Subquarter-micron lithography with dual-trench-type alternating PSM

Abstract

Lithographic characteristics of dual-trench type alternating phase-shifting mask (PSM), whose shifters are made of perpendicular trenches with different depth alternately, are evaluated numerically and experimentally. The structure of dual-trench type PSM could reduce the difference of adjacent peak intensities created by topography on the mask. Exposure characteristics of the mask varied with depth of deep and shallow trenches, and depth of both trenches should be controlled so as to have the optimum value. Mainly, the difference in depth of deep and shallow trenches caused varying "effective phase" and depth of shallow trench caused varying "effective transmission". The depth of focus using the mask was sensitive to the effective phase difference controlled by adjusting etched depth difference between both trenches, and insensitive to depth of shallow portion. From analysis of mask process margin, respecting acceptable error of depth of both trenches, it was found that the effective transmission error caused reduction of acceptable depth error.