High NA EUV stitching: mask performance is a key

High NA EUV lithography has become a reality. The high NA EUV scanner (EXE:5000) produces exposure fields of 26x16.5 mm² which is twice smaller than standard fields on other scanners. For certain use cases (e.g. when a die is larger than the High NA field) stitching between two exposure fields might be required. Stitching of vertical lines across two exposure fields has already been demonstrated in several publications. In this publication, we pay attention to photomask related aspects of stitching which are multifold. We draw attention to the need for mask resolution enhancement which will enable advanced OPC at stitching. We will show stitching behavior on both Tantalum and low-n masks and demonstrate low-n absorber reflectivity suppression by means of sub-resolution gratings which is required for stitching. We explore the behavior of the exposure field black border (BB) edge and formulate recommendations for specifications on BB edge control as well as pattern placement and pattern fidelity at the black border. Finally, we conclude that the mask performance is a key enabler of High NA stitching.