High-rate directional deep dry etching for bulk silicon micromachining

Cryogenic reactive ion etching (RIE) of silicon using SF₆ gas and a multilayer Ni/Al mask has been studied and high-rate, directional deep etching was demonstrated. The temperature of the cathode wafer stage was controlled at a low temperature (-60 degrees C) and a magnetic field was applied to enhance the plasma density. A high flow rate (1 sccm) of the etching gas and low pressure (0.5 Pa) produced by high-speed exhausting were effective for high-rate etching and directional etching. A trade-off of the RF power density was necessary between the etching rate and the etching selectivity to the mask with selectivity improving at higher etch rates. The system could be used to etch through a silicon wafer of 200 mu m in thickness. An etching rate of 0.8 mu m min^-1 and vertical walls as thin as 20 mu m were obtained in the through wafer etching. On the other hand, the etching rate was reduced in narrow deep grooves. This etching method is effective in fabricating three-dimensional silicon microstructures with high aspect ratio.