High-Performance SAW Resonators Based on Single-crystalline a-Plane AlN Thin Films on Sapphire Substrates

This work reports on high-performance surface acoustic wave (SAW) piezoelectric resonators based on a-plane Aluminum nitride (AlN) thin films. Rayleigh SAW resonators with different in-plane orientations on AlN thin film were investigated. The resonator with in-plane orientation parallel to the C-axis of the AlN thin film shows better performance, exhibiting an excellent maximum Bode_Q (Q
max
) of 3731 at 2.38 GHz, an impedance ratio of 39 dB, and a high phase velocity of about 5700 m/s. The fitted first-order temperature coefficient of frequency (TCF) values for the resonant frequency (f
r
) and anti-resonant frequency (f
a
) are −46.7 ppm/°C and −46.6 ppm/°C at the temperature range of 25°C to 145°C, respectively. More importantly, the Q
max
of the a-plane AlN thin film based SAW resonator is maintained well at 2682 even at 145°C. These results demonstrate that the a-plane AlN is a promising platform to achieve high-velocity, low-loss resonators and narrow band filters.