산소 기반 저온 폴리머 식각 공정에서의 온도 의존성과 이방성 제어

Abstract

Low-temperature, oxygen-based plasma etching of thick SU-8 films was systematically examined under both pure O2 and CO/O2 (2.4:0.6) gas environments, with substrate temperatures varied from room temperature down to –80 °C. In pure O2 plasmas, pronounced sidewall erosion and stepwise etch-rate fluctuations—attributable to photon-induced CO desorption, viscoelastic matrix rearrangements, and formation of volatile reaction products—resulted in significant undercutting and poor reproducibility at intermediate cryogenic temperatures. The introduction of CO to promote carbon-based passivation via the Boudouard reaction markedly reduced lateral etching by depositing a protective carbon film; however, this benefit sharply declined at temperatures ≤ –30 °C owing to sluggish carbon deposition kinetics. Furthermore, independent thermal annealing of SU-8 at 250 °C for 30 min prior to etching substantially suppressed abrupt, stepwise etch-rate variations—thereby enhancing process stability—likely due to increased crosslink density and removal of residual solvent. Optimal anisotropic profiles with minimal undercut were achieved near +10 °C, where carbon passivation and oxidative etching were balanced. These findings demonstrate that realizing high-aspect-ratio SU-8 structures with robust sidewall integrity necessitates careful, multivariable optimization of substrate temperature, gas chemistry, ion energy, and polymer pretreatment.