In KrF or ArF resist processing, chemically amplified resists are widely used for ULSI device fabrication. Decomposition (positive resist) or cross linking (negative resist) is amplified by an acid catalytic reaction during post-exposure baking (PEB). We previously demonstrated PEB and development simulations including T-top forming using a cluster model based upon percolation theory. The cluster formation process and the cell combination rule should be reflected in resist processing conditions and resist synthesis processes such as using some additives. So, we investigated the cell combination rule in cluster formation, varying resist process conditions. Using this cluster model, a new resist process model is developed. Molecular interaction dependence on resist sensitivity is investigated by a cluster formation chart. The strength of the molecular interaction affects resist sensitivity. This model is consistent with the sensitivity changes in Si-resist experiments and dielectric constant calculations of several ArF resists. From the cluster forming chart, we see that the resist matrix becomes gel or sol state near the critical point of development. This simulation result theoretically supports a salting out experiment analysis, and shows a large cluster near the critical point. This fact is verified by AFM surface observations, which are related to resist surface roughness. Consequently, cluster structure is formed due to the molecular interaction, leading to resist dissolution rate features such as sensitivity.