A Micromechanical Parallel-Class Disk-Array Filter

A method for realizing a high-order on-chip filter response that combines two fourth-order (i.e., two-pole) micro-mechanically-coupled MEMS-based disk-array sub-filters in a parallel configuration has been used to demonstrate a 163-MHz eighth-order (i.e., four-pole) bandpass filter with a 0.16%-bandwidth insertion loss of 2.73 dB, a 20 dB shape factor of only 1.78, and a termination impedance of only 4 kOmega, which is considerably smaller (i.e., better) than that achievable by a parallel combination of single disk resonators. The key to achieving such performance is the use of two capacitively-transduced disk-array composite sub-filters instead of the usual two stand-alone resonators used in previous parallel filter renditions. Here, the use of 275 mum times 360 mum sub-filters, each comprised of coupled radial-contour mode disk arrays, allows a combining of in-phase outputs to substantially raise output currents towards a 50times reduction in motional impedance versus that of a single disk resonator under the same operating conditions. This then lowers the termination impedance required by the overall filter, facilitating the ease with which other components can match to it, and thereby alleviating one of the primary drawbacks of capacitively-transduced micromechanical filters. By utilizing more than 128 disks and mechanical links, the 560 mum x 360 mum filter of this work comprises a true medium-scale integrated (MSI) mechanical circuit on a chip.