Mask optimization approach for wafer LCDU improvement in ArF lithography

Extreme ultra violet lithography is one of the most promising technologies for next-generation and already applied to critical layers for imaging 7-nm node and beyond. On the other hand, immersion ArF (iArF) lithography also continues to be applied to some critical layers by utilizing Multiple Patterning (MP). High accurate overlay control is required to reduce Edge Placement Error (EPE). In general, global errors on mask such as Critical Dimension Uniformity (CDU) and Image Placement (IP) are known as critical factors affecting EPE. Recently, the local variations on wafer are also discussed as non-negligible factors, especially for advanced technology node. Local CDU (LCDU) is one of the most typical local variations, therefore its requirements are getting more severe. In this paper, the mask impact on wafer LCDU in ArF lithography was investigated. In order to characterize the mask contribution, we designed the mask which has the patterns with various mask LCDU and lithographic performances. According to these evaluations, it was confirmed that mask LCDU, Normalized Image Log Slope (NILS) and Mask Error Enhancement Factor (MEEF) are major contributors to wafer LCDU. Based on the results, we explored wafer LCDU improvement by mask optimization and demonstrated its benefit on wafer.