Visualization of a gas flow instability in spin coating systems

Flow visualization in the gas above the surface of a rotating wafer in an industrial spin coating apparatus demonstrates the presence of an Ekman‐like flow instability that adversely affects the uniformity of the dried film. Experiments performed with a 6‐in.‐diam wafer and typical operating conditions show 6–8 spiral vortices around the wafer oriented with negative angle, as is indicative of the type II flow instability predicted by linear stability analysis. The critical Reynolds number for onset is in reasonable agreement with the theory.