Predicting pattern transfer distortions during EUV mask fabrication

The stringent requirements placed on sub-100 nm feature lithography require the development of a low distortion mask. The distortions due to the fabrication process of extreme ultraviolet lithography masks have been predicted using finite element methods. The investigation included the simulation of the mask blank fabrication, pattern transfer and mounting. Film stress gradient effects for the multilayer stack, absorber layer and resist layer were also evaluated, considering both in-plane and out-of-plane distortions. To assist in optimizing the final substrate and pattern dimensions, a number of different substrate formats and materials were considered. To determine the effects of pattern density, different types of patterns were modeled, including uniform coverage, a 'half-and-half' pattern and an orthotropic feature pattern. Reticle placement errors were less than 2 nm at the mask level (0.5 nm at the wafer level) for nearly all cases studied.