Identification of new LTO HVPSAW orientations considering finite thickness electrodes

Abstract

The continuing trend towards higher operating frequencies in RF communications has strained SAW filter fabrication capabilities, focusing recent efforts on identifying low-attenuated high velocity pseudo-surface acoustic wave (HVPSAW) orientations of quartz, lithium niobate (LNO), lithium tantalate (LTO), and lithium tetraborate (LBO). In particular, LTO is a well-established SAW substrate widely used for wireless applications, due to its relatively high electromechanical coupling (K 2 ) and moderate temperature sensitivity in comparison to the other substrates previously mentioned. Although HVPSAW propagation directions have been recently identified in the literature for these materials, several show diminished coupling or prohibitively high propagation losses. Others imply the use of heavy mechanical loading by the use of thick electrodes to achieve reduced device loss. This paper reports on the investigation of LTO HVPSAW excitation and propagation properties, including: phase velocity, propagation loss per wavelength (λ), and electromechanical coupling for arbitrary orientations under periodic aluminum (Al), gold (Au), and platinum (Pt) electrodes of finite thickness h. The harmonic admittance (HA) of periodic electrodes is calculated using Chebyshev polynomial charge and stress basis functions in conjunction with finite element method using parallel computing techniques. The determination of well-defined complex poles in the HA curve as a function of crystal rotation leads to the identification of low-attenuated and highly coupled HVPSAW orientations. For example, along selected orientations of the LTO (0°, 120°, ψ) plane, HVPSAWs are identified which exhibit phase velocities (Vsc) around 5.0 km/s, propagation losses as low as 0.014 dB/λ, and K 2 as high as 6.997%, for Al, Au, and Pt electrodes with thicknesses around h/λ = 9%, h/λ = 3.25%, and h/λ = 3.5% respectively. For the same electrode materials and thicknesses, the HVPSAW propagating under uniform film layers show propagation losses around 0.3 dB/λ. Therefore, the resulting decrease in propagation loss by more than one order of magnitude demonstrates the importance of the grating structure in achieving low loss operational HVPSAW devices. The reported work identifies new LTO HVPSAW orientations, exhibiting high phase velocity, low propagation loss, and high electromechanical coupling, suitable for high frequency, low loss communication and sensor applications.