Design of the comb-drive structure to reduce asymmetry lateral plasma etching on the cavity SOI substrate for MEMS fabrication

Abstract A conventional microelectromechanical system (MEMS) fabricated using deep-reactive ion etching (DRIE) on a silicon-on-insulator (SOI) wafer and wet etching with hydrofluoric acid (HF) encounters challenges related to isotropic etching, such as microstructure stiction, etched byproduct contamination, and over-etching of the side protective oxide layer. Even with the utilization of cavity SOI (C-SOI) wafers, certain movable structures can still experience damage due to asymmetry in lateral plasma etching caused by the uneven distribution of plasma ions and etchant radicals. This investigation examines three different comb designs to reduce the asymmetry lateral etching caused by DRIE. Factors such as the presence of a pedestal under comb branches and the area of the proof-mass are taken into consideration. At 100 µm of depth, the comb size, line width, and space are 15.0, 7.3 and 5.0 µm, respectively. The overlapping length of combs is 15 µm of the total comb bar length. The proof-mass has an area variation of 300 x 900 µm2, 600 x 900 µm2, and 1200 x 900 µm2, respectively. The Deep Si etching process employs time multiplex etching (Bosch process), which involves the deposition of C4F8 fluorocarbon film and etching with SF6, controlled by inductively coupled plasma (ICP) power to generate a high-density charge. The direct current voltages during the deposition and etching stages are 0 volts and 170 volts for Si bulk, and 0 volts and 3 volts for C-SOI. Our results indicate that the comb designs do not effectively mitigate the asymmetry in lateral etching on the comb structures in this configuration. However, we have observed that the location on the wafer determines the asymmetry lateral etching.