Evaluation of the effect of mask-blank flatness on CDU and DOF in high-NA systems

The purpose of paper is to investigate the impact of mask blank flatness on critical dimension uniformity (CDU) and depth of focus (DOF) in the wafer printing process with a test pattern designed for 65nm node technology. In this experiment we use 3 test masks with different flatness (0.3T, 0.5T and 1T), and the same test pattern array. The mask flatness was measured with a Tropel® UltraFlat TM 200, and the focus error is extracted from the CD data of the focus and energy matrix (FEM) analysis. The goal of the study is to quantify the mask flatness influence on the high-numerical aperture (NA) lithographic process.