Understanding Molecular Level Effects during Post Exposure Processing

The perpetual advancement of materials and equipment for microlithography has resulted in reduction of critical dimensions to scales approaching the size of the molecules that constitute a photoresist. As a result, molecular scale effects such as line edge roughness have become an increasing concern for resist manufacturers and process engineers alike. Computer simulation of lithography has become an integral tool for both process optimization and development of new technologies. However, these simulation tools are generally based upon a continuum approximation of the resist material, and are therefore unable to investigate molecular level variations. In this work we investigate the increasing importance of molecular level effects, especially in terms of the contributions of the post exposure bake (PEB) to feature roughness. A linkage has been made between a previously reported mesoscale simulation of the post exposure bake (Schmid et al, Proc. SPIE – Int. Soc. Opt. Eng., 3999) and PROLITH® 7.0 (KLA Tencor, Corp.). The mesoscale simulation models discrete transport and reaction events during the post exposure bake to determine solubility variations on the scale of a single oligomeric chain. These solubility variations are then imported into PROLITH and transformed into photoresist topography using the familiar Mack dissolution model. This method has been used to simulate line-edge formation in an APEX®-type resist. It is found that the distribution of photoproducts produced during exposure can lead to significant solubility variations during the PEB. These solubility variations can become manifest as roughness of resist feature topography.