Estimation of EUV Mask Flatness for Allowable Pattern Shift

摘要

An isolated local non-flatness model was devised to investigate the influence of a non-flat mask on pattern shift. The causes of pattern shift were divided into two types: out-of-plane displacement (OPD) of the mask, and the global height variation (GHV) resulting from OPD. GHV induces a pattern shift that depends on the incident angle of the EUV beam. Reducing the incident angle was found to mitigate pattern shift on a wafer. Although the pattern shift due to OPD is negligible when the P-V flatness is more than 100 mm, the effect of GHV is not negligible. To keep the pattern shift on a mask below 12 nm, the upper limit on the non-flatness of the mask is a P-V flatness of less than 100 nm and a period of more than 300 mm. In addition, the global slope must be less than 1 μrad for an EUV mask substrate.