Study of haze in artificially grown single crystal CaF2

Abstract

The artificially grown calcium fluoride is used as materials of the optics such as the lenses of the illumination optical system and the projection optical system of the lithography equipment that use the sources of light such as excimer lasers. Such calcium fluoride is required high transmittance. However, there are very small scatterers and absorbers inside the crystal and they cause degradation of transmittance. In this study, we examined these defects and clarified the process how they occur. Haze is characteristic optical defect in the artificially grown calcium fluoride. It is thought that haze is an aggregation of very small scatterers and this scatterer is void or calcium oxide crystal. When we irradiate the light into a crystal with much haze, the path of the light looks white. However, we were not able to clarify neither the structure nor components of haze. First, we examined how the scatterers were distributed by an infrared tomography method. The result pointed out that the scatterers were located along sub-grain boundary and dislocation network. We prepared a surface sample for TEM (Transmission Electron Microscopy) with FIB (Focused Ion Beam) from the point where it seemed that the scatterers were located in the dislocation network, and observed it with TEM and analyzed grain boundary region and the grain inside with EDS (Energy Dispersive X-ray Spectroscopy). From the EDS spectrum of the grain boundary region, a very small amount of oxygen was detected, but no oxygen was detected from the grain inside. This suggests that oxygen is located in the grain boundary. From these results, it is suggested that scatterers of haze are made of oxygen voids or calcium oxides crystals.