Process latitude measurements and their implications for CD control in EUV lithography

We have exposed 10 wafers on the Engineering Test Stand (ETS), the 0.1 NA EUV scanner at Sandia National Laboratories in Livermore, CA. The EUV reflective mask was fabricated in-house using a Ta-based absorber stack on Mo/Si multilayers. The printed wafers contained different line sizes and pitches, line-end shortening measurement structures, contact holes, and patterns for estimating absorber defect printability. The depths of focus of each feature are typically 2 mum due to the small NA of the scanner, and these should decrease by at least a factor of 6.25 as the NA's increase to 0.25. The data from measurements of line size through pitch and line-end shortening test structures indicate that both 1D and 2D optical proximity correction will be required. Defects that are either notches in or protrusions from absorber lines are the first to print, and they begin to print when they reach approximately 15 nm (1X) in size. This size threshold is in accordance with the 2003 ITRS specifications. We also report the first printing of SRAM bitcells with EUV lithography.