Modeling of ultra thin resist film structure after spin-coating and post-application bake

Film thinning during spin-coating can be split into two stages. At first, film thickness is controlled by the hydrodynamic flow of the resist during spinning before solvent evaporation becomes the dominant thinning process. Modeling solvent evaporation is rather simple if one assumes a constant diffusion coefficient of the solvent in the resist film; however such a treatment is incomplete. This paper presents a model of the solvent evaporation during the spinning and the post-application bake (PAB) stages, taking into account a non-constant solvent diffusion coefficient and the variation of the fraction of free volume in the film. In particular, the temporal evolution of the film thickness during spinning and PAB is simulated. After spin-coating and PAB, evidence of gradients in the species concentration is shown. A dry skin is formed at the free surface of the resist and a non-uniform concentration depth profile of the film is expected.