Tritone PSM and its performance

The halftone phase shift mask (PSM) gives a significant improvement on DOF for hole pattern printing on wafer. However, we need to be careful when we use it on other patterns such as line and space or patterns surrounded by bright field, because sub-peak effect could cause deterioration of those patterns when they are printed on wafer. Toppan presents a new structure in which Cr film is partially placed on the half tone surface. With this structure, so called Tri-tone PSM, we can block harmful light transmission for non-hole patterns, while keeping the full characteristics of PSM effect on targeted patterns. This structure could also prevent the line shortening problem. Using this structure we can get good printability on smaller geometry without using optical proximity correction (OPC) patterns (serif, etc.). Of course this method can offer a perfect shielding of the opaque ring which prevent the wafer damage caused by light leakage. By using this Tri-tone structure, the application of half tone PSM, which is so far limited to hole patterns, would be extended to line patterns.