Recent performance of EUV mask blanks with low thermal expansion glass substrates

摘要

A high flatness of 50 nm, zero defects at more than a size of 30 nm and a high reflectivity of more than 66% for extreme ultraviolet (EUV) light are critical issues related to EUV mask blanks. In this paper, progress on these issues and the recent performance of EUV blanks is reported. Steady progress in defect reduction was achieved in the past six years by improving fabrication processes. When inspected by a Lasertec M1350, defect quality as low as 0.02 defects/cm~2 at 70-nm sensitivity was demonstrated on a multilayer (ML) blank with a quartz (QZ) substrate. A QZ substrate with a high flatness of around 90 nm peak-to-valley (P-V) on both sides and a high defect quality of 0.006 defects/cm~2 at 60-nm sensitivity was obtained using a newly developed polishing process consisting of local polishing, touch polishing and cleaning. The cleaning process was developed for low thermal expansion (LTE) glass to reduce the defects associated with it. Using the cleaning process, the ULE~(TM) substrates showed defectivity similar to the QZ substrates. An average flatness of 117 nm P-V, and best flatness of 84 nm P-V on the front side and 56 nm P-V on the back side were obtained on ULE substrates using the new polishing process. Multilayer (ML) blanks with a high defect quality of 0.08 defects/cm~2 at 80-nm sensitivity were produced on a ULE substrate. The ML blanks, consisting of 50 bi layers, have high peak reflectivity of more than 66% and excellent uniformity of less than 0.04 nm in centroid wavelength, which meets the desired specifications.