Top surface imaging provides one process alternative for 193 nm lithography and is an important aspect of the EUV imaging strategy. This paper describes an effort to characterize the fundamental mechanisms underlying the silylation process. The extent of silylation has been measured as a function of exposure dose using FTIR spectroscopy. These studies demonstrate that for certain polymers, the extent of silylation has a nonlinear dependence on exposure dose. Simulation modeling has demonstrated how this nonlinearity may contribute to improved contrast. The influence of resist optical density on the shape of the silylated image has also been simulated, and these results have helped guide the design of polymers with the optimal optical density. The TSI processes function on the basis of photogenerated differences in the reactive transport of the silylating agent into and through the resist film. We are exploring alternative mechanisms for photo-induced changes in the permeability of the silylating agent. The results of these studies will be presented.