ANISOTROPIC ETCHING OF POLYMERS IN SO2/O2 PLASMAS - HYPOTHESES ON SURFACE MECHANISMS

A study of the anisotropy of the etching of resists in SO2-based plasmas is performed in a distributed electron-cyclotron-resonance plasma excited at 2.45 GHz with independent radio frequency biasing at 13.56 MHz. Emphasis is put on the comparison of the profiles and etch rates obtained in pure oxygen and SO2-containing plasmas as a function of substrate temperature and ion bombardment energy. For a constant ion bombardment intensity, a significant decrease in the etch rate obtained using pure SO2 plasmas is observed as compared to pure O2 plasmas. The evolution of the etch rate with ion energy also shows quite different behavior. In contrast to pure oxygen plasmas, perfect anisotropic profiles can be obtained on cooled substrates with SO2-containing gas mixtures. A likely explanation for these results is to assume that sulfur acts as a passivating layer at temperatures close to or below room temperature. The surface mechanisms involved in this sulfur passivation and the conditions required to obtain an effective sidewall passivation are discussed. Based on similar mechanisms, the operating conditions required to obtain perfect anisotropic profiles in pure oxygen plasmas are deduced and compared to the experimental data.