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A fully monolithic oscillator achieved via MEMS-last integration of low temperature nickel micromechanical resonator arrays over finished foundry CMOS circuitry has been demonstrated with a measured phase noise of -95 dBc/Hz at a 10-kHz offset from its 10.92-MHz carrier (i.e., output) frequency. The use of a side-supported flexural-mode disk resonator-array to boost the power handling of the resonant tank is instrumental to allowing adequate oscillator performance despite the use of low-temperature nickel structural material. Because the fabrication steps for the resonator-array never exceed 50 o C, the process is amenable to not only MEMS-last monolithic integration with the 0.35 µm CMOS of this work, but also next generation CMOS with gate lengths 65 nm and smaller that use advanced low-k dielectric material to lower interconnect capacitance.

Substantial improvements in the far-from-carrier phase noise of oscillators referenced to stand-alone (as opposed to arrayed) capacitively transduced micromechanical disk resonators have been attained via the use of atomic layer deposition (ALD) to tune the electrode-to-resonator capacitive gaps. Specifically, ALD of about 30nm of hafnia (HfO 2 ) onto the surface of a released 60-MHz micromechanical disk resonator to reduce its effective resonator-to electrode gap size from 92nm to 32nm provides an increase in power handling leading to more than 15-20dB reduction in the far-from-carrier phase noise of an oscillator referenced to this resonator. This ALD-enabled nano-scale gap tuning provides a simple and effective method to satisfy increasing demands for higher short-term stability in frequency references for electronic applications.

Micromechanical resonant displacement gain stages have been demonstrated that employ directionally engineered stiffnesses in resonant structures to effect displacement amplification from a driven input axis to an output axis. Specifically, the introduction of slots along the output axis of a 53-MHz wine-glass mode disk resonator structure realizes a single gain stage with a measured input-to-output displacement amplification of 3.08x. Multiple such mechanical displacement gain stages can then be cascaded in series via half-wavelength beam couplers to achieve multiplicative gain factors; e.g., two cascaded gain stages achieve a total measured gain of 7.94x. The devices have also been operated as resonant switches, where displacement gain allows impact switching via actuation voltages of only 400mV, which is 6x smaller than for previous resoswitches without displacement gain. The availability of such high frequency displacement gain strategies for resonant switches may soon allow purely mechanical periodic switching applications (such as power amplifiers and power converters) with much higher efficiencies than current transistor-based versions.

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