Design and Optimization of Tunable Silicon-Integrated Evanescent-Mode Bandpass Filters

This paper reports on the design and optimization of MEMS-tunable evanescent-mode cavity-based bandpass filters with continuously variable center frequency within an octave tuning range. The devised filters are manufactured using silicon-micromachining techniques that enable their actualization for frequencies located in the millimeter-wave (30-100 GHz) regime. An RF design methodology that takes into consideration all microfabrication-induced constrains-e.g., nonvertical wall profiles and finite MEMS deflection-enables high unloaded factor (Q_u) and also minimizes bandwidth (BW) variation within the octave tuning range is reported. Furthermore, a new passively compensating package-integrated input/output feeding structure that enables optimal impedance matching over the entire tuning range is also presented. In order to evaluate the devised RF design methodology, a filter prototype was manufactured and measured at Ka-band. It exhibits a measured frequency tuning between 20 and 40 GHz (2:1 tuning range), relative BW between 1.9 and 4.7%, insertion loss between 3.1 and 1.1 dB, and input reflection below 15 dB. This paper also explores important tradeoffs between mechanical stability and insertion loss by comparing creep-resistant to pure-Au tuning diagrams.