolge höherer Harmonischer der Betriebsfrequenz der SAW-Vorrichtungen durch Kompensationsmittel elektrisch beseitigt, welche zumindest eine zweite Signalleitung mit Mitteln zum Erzeugen eines Aufhebungssignals, das im Vorzeichen oder in der Phase vom Parasitärsignal verschieden ist, oder eine Nebenschlussleitung zum elektrischen Verbinden des SAW-Wandlers mit einer rückseitigen Metallisierung des SAW-Chips umfassen.
In a SAW device comprises a SAW chip bearing a SAW transducer arranged within a first signal line parasitic signals due to higher harmonics of the operating frequency of the SAW devices are electrically eliminated by compensating means comprising at least one second signal line having means for producing a cancelling signal different in sign or phase to the parasitic signal, or a shunt line to electrically connect the SAW transducer to a back side metallization of the SAW chip.
High-precision signal processing algorithm to evaluate SAW properties as a function of temperature
(2013)
This paper presents a signal processing algorithm which accurately evaluates the SAW properties of a substrate as functions of temperature. The investigated acoustic properties are group velocity, phase velocity, propagation loss, and coupling coefficient. With several measurements carried out at different temperatures, we obtain the temperature dependency of the SAW properties. The analysis algorithm starts by reading the transfer functions of short and long delay lines. The analysis algorithm determines the center frequency of the delay lines and obtains the delay time difference between the short and long delay lines. The extracted parameters are then used to calculate the acoustic properties of the SAW material. To validate the algorithm, its accuracy is studied by determining the error in the calculating delay time difference, center frequency, and group velocity.
Nonlinearity can give rise to intermodulation distortions in surface acoustic wave (SAW) devices operating at high input power levels. To understand such undesired effects, a finite element method (FEM) simulation model in combination with a perturbation theory is applied to find out the role of different materials and higher order nonlinear tensor data for the nonlinearities in such acoustic devices. At high power, the SAW devices containing metal, piezoelectric substrate, and temperature compensating (TC) layers are subject to complicated geometrical, material, and other nonlinearities. In this paper, third-order nonlinearities in TC-SAW devices are investigated. The materials used are LiNbO 3 -rot128YX as the substrate and copper electrodes covered with a SiO 2 film as the TC layer. An effective nonlinearity constant for a given system is determined by comparison of nonlinear P-matrix simulations to third-order intermodulation measurements of test filters in a first step. By employing these constants from different systems, i.e., different metallization ratios, in nonlinear periodic P-matrix simulations, a direct comparison to nonlinear periodic FEM-simulations yields scaling factors for the materials used. Thus, the contribution of the different materials to the nonlinear behavior of TC-SAW devices is obtained and the role of metal electrodes, substrate, and TC film are discussed in detail.
Recently a P-matrix and COM formalism was presented, which predicts third order intermodulation (IMD3) and triple beat with good accuracy and needs only a single nonlinearity constant. This formalism describes frequency dependence correctly. In this work the dependence of this nonlinearity constant on metalization ratio is investigated for aluminum metalization on LiTaO 3 (YXl)/42°. By comparison to test devices the nonlinearity constant is shown to be largely independent of metalization ratio. The nonlinear effect, however, strongly depends on metalization ratio, which is well described by the model. The linearity of a duplexer is optimized by reduction of metalization ratio and redesign of Tx branch topology.
The growing complexity in RF front-ends, which support carrier aggregation and a growing number of frequency bands, leads to tightened nonlinearity requirements in all sub-components. The generation of third order intermodulation products (IMD3) are typical problems caused by the non-linearity of SAW devices. In the present work, we investigate temperature compensating (TC) SAW devices on Lithium Niobate-rot128YX. An accurate FEM simulation model [1] is employed, which allows to better understand the origin of nonlinearities in such acoustic devices.