Effect of the duty ratio of line and space in phase-shifting lithography

This article discusses the impact of the line and space duty ratio and optical coherence on image quality for i‐line projection systems used in conjunction with alternated type phase shifters on projection masks. With a negative process, it was found that the alternated phase‐shifting method improve the depth of focus (DOF) for space patterns with a width smaller than the linewidth. On the contrary, there was no effect for narrow line patterns when the space width was larger than twice the linewidth. It was also found that the DOF became larger when the coherence became higher using both the alternated and the conventional mask for line patterns with a width smaller than the space width. It is concluded that high degree of optical coherence must be chosen for the phase‐shifting method. The alternated phase‐shifting method was applied to a bit line layer of 64 Mbit DRAM, and a DOF of 1.2 μm was obtained for both memory cells and peripheral patterns in which the minimum feature size is 0.35 μm. It is suggested that a negative i‐line resist with a large focus latitude at a high degree of optical coherence will be indispensable in applying phase‐shifting lithography to device fabrication.