Influence of molecular weight of resist polymers on surface roughness and line-edge roughness

The effect of molecular weight on the roughness of the dissolution front (surface roughness) and line-edge roughness (LER) was investigated in a chain-scission-type of positive-tone electron-beam resist. Surface roughness and LER were measured by directly observing the surface of flood-exposed resists after development and the sidewall of patterns, respectively, with an atomic force microscope. It was clarified that the molecular weight dependencies of surface roughness and LER are quite different. Surface roughness was found to increase with molecular weight. This is because the size of aggregates protruding from the surface as a result of the development process becomes larger as the molecular weight increases. In contrast, LER was found to be larger in a low-molecular-weight resist than in a high-molecular-weight resist. Moreover, the dependence of LER on the latent-image profile is different for low- and high-molecular-weight resists. For low-molecular-weight resist, LER is independent of the steepness of the latent image; whereas, for high-molecular-weight resist, it increases rapidly as the steepness decreases. These results can be explained by the relationship between the size of aggregates and the width of the transition zone between low- and high-dose regions of the latent-image profile. This strongly suggests that low-molecular-weight resists are not necessarily advantageous in reducing LER.