STW resonator on (0?, 22?, 90?) cut of langasite with Al electrodes

In this paper we present the measurement results for surface transverse wave (STW) resonators on langasite (0°, 22°, 90°) with aluminum electrodes. This STW cut of langasite provides temperature stable delay of STW with high coupling coefficient and potentially provide better Q-factor than standard SAW resonators, since pure shear vibrations in STW result in less loss than those of Rayleigh type waves with compression components. We fabricated resonators with wavelength (2p) of 6μm, 7.2μm, and 9 μm; m/p = 0.5; W = 400p, NIDT=130, Nr=400. The thickness of aluminum hAl = 288 nm. The experimentally obtained values of Q-factor <;3000 were lower than expected and required explanation. Since the shear wave velocity in this cut of langasite is smaller than the corresponding BAW velocity in aluminum, the aluminum electrodes accelerate waves. The Bragg reflection, on the other hand, creates “stop-bands,” on the left edge of which the phase velocity of the wave decreases. That is, the surface wave exists only because of the Bragg stopband phenomenon. In the case of a uniform Al layer there is no localized near-surface shear wave (no Love wave with Al layer on LGS) and energy is lost in the direction of the busbars, thereby reducing the quality factor.