Generation of optical absorption bands in CaF2 single crystals by ArF excimer laser irradiation: Effect of yttrium impurity

Abstract

Generation of optical absorption bands was observed in CaF2 single crystals with different Y impurity concentrations, excimer grade (yttrium impurity; <1 ppm) and UV grade (5 ppm), by irradiation with ArF excimer laser light at room temperature. The transmittance at the wavelength of 193 nm gradually decreased from 100% down to a saturation level of ∼95% for the UV grade specimen during the ArF excimer laser irradiation. This degradation process was found to obey first order kinetics. On the other hand, no significant decrease in transmittance was observed for the excimer grade specimen. “Four-band structure” which was a characteristic of Y-associated F centers (YFCs) and a band peaking at ∼6.6 eV were observed in the spectrum of the irradiated UV grade CaF2. The 6.6 eV band, which substantially controlled transmission loss at the wavelength of 193 nm and was generated via one photon process, was tentatively attributed to YFCs from growth kinetics on the laser irradiation and decay kinetics upon annealing. Neither intrinsic F centers nor H centers were observed by the ArF laser light irradiation at room temperature. These results demonstrated that a trace amount of Y impurity enhanced the formation of F centers perturbed by a nearest Y impurity and that an absorption band of the resulting YFCs controlled the transmission loss at the wavelength of 193 nm.

Irradiation Damage of CaF2 with Different Yttrium Concentrations under 193 nm Laser

“…Since the yttrium ions has low-lying s and d valence orbitals which overlap the F-center wave functions, Orbital hybridization as shown in Fig. 3(b) occurs [21] , which leads to the emergence of color center absorption peaks. In addition, since the F center has a more robust localized structure and symmetry than the H center, the diffusion energy barrier of the F center is much higher than that of the H center in the CaF2 crystal [6] .…”

“…Among the impurity ions, the Y ion was the main impurity element, Y is easily incorporated into CaF2 lattice due to very similar ionic radius and chemical properties to Ca. Mizuguchi and coworkers [21] have shown that the damage resistance of CaF2 crystals doped with only 5×10 -6 of Y decreased strongly comparing to pure CaF2 crystals (Y content 1×10 -6 ). All these studies show that impurity ions promote the formation of calcium colloids.…”

Vacuum ultraviolet spectroscopy of Ce3+-doped SrMgF4with superlattice structure

“…Schematically, the photochemical reaction [19] may be described by PC + RE 3+ ⇐⇒ Vis,kT + RE 2+ The PC + centre ionized by UV irradiation was identified as an F A centre (an electron trapped at a fluorine vacancy associated with a nearest neighbour RE 3+ ion) by the ESR technique [20]. Recently, generation of the same absorption bands in CaF 2 containing Y 3+ (∼5 ppm) by ArF excimer laser irradiation was reported [23]. As ESR signals from the F A centre were observed simultaneously, the centre corresponding to the induced absorption bands seems to be the F A centre.…”