Gigahertz-band electroacoustic devices based on AlN thick films sputtered on Al2O3 at low temperature

Piezoelectric aluminum nitride films, 1.4–6.2 μm thick, have been grown on z-cut Al2O3 substrates by reactive rf sputtering at 180 °C. The films were clear, uniform, stress free, and highly adhesive to the substrate. Surface acoustic wave (SAW) delay lines with harmonic modes operating at frequencies up to about 2.4 GHz were obtained using only conventional optical lithography at 7.5 μm line width resolution. The phase velocity and the electromechanical coupling factor of SAWs propagating along zx-Al2O3/AlN and zy-Al2O3/AlN structures were investigated and found to be in good agreement with the theoretical predictions. The Al2O3/AlN first-order temperature coefficient of frequency was estimated for different film thickness to wavelength ratio values, and the temperature-compensated SAW delay line was obtained.