The air flow distribution in a spin coater has a critical effect on film thickness uniformity. The photoresist liquid contains a volatile solvent that evaporates during the spin coating process, and the final film thickness uniformity is determined by material spin-off and solvent evaporation. The solvent evaporation rate depends on the difference in partial pressures between the solvent in the air boundary layer next to the film surface and the bulk air over the surface, and on convective transport of air flowing over the wafer. If air flow near the wafer surface is different from the ideal flow induced by an infinite spinning disk, then the mass transfer coefficient at the wafer surface will change with radial position and the resist film thickness will not be radially uniform.