Tunable piezoelectric MEMS resonators for real-time clock

This paper reports on the design, simulation and characterization of small form factor, tunable piezoelectric MEMS resonators for real time clock applications (32.768 kHz). The structures were fabricated on a thin-film AlN-on-SOI substrate to enable piezoelectric actuation of an out-of-plane flexural mode, as well as electrostatic frequency tuning by utilizing the handle layer as a DC voltage electrode. Resonators of only a few hundred of μm in size exhibit greater than 3100 ppm of tuning using voltages no larger than 4 V; this tuning sufficiently compensates for frequency variations across temperature from -25 to 100°C. The devices exhibit low motional impedance that is completely independent of the tuning potential.