The present invention relates to the formation of semiconductor devices.
During semiconductor wafer processing, features of the semiconductor device are defined in the wafer using well-known patterning and etching processes. In these processes, a photoresist (PR) material is deposited on the wafer and then is exposed to light filtered by a reticle. The reticle is generally a glass plate that is patterned with exemplary feature geometries that block light from propagating through the reticle.
After passing through the reticle, the light contacts the surface of the photoresist material. The light changes the chemical composition of the photoresist material such that a developer can remove a portion of the photoresist material. In the case of positive photoresist materials, the exposed regions are removed, and in the case of negative photoresist materials, the unexposed regions are removed. Thereafter, the wafer is etched to remove the underlying material from the areas that are no longer protected by the photoresist material, and thereby define the desired features in the wafer.
Various generations of photoresist are known. Deep ultra violet (DUV) photoresist is exposed by 248 nm light. To facilitate understanding, FIG. 1A is a schematic cross-sectional view of a photoresist mask 112 over a silicon etch layer 108. The photoresist mask 112 has mask openings 122. The silicon etch layer 108 may be over a substrate 104 with one or more layers disposed between, or the silicon etch layer may be the silicon substrate. FIG. 1B is a schematic cross-sectional view of the photoresist mask 112 and silicon etch layer 108, after features have been etched into the silicon etch layer 108. The etch process causes mask undercut 116, which causes the resulting silicon lines to be thinner than the original mask. It has been found that the deeper the etch, the more severe the undercut.