Transparent and visible light-insensitive acrylic photoresist for negative tone optical lithography

The study presented in this paper describes an optimized negative tone optical resist based on ultraviolet (UV)-induced cross-linking of polymethyl methacrylate (PMMA). This simple resist is made by dissolving PMMA and a photo-cross-linking agent—Irgacure 379, in ethyl lactate. Irgacure cross-links PMMA when exposed to i-line radiation (365 nm wavelength). However, as it does not absorb at longer wavelengths, this resist is in-sensitive to visible light. The outstanding optical transparency and high resolution capability of PMMA make this resist system useful for many applications. While PMMA has low resistance to erosion during dry etching processes, this resist allows plasma erosion resistance to be enhanced by a postpatterning blanket UV exposure process. Furthermore, being based on PMMA and similar acrylic polymers that are amenable to patterning through electron beam lithography, this resist system can also be used for processes that combine optical and electron beam lithography on the same resist film.