Dry etching of high aspect ratio silicon microstructures in high density plasma sources for microelectromechanical systems.

Abstract

Electron cyclotron resonance (ECR) and inductively coupled plasma (ICP) sources have been used to generate a Cl₂ plasma. Trenches > 100 mum deep have been etched in Si. High microwave power was used to achieve Si etch rates of 1.09 mum/min in a Cl₂ plasma and 1.89 mum/min in a Cl₂/SF₆ plasma, while maintaining the vertical profile and smooth surface. Features with aspect ratios >30 have been fabricated. An electron beam lithography system was used to pattern features as small as 0.1 mum. An etch has been developed to fabricate released 2 mum wide cantilevered single crystal Si beams as long as 500 mum with 0.1 mum gaps between them. Comb driven clamped-clamped beam resonators that are 3.1 mum thick with 0.2 mum gaps between comb fingers have been fabricated and tested. A frontside-release etch-diffusion process has been developed to create released, single crystalline Si microstructures. Resonant microstructures 10 to 55 mum thick have been fabricated using this process. For typical clamped-clamped beam resonators that were 24 mum thick, 5 mum wide, and 400 mum long, with 2 mum comb gaps, a resonant frequency of 90.6 kHz and a quality factor of 362 were measured in air. A simple process has been developed which combines thick single crystal Si micromechanical devices with a conventional bipolar complementary metal oxide semiconductor (BiCMOS) integrated circuit process. Clamped-clamped beam Si resonators that were 500 mum long, 5 mum wide, and 11 mum thick have been fabricated and tested with a CMOS transimpedance amplifier on the same chip. A typical resonator had a resonant frequency of 28.9 kHz and a maximum amplitude of vibration at resonance of 4.6 mum in air. An accelerometer has been designed, simulated, fabricated, and tested using the technology described. As the gaps are scaled down to 0.1 mum the designed detectable acceleration is 4.40 mug/Hz<super>1/2</super>, limited by thermo-mechanical noise. The fabricated accelerometer with 1 mum beams and 0.2 mum comb gaps had a spring constant of 0.127 N/m, close to the simulated value of 0.146 N/m and a sensitivity of 6.3 fF/g, close to the calculated sensitivity of 7.0 fF/g,