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-semiconduc tor (BiCMOS) integrated circuit process. This merged process allows the int egration of Si mechanical resonators as thick as 11 mu m with any integrate d circuit process with the addition of only a single masking step. The proc ess 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 so urce which allowed deep trenches to be fabricated with high aspect ratios a nd smooth sidewall surfaces. Clamped-clamped beam Si resonators that were 5 00 mu m long, 5 mu m wide, and 11 mu m thick have been fabricated and teste d. A typical resonator had a resonance frequency of 28.9 kHz and a maximum amplitude of vibration at resonance of 4.6 mu m in air. The average measure d resonance frequency across a 4-in-diameter Si wafer was within 0.5% of th at 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 2.0x10(-5) Ohm(-1) for a gate voltage of 4 V .