Fully Monolithic CMOS Nickel Micromechanical Resonator Oscillator for Wireless Communications.

Abstract

A nickel surface-micromachining technology offering various electrode-to-resonator gap materials is presented that is particularly suitable for high-Q, low impedance MEMS-based vibrating resonators. The low temperature of this nickel fabrication technology makes it amenable to post-processing over finished foundry CMOS wafers, even those using advanced low-k, low temperature dielectrics around metallization to decrease inter-connect capacitance. Such a MEMS-last process technology is used in this work to dem-onstrate a fully monolithic MEMS-based oscillator comprised of a nickel disk resonator array surface-micromachined over foundry CMOS. To achieve resonator motional resistances below 5.8 k with adequate quality factor, a mechanically-coupled array of resonators is used that actually realizes a multi-pole fil-ter structure, from which a single mode can be selected and other modes can be sup-pressed by proper electrode phasing. To attain higher frequencies, a nickel wine-glass mode disk resonator with a nitride capacitive transducer gaps was demonstrated at fre-quencies approaching 60 MHz with Q’s as high as 54,507, which is the highest to date for any micro-scale metal resonator in the VHF range. To boost frequencies to the UHF range, vibrating nickel micromechanical spoke-supported ring resonators were demon-strated at 425.7 MHz with Q’s as high as 2,467. These devices employed an anchor iso-lating spoke-supported ring geometry along with notched support attachments between the ring structure and supporting beams to achieve the highest reported vibrating fre-quency to date for any micro-scale metal resonator. Finally, a fully monolithic oscillator was achieved using MEMS-last integration to fabricate a resonator array of nine nickel flexural-mode disks over foundry CMOS cir-cuitry. The oscillator demonstrated a measured phase noise of -95 dBc/Hz at a 10 kHz offset from its 10.92-MHz carrier frequency, which is adequate for some low-end timing applications. This, together with its low power consumption of 350 μW, and the potential for full integration of integrated circuits and MEMS devices onto a single chip, makes the fully monolithic CMOS nickel micromechanical disk-array resonator oscillator presented here a reasonable on-chip replacement for quartz crystal reference oscillators in low-end applications.