Physics of Spin Casting Dilute Solutions

TL;DR

This paper models the evolution of composition profiles during spin casting of dilute solutions, revealing key physical insights and power laws linking process parameters to outcomes, with relevance for thin film deposition.

Contribution

It introduces a diffusive-advective model for spin casting, highlighting the role of the Sherwood Number and providing quantitative insights into process stages.

Findings

Power laws linking process parameters to composition evolution

Identification of the Sherwood Number as a key process parameter

Analysis applicable to both monolayer and thicker film depositions

Abstract

We analyze the evolution of the vertical composition profile during hydrodynamic-evaporative film thinning as it typically occurs during spin casting mixtures of non-volatile solutes and volatile solvents. We assume that the solvent dominates the hydrodynamic-evaporative film thinning. The internal spatio-temporal evolution of the composition is analyzed with a diffusive-advective approach. The analysis provides transparent physical insights into the influence of the experimental conditions on the evolution of the internal composition. We present power laws that link the process control parameters to the composition evolution, process duration, and final solute coverage. The analysis reveals a characteristic Sherwood Number as fundamental process parameter. It identifies for which stages of the process our analysis is quantitatively relevant and discloses the dominance of either diffusion or evaporation. The analysis is valid for dilute solutions e.g., for the deposition of solute (sub)monolayers. But it is also relevant for the deposition of thicker (polymer) films.