Abstract:

Radiation resistance of CaF₂ crystal is one of the critical properties in the application of deep ultraviolet lithography, but the damage process under 193 nm laser irradiation is still unclear. This paper reports the damage behavior of CaF₂ crystals under 193 nm laser irradiation and the key defect factors affecting the damage. Through the 193 nm laser irradiation experiment, it is found that the crystal damage is mainly manifested as the radiation-induced color centers inside the crystal and the radiation-induced damage pits on the surface. Irradiation-induced color centers were analyzed by UV-visible spectrophotometer, and linear fitting was performed between absorption coefficients of different color centers and Y impurity contents. The results show that Y ion has a low-order orbit that overlaps with the F center structure wave function, and hybridizes to form a stable structure. There is a linear relationship between Y ions contents and intrinsic color centers of CaF₂ crystals, confirming that Y element is the key impurity ion affecting the formation of color centers. Energy dispersive X-ray spectrometer (EDS) results show that the content of calcium in the damage pits increases and the content of fluorine decreases, which confirms that the diffusion of H centers and the aggregation of F centers lead to irradiation damage. Electron backscatter diffraction (EBSD) results show that surface irradiation damage occurs preferentially at dislocations. Therefore, reducing the impurity content and dislocation density is an important way to improve the anti-irradiation damage performance of calcium fluoride crystals under 193 nm laser.

Key words: CaF₂ crystal, color center, laser irradiation damage, impurity content, dislocation density