A merged process for thick single-crystal Si resonators and BiCMOS circuitry

A simple process has been developed which combines thick single-crystal Si micromechanical devices with a bipolar complimentary metal-oxide-semiconductor (BiCMOS) integrated circuit process. This merged process allows the integration of Si mechanical resonators as thick as 11 μm with any integrated circuit process with the addition of only a single masking step. The process does not require the use of Si on insulator wafers or any type of wafer bonding. The Si resonators were etched in an inductively coupled plasma source which allowed deep trenches to be fabricated with high aspect ratios and smooth sidewall surfaces. Clamped-clamped beam Si resonators that were 500 μm long, 5 μm wide, and 11 μm thick have been fabricated and tested. A typical resonator had a resonance frequency of 28.9 kHz and a maximum amplitude of vibration at resonance of 4.6 μm in air. The average measured resonance frequency across a 4-in-diameter Si wafer was within 0.5% of that predicted by theory. Working NMOS transistors were fabricated and tested on the same chip as the resonator with measured threshold voltages of 0.6 V and an output conductance of 2.0×10 ^-5 Ω^-1 for a gate voltage of 4 V