Controlled contamination of optics under 157-nm laser irradiation

Contamination rates of CaF2 substrates in the presence of trace levels of toluene vapor and oxygen under 157-nm irradiation have been studied to determine conditions which prevent contamination films from depositing on optical elements in lithographic projection systems. A 2-3 monolayer thick deposit, causing a 1-2% transmission drop per surface, can readily form over a range of contaminant levels in the sub-ppm range and typical background oxygen levels. In addition, stable partial surface coverage can be supported with either lower concentrations of contaminant or conversely much higher levels of oxygen. Contamination rates are also higher at lower fluences, and thus contamination effects are expected to impact the projection optics more severely than beam delivery and illumination components. Finally, a permanent degradation in transmission of coated optics has been observed on anti-reflective coatings exposed to sub-ppm levels of toluene. Taken together, the results suggest that even with hydrocarbon based contaminants, where oxygen can be introduced into the beam-line in trace levels (i.e., hundreds of ppb) without significantly degrading transmission, toluene contaminant levels will have to be maintained in the ppb range or below.