Reduction of edge beading defects on rectangular substrates using a rotation about an axis perpendicular to the spin axis.

This article focuses on the coating dynamics of a binary liquid film on a rectangular substrate that simultaneously spins and rotates about perpendicular axes, a process relevant to spin coating in industries such as micro-electromechanical systems. The study derives lubrication-based governing equations incorporating surface tension, body forces from spinning and rotation, viscosity changes, and evaporation effects, which are solved numerically using an alternating direction implicit (ADI) method. Numerical simulations demonstrate that substrate rotation induces elevated gravity, enhancing solvent evaporation and viscosity, which significantly reduces edge beading defects—thicker film accumulation near substrate edges—compared to conventional spinning, especially for thinner coatings. Additionally, surface tension gradients during the drying phase further mitigate edge beading, particularly in corner regions. The findings suggest that combining rotation with appropriate coating liquids can improve film uniformity by minimizing edge defects in spin-coated rectangular substrates.