Microstructure and radiation interactions of optical interference coatings for 193 nm applications

In this paper, we report on our investigations of radiation induced processes in optical interference coatings for 193 nm applications with respect to the microstructure of the coating. Experimental studies revealed that fluoride coatings contribute the main source for radiation induced optical changes during its exposure to 193 nm laser irradiation due to their porous microstructure. NIR spectroscopy could identify the origin of optical changes in interference coatings as a reversible hydrocarbon contamination which occurs within the coatings from storage in air atmosphere. Additionally, Laser Induced Damage Threshold measurements show a direct influence of the hydrocarbon contamination on the radiation durability of the multilayer systems during laser exposure. Experiments were carried out by using several characterization techniques including DUV spectrophotometry, ATR-IR-spectroscopy, x-ray diffractometry, and the determination of the '1-on-1' laser induced damage threshold. Test m ethods were applied to DUV coatings before and after exposure to 193 nm radiation with irradiation doses of up to 108 laser pulses at a fluence of 70mJ/cm2, Test samples consisted of several coating designs, primarily of high reflective multilayer systems.