In the photolithographic process, a polymeric photo resist layer is formed on a thin film to be etched and then exposed to actinic radiation through a photo mask, e.g., by contact printing. Actinic radiation renders one portion of the photo resist relatively more soluble, and the other portion relatively less soluble. The more soluble portion of the photo resist is removed, e.g., by solubilization with a suitable solvent, uncovering portions of the thin film. The uncovered portions of the thin film are then removed by etching, leaving behind a facsimile or reverse facsimile of the photo mask pattern. However, variations in the thickness of the applied polymeric photo resist layer generate imperfections in the photo mask pattern. One way to ensure a uniformly thick polymeric photo resist layer is to apply highly viscous resist.
Techniques which have been developed for formation of the film of the photometric photo resist include: meniscus coating (see U.S. Pat. No. 5,270,079 incorporated herein by reference), slot coating (see U.S. Pat. No. 4,696,885 incorporated herein by reference), and patch coating (see U.S. Pat. No. 4,938,994 incorporated herein by reference). However, these methods have many disadvantages; for example, there is excess waste of the photo resist, only substrates of limited size may be accommodated, they produce a film coating of non-uniform thickness, and they produce an edge bead build up at the end of the coating. In particular, the meniscus coating method allows too much solvent evaporation and patch coating is a complicated, unproven technique.
Capillary coating is a superior method that applies a more uniformly thick layer of photo resist to substrates, does not produce an edge bead build up, can handle larger substrate sizes, and does not allow solvent to evaporate. Referring to FIG. 1, a capillary apparatus is shown wherein the substrate (1) is held by a vacuum chuck (2). The vacuum chuck (2) is positioned above a coat head (3) of photo resist. The coat head (3) of photo resist is in fluid communication with a photo resist reservoir (4). The photo resist travels from the reservoir (4) up through the coat head (3) to an orifice at the top of the coat head where it forms a meniscus. The vacuum chuck (2) is then made to bring the substrate (1) into contact with the meniscus. The vacuum chuck (2) then moves the substrate (1) laterally, relative to the coat head (3). As the vacuum chuck (2) is moved horizontally relative to the coat head (3), a layer (5) of photo resist is coated onto the substrate (1). This method provides a uniformly thick layer because the flow of resist onto the substrate is adequately controlled by the capillary action. However, capillary action is only effective for fluids having a viscosity of less than thirty centipoises, because the capillary action is not strong enough to pull highly viscous liquids up through the coat head (3).
Therefore, there is a need for a capillary type method of applying fluids having viscosities of greater than thirty centipoises.