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 plasm as is performed in a distributed electron-cyclotron-resonance plasma e xcited at 2.45 GHz with independent radio frequency biasing at 13.56 M Hz. Emphasis is put on the comparison of the profiles and etch rates o btained in pure oxygen and SO2-containing plasmas as a function of sub strate temperature and ion bombardment energy. For a constant ion bomb ardment intensity, a significant decrease in the etch rate obtained us ing pure SO2 plasmas is observed as compared to pure O2 plasmas. The e volution of the etch rate with ion energy also shows quite different b ehavior. In contrast to pure oxygen plasmas, perfect anisotropic profi les can be obtained on cooled substrates with SO2-containing gas mixtu res. A likely explanation for these results is to assume that sulfur a cts as a passivating layer at temperatures close to or below room temp erature. The surface mechanisms involved in this sulfur passivation an d the conditions required to obtain an effective sidewall passivation are discussed. Based on similar mechanisms, the operating conditions r equired to obtain perfect anisotropic profiles in pure oxygen plasmas are deduced and compared to the experimental data.