Statistical limitations of printing 50 and 80 nm contact holes by EUV lithography

Nanoscale photolithography requires accurate formation of very small resist images using high energy photons and a high sensitivity resist. Historically it has been presumed that the primary technical challenges for design of a photoresist that will image with high accuracy under these conditions are shot noise effects, i.e., statistical variation of overall photon intensity from place to place on the wafer, and line-edge roughness. While these issues have been examined to various degrees, other aspects of the resist response have not received comparable attention. These include the effect of the statistics of the chemical reactions during post-expose bake and dissolution on resist images, and the effect of the fluctuations of the spatial distribution of photons within a printed feature. In order to examine the impact of all of these we have carried out a series of simulations of resist image formation as a function of dose for arrays of 50 and 80 nm contact holes printed with EUV (13.4 nm) radiation using an experimentally validated reaction-diffusion model that mimics ESCAP-class photoresists. The results show that intensity and spatial fluctuations of the light as well as simple variations in the local chemistry all affect contact size. The simulations indicate that the degree of precision contemplated for printed nanoscale features will require an extraordinary level of control over the resist chemistry and processing.