Improvement of total quality on EUV mask blanks toward volume production

Abstract

Total quality on EUV mask blanks have to be improved toward future volume production. In this paper, progress in EUV blank development and improvement in flatness, bow and ML blank defects as critical issues on EUV blanks were reported. Steadily progress in flatness improvement was made in the past five years by improving polishing processes. A LTE substrate with a high flatness of 78 nm PV in 142 mm square area was achieved in average. Annealing process was developed to make small bow of less than 600 nm after ML coating. It was confirmed that annealed ML blank has stable performance in bow and centroid wavelength values through mask making process. Small bow of less than 300 nm was successfully demonstrated using annealing process and a CrN back side film with high compressive stress. Low defects of 0.05 defects/cm² at 70 nm SiO₂ sensitivity inspected by a Lasertec M1350 was demonstrated on a multilayer (ML) blank with a LTE substrate as best. Small defects over 50 nm in a M7360 were effectively reduced by improvement of polishing process consisting of local polish, touch polish and cleaning.