EUV mask blank activities at LETI: defect detection at 80 nm

In this paper, defect counting results are presented both on silicon substrates and on EUV mask blanks with their equivalent PSL size distribution. The measurements are achieved with our counting defect prototype COMNET but also with a commercial tool. Improvements of COMNET have been performed to reach sensitivities (PSL equivalent size on silicon) of 80 nm on silicon substrate, 90 nm on EUV mask blank and 125 nm on glass substrate. A first analysis about COMNET repeatability is presented and the impacts of known errors on defect number and size values are evaluated. Typical counting data measured on a EUV mask blank having an added defect density inferior to 1 def/cm2 (defect size superior to 90 nm) are presented. The analysis of these data shows two main populations of added defects which are characteristic of the deposition process: one close to 100 nm and the other in the micron range. Each group represents half of the total number of added defects. The origins of these defects are finally discussed on the basis of specific experiments chosen to highlight the possible role of the different process steps.