A detailed study of film formation and directional etching for high-resolution trilevel resist was performed. The top layer was CMS and AZ-1350. The intermediate layer was ion beam sputter- deposited SiO2. The bottom layer was AZ-1370. High-temperature baking of the bottom layer prevents crack generation in the SiO2 intermediate layer. Reactive ion etching in O2 gas plamsa with a carbon electrode was used to pattern the bottom layer and gave a residue-free etched surface. A study was made of undercutting during O2 etching with power, temperature, and pressure varied over wide ranges. The results agree with spectroscopic observations of the variation in the ratio of excited neutral species to ionic species in the plasma. Low pressure and low substrate temperature during the bottom layer etching were found to be necessary for a low undercut. The viability of the trilevel resist for submicron patterning was verified.