High-κ dielectrically transduced MEMS thickness shear mode resonators and tunable channel-select RF filters

Electrically coupled, high quality factor (Q), tunable channel-select ladder filters comprised of dielectrically transduced thickness shear mode resonators are presented using integrated circuit compatible bulk micromachining technology. The filters are fabricated on the 3.2 μm thick device layer of heavily doped SOI wafers with a 20 nm thick hafnium dioxide film sandwiched between the polysilicon electrodes and the silicon device layer. The ladder filter consists of shunt and series resonators operating in the half-wave thickness shear vibration mode. Dielectric transduction provides a k2 reduction in motional impedance relative to air-gap electrostatic transduction. Each constituent resonator of the filter can be excited at above 810 MHz resonant frequency with Q of 7800 in air and a motional impedance (RX) of 59 Ω. The ladder filter demonstrates a center frequency tuning range 8–817 MHz and an adjustable bandwidth from 600 kHz to 2.8 MHz, while maintaining an insertion loss <4 dB, stop-band rejection >30 dB and pass-band ripple <2 dB. By having a tunability feature, RF MEMS filters can accommodate various signal waveforms with bandwidth range 0.1–5 MHz. In addition, errors due to fabrication can be compensated and capacitive loading in receiver architecture can be minimized.