Resolution enhancement for lensless mask metrology with RESCAN

The EUV photomask is a key component of the lithography process for semiconductor manufacturing. A critical defect on the mask could be replicated on several wafers, causing a significant production yield reduction. For this reason, actinic patterned mask inspection is an important metrology component for EUV lithography. The RESCAN microscope is a lensless imaging platform dedicated to EUV mask defect inspection and metrology. The resolution of the tool is about 35 nm, which is similar to that of state-of-the-art EUV microscopes. To improve the resolution of RESCAN, we designed an upgraded optical layout for the illumination system and we developed a coherent diffraction imaging-compatible method to synthesize a custom pupil structure. This new scheme will enable a lensless EUV microscope with a resolution down to 20 nm and thereby allow mask review capabilities for future technology nodes with EUV lithography.