Cryogenic dry etching for high aspect ratio microstructures

Cryogenic reactive ion etching (RIE) has been used to fabricate microstructures. The cryogenic system has a cathode stage that is temperature controlled from 0 to -140°C. A magnetic field and a narrow gap between electrodes are introduced to increase plasma density. The etching behavior of silicon and polyimide film has been investigated. Directional etching was achieved at low temperature. The etch rate was increased by using Sm-Co permanent magnets and also a high flow rate of the etching gas under constant pressure. In the case of silicon wafer etching, a maximum etch rate of 1.6 μm/min was achieved with normalized side etch of less than 0.02 at cathode temperature of -120°C. Etching selectivity was over 900 at a power density of less than 4.0 W/cm². In the case of the polyimide film etching, normalized side etch of less than 0.01 with an etch rate of 0.8 μm/min was achieved at -100°C. The cryogenic RIE system can be used to fabricate 3-D silicon and polyimide structures with high aspect ratio