Design and implementation of a low-power hybrid capacitive MEMS oscillator

This paper reports on the design and implementation of a low power MEMS oscillator based on capacitively transduced silicon micromachined resonators. The analysis shows how design parameters of MEMS resonator impact on the power requirement of the oscillator, particularly with a view towards informing the impact of device and interface parasitics. The analysis is based on resonators fabricated in a 2-μm gap SOI-MEMS foundry process. The sustaining circuit, which is based on a Pierce topology, is fabricated in a standard 0.35 μm process. An automatic gain control (AGC) is adopted to suppress the mechanical non-linearity so as to improve oscillator frequency stability. The 110-kHz MEMS and CMOS dies are assembled within a standard ceramic package and electrically integrated through wire bonds. The oscillator core consumes 400 nA (900 nA with parasitic readout loading) at 1.2-V dc supply while demonstrating a frequency stability of less than 0.5 ppm. The work provides a thorough analysis and design guidelines for both MEMS and CMOS circuit design with a view towards minimizing overall power consumption. The implications of the results reported in this paper are towards enabling a new class of low power resonant MEMS sensors that utilize the oscillator as a front-end building block.