High aspect ratio single crystal Si microelectromechanical systems

A review of fabrication techniques and testing of single crystal Si resonant devices with high aspect ratio capacitive transduction mechanisms has been presented. Deep trenches have been etched in single crystal Si using a Cl₂ plasma generated by an electron cyclotron resonance (ECR) and an inductively coupled plasma (ICP) source. This etching has been extended to the fabrication of resonant devices thicker than 50 μm using a frontside-release process and these devices have been electrically tested. The thick devices allow larger capacitance between drive and sense plates, which in turn reduces required driving voltage and increases sensing current. In addition, an etching condition has been developed which can etch trenches as narrow as 0.1 μm to depths greater than 3 μm. This etch has been used to fabricate comb driven resonators with high aspect ratio gaps (>30) between comb fingers. Finally, a fabrication method to integrate these single crystal Si mechanical devices with a conventional circuit process with only one additional masking step has been developed. 11 μm thick clamped-clamped beam comb driven resonators have been fabricated and tested on the same chip with working CMOS transimpedance amplifiers. The resonator had a resonant frequency of 28.9 kHz and a maximum amplitude of vibration of 4.6 μm, while the amplifier had a 3-dB frequency of 150 kHz and a power dissipation of 1.25 μW.