Development of Polymers in O2 Plasmas: Temperature Effects and Transition to Imperfect Anisotropy

A parametric study of the etching of a photoresist is performed in an O2 microwave multipolar plasma using a trilevel resist system. The etch rate and the anisotropy evolutions are reported as a function of ion energy, oxygen pressure and substrate temperature, which are important parameters likely to affect anisotropy. A transition from isotropic to imperfect anisotropic etching occurs when the ion energy is increased. However, residual lateral etching always subsists at room temperature, even at the lowest oxygen concentrations. Above room temperature, in contrast with ion-induced etching, the spontaneous lateral etch rate increases with temperature, suggesting a thermally activated reaction process. The phenomena are interpreted in terms of oxygen coverage on the polymer surface, with a threshold coverage being required for purely spontaneous chemical etching to occur, and on the assumption of a photon-induced desorption of volatile products proportional to the oxygen coverage.