Current optical lithography methods are nearing theoretical limits that prevent their use in the production of circuits for future nodes. A proposed solution is to increase the index of refraction of the transmission medium between the final lens of the exposure system and the wafer. When a liquid is used in this lens-wafer gap, the process is known as immersion lithography. A major concern is air bubbles in the liquid, since they are sources of index discontinuities. This article investigates the potential for trapping air as the free surface of the fluid front moves over features associated with wafer topography during the filling and scanning process. Optical simulations have shown that even very small bubbles located near or on the resist can significantly impact the imaging process. Therefore, the ability to predict the characteristics of the flow, liquid, and features that lead to air entrainment during filling is important. Modeling techniques were developed in order to create models that were capable of resolving the flow characteristics over 100 nm scale features without the need to simulate the entire macroscopic flow through the lens-wafer gap. Results of these models show that bubble formation occurs only for extreme geometries and flow conditions. In actual production the velocities, contact angles, and feature profiles are well out of these extreme ranges, and will not cause bubble formation. (C) 2004 American Vacuum Society.