Effect of developer molecular size on roughness of dissolution front in electron-beam resist

Abstract

This article describes the generation of roughness at the dissolution front of electron-beam positive-tone resist. The effect of a developer solvent molecule on the surface roughness as well as on the dissolution rate is investigated from the viewpoint of the size of a solvent molecule. The relationship between the dissolution rate and solvent molecular size is represented by two straight lines with different slopes in a homologous series of alkyl acetate solvents. A bending point, which corresponds to a critical molecular size, exists between ethyl and propyl acetate. This indicates that the dissolution behavior is largely different between acetates that are larger or smaller than the critical molecular size. The size of a solvent molecule is the dominant factor determining the degree of surface roughness. For a solvent molecule larger than the critical molecular size, the roughness becomes large because polymer aggregates appear on the dissolution front. For a smaller solvent molecule, on the other hand, no aggregates appeared and the dissolution front is flat and smooth. The critical molecular size is about the same as the average size of voids (free volume holes) in resist films. These results indicate that the roughness strongly depends on how a solvent molecule penetrates the resist film through void regions inhomogeneously distributed in the resist polymer matrix due to polymer aggregation.