Post-CMOS processing for high-aspect-ratio integrated silicon microstructures

We present a new fabrication sequence for integrated-silicon microstructures designed and manufactured in a. conventional complentary metal-oxide-semiconductor (CMOS) process. The sequence employs a post-CMOS deep silicon backs side etch, which allows fabrication of high aspect ratio (25:1) and flat (greater than 10 mm radius of curvature) MEMS devices with integrated circuitry. A comb-drive resonator, a cantilever beam array and a z-axis accelerometer were fabricated using this process sequence. Electrical isolation of single-crystal silicon was realized by using the undercut of the reactive ion etch (RIE) process. Measured out-of-plane curling across a 120-mum-wide 25-mum-thick silicon released plate was 0.15 mum, which is about ten times smaller than curl of the identical design as a thin-film CMOS microstructure. The z-axis DRIE accelerometer structure is 0.4 nun by 0.5 mm in size and has a 25-mum-thick single-crystal silicon proof mass. The measured noise floor is 1 mG/rootHz, limited by electronic noise. A vertical electrostatic spring hardening effect was theoretically predicted and experimentally verified.