Implementation of a CMOS-MEMS Filter Through a Mixed Electrical and Mechanical Coupling Scheme

We propose a novel filter coupling scheme, which combines the merits of mechanically and electrically coupled methods to enable a well-defined narrow bandwidth and a decent stopband rejection in a CMOS-MEMS bandpass filter. In the proposed design, a previously developed free-free beam arrayed resonator is employed as a basic resonant tank for the filter concept validation. By the use of several design strategies, including oxide-rich structure, arrayed design, and deep-submicrometer transducer's gap, the constituent resonant tanks feature small motional impedance (R
_m
), making filter termination feasible. Furthermore, we also provide the complete equivalent circuit and numerical modeling for such a fourth-order bandpass filter under the proposed coupling scheme. Compared with the traditional coupling topologies, the presented coupling concept is capable of achieving significant feedthrough suppression and precise bandwidth control, only at the cost of slight in-band loss. As a result, we successfully demonstrate a terminated CMOS-MEMS filter centered at 8.58 MHz with a narrow passband of 35 kHz and a stopband rejection greater than 20 dB. The proposed design concept of mixed coupling scheme can also be implemented in other MEMS fabrication platforms.