Laser damage of calcium fluoride by ArF excimer laser irradiation

Abstract

Artificially grown excimer grade calcium fluoride is one of key optical materials used in microlithography applications. Such calcium fluoride is required for optical components requiring high laser durability and laser induced bulk damage threshold (LIDT). The mechanical properties of calcium fluoride can vary depending on the crystal axis, <111>, <110> and <100>. For example, material hardness is highest in the {100} crystal orientation. Furthermore, it is also known to cleave in the {111} plane. Therefore there is a possibility of a property that originates in such a crystal structure that influences LIDT. In this study, we investigated the relationship between crystal structure, laser durability and LIDT. The influence in the relation between the polarization plane of the ArF excimer laser and the crystal orientation of calcium fluoride in regards to LIDT was examined. The samples were all prepared from the same CaF2 crystal with optical axis's of <111>, <110> and <100>. The azimuth of the samples was measured by the reflection Laue method. For the experiment, the samples were rotated to the polarization plane of the ArF excimer laser, and the change in the number of irradiation pulses that damage was observed and measured. As a result, we determined the position of the crystal orientation of the calcium fluoride relative to the polarization plane of the ArF excimer laser that produced the highest LIDT.