Accelerated testing of high fluence protective coated optics

Excimer lasers are the commonly used light sources for photo-lithography industries; one challenge is to minimize the production interruption by providing a reliable source of DUV (193 nm) photons. This requires CaF₂ optics with high transmission and optical uniformity at 193 nm over a long lifetime. However, CaF₂ crystal can possess defects due to mis-orientation and dislocations in the sub-grain boundaries, which can act as an absorption or scattering center leading to transmission loss and thermal stress induced birefringence in the CaF₂ crystal. In addition, the CaF₂ surface can suffer from different mechanical stress due to cleaning and finishing processes leading to formation of surface imperfections/fractures known as sub-surface damage. This sub-surface damage layer is prone to damage under high fluence 193 nm exposure and can lead to fluorine escape from the CaF₂ lattice. Protective coatings for CaF₂ optics have been developed to prevent surface fluorine depletion from 193 nm exposure. However, these protective coatings can also develop defects due to imperfections in the coating fabrication process and/or photochemical reaction initiated by 193 nm photons in presence of traces of oxygen, water vapor or carbon dioxide. Therefore, to provide uninterrupted 193 nm photons a robust protective coating is required to extend the lifetime of CaF₂ optics. Generally, field learnings for high fluence protective coating can require from 6 months to a year of normal operation and thus validation of protective coatings based on field data alone can hinder the adoption of improved technology. To expedite this selection process, an accelerated 193 nm exposure setup was built to test high fluence protective coatings from different suppliers at various elevated fluences for a short period of time (~2-3 weeks). This setup was successful in screening the best high fluence protective coating under highly accelerated 193 nm exposure. Additionally, based on the relative performance of the protective coatings under accelerated conditions and use case in-laser fluence conditions, the lifetime for the high fluence protective coatings were estimated for the use case scenario.