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The thickness uniformity of a spin‐cast film is governed by the air flow through the spin coater, particularly the boundary layer flow above the surface of the spinning wafer, which controls solvent evaporation from the dry film. Laser Doppler velocimetry (LDV) and hot wire anemometry (HWA) are used to map the flow field throughout an industrial spin coater and to study flow instabilities in the boundary layer for various combinations of wafer spin speed and exhaust flow rate. The flow field measured by LDV compares well with a numerical simulation of laminar, axisymmetric, and steady air flow throughout the coating bowl. However, Ekman spiral flow instabilities of both type I (positive spiral angle) and type II (negative spiral angle) were found by HWA in the boundary layer near the surface of the spinning wafer. The type‐II spirals form at Reynolds number in the range 2000–2500 and the type‐I spirals form at Reynolds number in the range 80 000–85 000. It is the type‐II spirals that are responsible for disrupting the air flow in the boundary layer flow and that cause nonuniform drying of spin‐cast films. © 1995 American Institute of Physics.