Substrate Effect on the Vertical Density Profile of Thin Photoresist Film

Vertical electron density (ED) profiles of photoresist film on three different substrates (bare, HMDS-primed, underlayer-coated Si wafer) were obtained using X-ray reflectivity analysis. Small amounts of density fluctuation at each depth of film were analyzed using distorted wave Born approximation fitting. Near the interface, the ED profiles of pure resin on three different substrates differed among substrates. In the bare Si wafer, ED decreased near the interface, whereas ED in the underlayer-coated Si wafer ED was homogeneous. These ED distributions near the interface were strongly dependent on the interfacial energy between resin and substrate. The approximate distributions of photoacid generators (PAGs) were obtained from the comparison of the ED distributions of pure resin and of photoresist containing PAG. When PAG was introduced, the ED profiles showed comparatively uniform increase. However, near the interface, the bare Si wafer showed a large depletion in ED, whereas the underlayer-coated Si wafer showed homogeneous ED. PAG distribution inside photoresist film was affected by both the ED distribution of pure resin and by the interaction between PAG and the substrate.