thin, uniform photoresist in positive, negative, lift-off, and polyimide chemistries, ideal when a few microns of resist or blanket surface protection is needed. Spin coating spreads liquid resist across a spinning wafer by centrifugal force. Spin coating begins by holding the wafer on a vacuum chuck and dispensing liquid photoresist onto its center. The wafer is then spun at high speed, and centrifugal force spreads the resist outward into a thin, even film while the solvent begins to evaporate. A soft bake afterward drives off the remaining solvent and stabilizes the film. The final thickness is set primarily by the spin speed and the resist viscosity, which makes spin coating highly repeatable on planar wafers. Centrifugal spreading. Rotation flings the dispensed resist outward into a thin, highly uniform film on a planar wafer, while most of the resist spins off the edge. Spin coating delivers excellent thickness uniformity, a smooth surface, and the highest resolution, which is why it is the standard method for planar lithography. Its limitations appear when the wafer is not flat. Over deep trenches, cavities, or other high topography, the resist thins on raised areas and pools in recesses, and it cannot coat steep sidewalls conformally. Spin coating also produces an edge bead and wastes a large fraction of the dispensed resist, most of which is spun off the wafer. Typical Applications - Standard front side lithography on planar wafers - Thin, uniform resist layers of a few microns - Blanket surface protection - Fine, high-resolution features - Positive, negative, lift-off, and polyimide resists
