New phase shifting mask with self-aligned phase shifters for a quarter micron photolithography

In order to markedly improve the resolution of photolithography without improving the resolution of exposure systems, the authors propose a simple and effective phase shifting mask technology. The mask has self-aligned phase shifters which do not require assistant patterns and or complicated design of the phase shifter patterns, which are essential to the conventional phase shifting mask. The mask with a phase shifter size of 0.5 μm reduces the width of photointensity to 60% of that without phase shifters, while keeping high contrasts. The authors have fabricated the phase shifting mask and obtained 0.2-μm line resist patterns with a high-contrast resist profile by a KrF excimer laser stepper with resolution capability of 0.4 μm. The proposed phase shifting mask method is extremely attractive for a future ULSI lithography tool in 256-Mb DRAM (dynamic RAM) and beyond.