EUV vote-taking lithography for mitigation of printing mask defects, CDU improvement, and stochastic failure reduction

摘要

Vote-taking lithography is a method for mitigating mask defects, which has been applied in the 1980s to enhance yield. Vote-taking sums up N different mask images with identical content, each at 1/N dose, to mitigate the defects on each individual mask. The fundamental assumption is that the mask defects do not correlate in position from mask to mask, and so each individual defect will be blended with good images from the other N - 1 masks. Vote-taking has recently been reconsidered for extreme ultraviolet (EUV) lithography, where it might provide a temporary solution for situations in which the detectivity conditions are not yet meeting expectations. This paper provides a thorough experimental assessment of the implementation of vote-taking and discusses its pro's and con's. Based on N = 4 vote-taking, we demonstrate the capability to mitigate different types of mask defects. We found additional benefits of blending different mask images, distinct from mask defect reduction. Experimental results will be shown that demonstrate improved critical dimension uniformity (CDU), both local CDU and intrafield CDU, reduced overlay errors, and smaller stochastic defect levels. Finally, we perform dedicated throughput calculations based on the qualification performance of ASML's NXE:3400B scanner. This work must be seen in the light of an open-minded search for options to optimally enable and implement EUV lithography. Although defect-free masks and EUV pellicles are without argument essential for most of the applications, we investigate whether some applications could benefit from vote-taking.