Design and Optimization of Bidirectional Tunable MEMS All-Silicon Evanescent-Mode Cavity Filter

This article presents a bidirectional electrostatic MEMS tuner that enables active voltage compensation of integrated evanescent-mode cavity-based tunable filters. Unlike conventional tunable resonators tuned by unidirectional electrostatic diaphragms, the bidirectional design can compensate for residual displacement errors. This corrective tuning can successfully counterbalance long-term viscoelastic relaxation commonly found in large-displacement MEMS diaphragms. Consequently, it can significantly improve stability and reliability without introducing fabrication complexity. A detailed RF-MEMS codesign and a specific optimization design flow are also discussed. A proof-of-concept, two-pole filter is fabricated, which demonstrates a measured bidirectional tuning range of 18.9-39.6 GHz. The forward (main) actuation tunes the filter from 21.3 to 39.6 GHz with 120 V, whereas the reverse (corrective) actuation tunes it from 21.3 down to 18.9 GHz with 2 V. The measured filter insertion loss varies from 3.14 to 0.78 dB. The extracted unloaded quality factor is 265-510, which is comparable with the state-of-the-art unidirectional filters of this type.