Phase controllability improvement for alternating phase shift mask

An alternating phase shift mask can improve resolution and lithographic latitude. However, Alt-PSMs have not yet become practical because of difficulty in their tight phase and defect control. In this paper, we focused on how to control both phase uniformity and phase mean value of etched quartz shifters. We found that a material of a dry-etching table (cover plate of work electrode) was strongly affected for phase uniformity. By choosing an adequate material, phase uniformity of 1.9% could be achieved. Micro-loading effect and loading effect degrade phase controllability. Loading effect was not observed in our etching conditions. But micro-loading effect was observed. Back exposure process was useful to prevent micro-loading effect. To improve mean value controllability, 2 step etching process was adopted. By using this method, mean Value could be controlled within +/- 2 degrees.