Roy H. Olsson

This paper reports a MEMS displacement sensor with a sensitivity of 50 fm/radicHz at frequencies as low as 10 mHz. The sensor is composed of a near-field optical nano-grating that modulates the reflectivity of incident light by 10%/nm. A reference detection low noise readout circuit has been designed that cancels laser relative intensity noise (RIN) to frequencies as low as 0.7 Hz. The low frequency bandwidth of the position sensing microsystem is further reduced to below 10 mHz using a new MEMS correlated double sampling technique that cancels low frequency RIN, drift, and circuit 1/f noise by greater than 77 dB.

This paper reports post-CMOS compatible aluminum nitride (AlN) MEMS resonators, filters, and resonant sensors for the miniaturization of radio-frequency transceivers and sensor systems. Utilizing a resonator with two closely spaced modes, 2^nd order MEMS filters occupying 0.06 mm² have been realized in a single device. Methods for tuning the bandwidth and center frequency of these filters lithographically have been demonstrated. A 0.5% bandwidth, 108.4 MHz dual mode filter has a measured insertion loss of 9.4 dB with 50 Omega termination which can be reduced to 4.7 dB by terminating the filter with 75 Omega. In order to scale MEMS resonators to higher frequencies without increasing the size or impedance, resonators selectively driven at a harmonic determined by interdigitated drive and sense electrodes have been demonstrated reaching frequencies of 796 MHz with impedances of approximately 100 Omega and quality factors in excess of 750 in air. In the same process resonant sensors based on AlN double-ended tuning fork (DETF) sensing beams have been demonstrated at 727 kHz with quality factors of 2160. An oscillator based on the DETF sensing beams achieves a phase noise of -81 dBc/Hz at 275 Hz offset from the carrier. A 100 ng mass coupled to a pair of DETF sensors achieves an acceleration sensitivity of 565 mG/radicHz for accelerations from 275 to 1100 Hz.

A MEMS bulk wave acoustic bandgap has been designed and experimentally verified. The acoustic bandgaps are realized by including tungsten (W) scatterers in a SiO₂ matrix. Wide frequency ranges where acoustic waves are forbidden to exist are formed due to the large density and acoustic impedance mismatch between W and SiO₂. The acoustic bandgap structures are fabricated in a 7-mask process that features integrated aluminum nitride piezoelectric couplers. Acoustic bandgaps in a square lattice have been measured at 33 and 67 MHz with up to 35 dB of acoustic rejection and bandwidths exceeding 35% of the midgap.