Inductively coupled plasma nanoetching of atomic layer deposition alumina

Al₂O₃ thin-film deposited by atomic layer deposition is an attractive plasma etch mask for Micro and Nano Electro-Mechanical Systems (MEMS and NEMS). 20-nm-thick Al₂O₃ mask enables through silicon wafer plasma etching. Al₂O₃ is also an excellent etch mask for other important MEMS materials such as silicon dioxide, silicon nitride, and diamond. In this report, we systematically study nanoscale plasma etching of Al₂O₃ with electron beam lithography and deep UV resist masks. The gas composition and pressure were tuned for optimal etching, and redeposition conditions were mapped. With a BCl₃ and Ar plasma chemistry, the Al₂O₃ etch rate was controlled to between 0.1 and 1 nm/min. The etch selectivity of Al₂O₃ over resist ranged between 1:4 to 1:1. Etch-rate was linearly dependent on the substrate bias power. The etch profile angle can be controlled to between 20 and 82° that almost preserved the resist profile angle. For Al2O3 patterned with deep UV lithography, the smallest structures were 220 nm. For electron beam lithography patterns, the smallest gratings were 18-nm-wide with 50-nm-pitch. Using alumina as a hard mask, we show aspect ratio of 7-10 for subsequent silicon plasma etching, and we etched diamond nanopillars in single crystalline diamond. Based on these studies, we provide guidelines for alumina plasma etching on the nanoscale.