Method of making a micromechanical device from a single crystal semiconductor substrate and monolithic sensor formed thereby

Abstract

A monolithic sensor including a doped mechanical structure is movably supported by but electrically isolated from a single crystal semiconductor substrate of the sensor through a relatively simple process. The sensor is preferably made from a single crystal silicon substrate using front-side release etch-diffusion. Thick single crystal Si micromechanical devices are combined with a conventional bipolar complimentary metal oxide semiconductor (BiCMOS) integrated circuit process. This merged process allows the integration of Si mechanical resonators as thick as 15 &mgr;m thick or more with any conventional 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 are etched in an inductively coupled plasma source which allows deep trenches to be fabricated with high aspect ratios and smooth sidewall surfaces. Clamped-clamped beam Si resonators 500 &mgr;m long, 5 &mgr;m wide, and 11 &mgr;m thick are disclosed. A typical resonator had a resonance frequency of 28.9 kHz and an amplitude of vibration at resonance of 4.6 &mgr;m in air. Working NMOS transistors are fabricated on the same chip as the resonator with measured threshold voltages of 0.6 V and an output conductance of 2.0×10 &OHgr; for a gage voltage of 4 V.