At-resolution stitching with high-reflectivity low-n masks

The semiconductor industry relies on developments in extreme ultraviolet lithography (EUVL) to sustain the continued dimensional scaling of integrated circuits. The recent introduction of the ASML EXE:5000 enables such scaling by facilitating high numerical aperture (High NA) EUVL. To achieve this, the scanner utilizes anamorphic optics that compress the exposure field along the y-axis by a factor of 2. Consequently, stitching of multiple images is required to produce large chips with High NA EUVL. Although at-resolution stitching is demonstrated with Ta-based masks, this is not the case for alternative absorber material masks, such as high-reflectivity low-n masks. Due to high absorber reflectivity, these masks require an additional design solution to enable at-resolution stitching, as discussed in this study. Through the local insertion of sub-resolution gratings (SRGs) in the mask design, we show that high absorber reflectivity can be reduced to the reflectivity level of a Ta-based mask. In a wafer exposure study at 0.33NA, we investigated stitching metrics to demonstrate the insertion of SRGs in the mask design as an effective design solution to suppress low-n absorber reflectivity in the stitching region. Experiments are ongoing to evaluate the current findings through the mirrors of the ASML EXE:5000 scanner for High NA EUVL.