Deep lateral anhydrous HF/methanol etching for MEMS release processes

As demonstrated earlier, gas-phase etching of sacrificial oxide with a vapor mixture of anhydrous HF (AHF) and methanol (CH₃OH) offers a clean, stiction-free, effective etching technique for microelectromechanical systems (MEMS) release. The use of AHF/methanol process in MEMS release operations is explored in the deep lateral etching of patterned silicon-on-insulator (SOI) substrates using a 200 mm multiwafer commercial module, which assures adequate process throughput. It was determined that highly selective, with respect to both Si and Si₃N₄, etching of SiO₂ can be accomplished by controlling pressure and wafer temperature. It was also observed that the vertical etch rates for both the AHF/methanol and the HF: H₂O solution was higher than the rates of lateral etches in confined geometries. Furthermore, the AHF/methanol and 1:10 HF: H₂O etch chemistries were directly compared in releasing silicon cantilevers up to 500 μm in length and a significantly faster, stiction-free process was observed in the former case. Adequate process reproducibility from wafer to wafer as well uniformity across the wafer was demonstrated.