Immersion lithography has been proposed as a method for improving optical lithography resolution to 50 nm. The premise behind the concept is to increase the index of refraction in the space between the lens and wafer by insertion of a high refractive index liquid in place of the low refractive index air that currently fills the gap. Because the liquid will act as a lens component during the lithographic process, it must maintain high uniform optical quality. One source of optical degradation may be due to changes in the liquid's index of refraction caused by a change in temperature. During the exposure process, energy is deposited onto the wafer, causing a rise in temperature. Consequently, any liquid in direct contact with elevated temperature portions of the wafer will also experience an increase in temperature. Two-dimensional computational fluid dynamics models were created to assess the thermal and fluid effects of the exposure process on the liquid temperature. This article presents the results of the numerical thermal and flow simulations. Both aligned and opposing flow directions were investigated for a range of inlet pressures that is consistent with what can be expected with active filling jets. (C) 2003 American Vacuum Society.