Contamination transport in immersion lithography

In immersion lithography at 193 nm, water is inserted between a resist-coated wafer and the final lens element to improve resolution and depth of focus. Experiments have shown that some chemicals in the resist, particularly the photoacid generators, are soluble and therefore will leach out of the resist layer when exposed to water. Diffusion of this contamination across the lens-wafer gap may, over time, build up on the lens and therefore degrade the performance of the tool. We present models that describe the transport of contaminants in the under-lens region of an immersion tool. The mass flux of contaminants onto the lens is quantified for a reasonable range of parameters under various 2-D steady-state and transient flow conditions. A critical mass flux is estimated to provide a context for interpreting these results; the critical mass flux is defined as the level of mass flux that might, over a period of one year, result in a layer of contamination that is sufficiently thick so as to affect the optical transmission of the system. (c) 2006 Society of Photo-Optical Instrumentation Engineers.