New thermally crosslinkable electron-beam resists: 3. Prediction of the gel clearing dose

A theoretical description of the electron exposure dose required to solubilize the gel fraction of pre-crosslinked positive resists was first proposed by Suzuki and Ohnishi. These authors proposed that the important factors affecting the sensitivity of pre-crosslinked resists are the weight-average molar mass of the primary polymer (M^°_W), the main-chain scission efficiency and the average number of crosslinked units per weight-average molecule (d). In this paper we re-examine these conclusions and show that, for the practical situation, d and M^°_W should not be regarded as independent variables. Using a functional relationship between d and molar mass, and expressions due to Charlesby for the change in the gel fraction beyond the gel point, equations are developed to predict the critical exposure dose for the cases where degradation occurs (i) exclusively via scission of the crosslinks and (ii) exclusively by scission of main-chain units. Experimental data for pre-crosslinked copolymers of monomethyl itaconate and methyl methacrylate show good agreement with values calculated assuming that degradation of the gel occurs only via main-chain scission. © 1992.