Electrostatically tunable piezoelectric-on- silicon micromechanical resonator for real-time clock

This paper reports on the design, fabrication, and characterization of a small form factor, piezoelectrically transduced, tunable micromechanical resonator for real-time clock (RTC) applications (32.768 kHz). The device was designed to resonate in an out-of-plane flexural mode to simultaneously achieve low-frequency operation and reduced motional resistance in a small die area. Finite element simulations were extensively used to optimize the structure in terms of size, insertion loss, spurious-mode rejection, and frequency tuning. Microresonators with an overall die area of only 350 × 350 μm were implemented on a thin-film AlN on silicon-on-insulator (SOI) substrate with AlN thickness of 0.5 μm, device layer of 1.5 μm, and an electrostatic tuning gap size of 1 μm. A frequency tuning range of 3100 ppm was measured using dc voltages of less than 4 V. This range is sufficient to compensate for frequency variations of the microresonator across temperature from -20°C to 100°C. The device exhibits low motional impedance that is completely independent of the frequency tuning potential. Discrete electronics were used in conjunction with the resonator to implement an oscillator, verifying its functionality as a timing reference.