The photoresist (PR) thermal flow process is to enhance the resolution of lithography for the patterning of sub-200 nmdiameter contact holes as the required critical dimension (CD) approaches the physical limits of conventional lithography. It is difficult to predict the results of the application of this process, so we develop a new viscous PR flow model, which is verified to be applicable for various PRs from experimental results and is applied to the optimization of the layout design and process parameters. Using the model and simulation, we demonstrate the close agreement between the predicted and obtained results using with vertical scanning electron microscope (VSEM) to study the top corner rounding profile of a PR, and present a method for predicting the effect of changes in the dominant variables such as contact diameter, surrounding bulk density, and temperature. This model is also integrated with a lithography simulator. The layout design and process conditions for patterns with various contact diameters are optimized using our new methodology. The viscous flow model linked to the lithography simulator can be effectively used for predicting the contact patterning process and optimizing the layout, as well as, for analyzing defects.