Controlled contamination studies in 193-nm immersion lithography

Abstract

In liquid immersion lithography the last optical element is in intimate contact with the liquid for extended periods of time, and therefore is at risk of being contaminated by impurities in the liquid. The purity of the liquid must be kept under stringent control compared to "dry" lithography, since the density of liquid is ~ 1000 times higher than that of gas. Thus, 1 part per billion contaminant in the liquid may have an equivalent effect on the optics to 1 part per million in gas. The risk is that the combination of high contaminant density, short wavelength, and large laser dose will conspire to contaminate the optics, change its transmission, and possibly cause increased flare. In order to clarify the potential for such effects, we have begun a set of experiments with controlled contamination. In these studies, a 193-nm laser irradiates a sample in the presence of flowing clean water into which controlled amounts of contaminant have been injected. The sample is either bare fused silica or calcium fluoride protected with thin films. Results will be presented with organic contaminants such as isopropanol and acetone. These results will include an analysis on the implications for controlling water purity.