Low-loss HR coatings on fused silica substrates for 193 nm micro-lithography applications

High reflective coatings for 193nm wavelength and 45° incidence were developed which combines the advantages of all-oxide and all-fluorides layer stacks. Using plasma-assisted evaporation very smooth and dense Al 2 O 3 / SiO 2 multilayers showing small light scatter were deposited onto fused silica substrates. In the same coating process followed metal fluoride stacks, which could reduce the resulting coating absorption at 193nm. The non-polarized reflectance of combined stacks at 193nm is R>98.5% at 45° and R>98.0% for an angle range of 42°-48°. As the number of fluoride layers could be drastically reduced compared to all-fluoride coatings any formation of micro-cracks could be avoided. The stress of the oxide/fluoride stacks was less than 40MPa.