Rapid at-wavelength inspection of EUV mask blanks by photoresist transfer

We have developed a new technique for at-wavelength inspection of EUV mask blanks. In this technique a thin layer of EUV photoresist is applied directly to a mask blank which is then flood exposed with EUV light and partially developed. We have demonstrated using atomic force microscopy (AFM) that a change in reflectance of only 12% can cause an approximately 50 Angstrom mound in ZEP photoresist. We propose that such mounds could be detected by existing optical inspection tools, and have demonstrated the detection of similar features in ZEP (created using electron beam lithography) by an automated darkfield optical scanning instrument. The greatest advantage of the technique is speed, since both steps (EUV flood exposure and optical inspection) can be done rapidly. Difficulties, such as contamination and resist roughness, are discussed along with possible solutions.