SINGLE CRYSTAL OF MAGNESIUM FLUORIDE, OPTICAL MEMBER AND OPTICAL ELEMENT COMPRISING THE SAME

摘要

A single crystal of magnesium fluoride having a large diameter and excellent optical properties such as internal transmittance and long term laser durability, and suited for use as optical elements for exposing apparatus. The single crystal of magnesium fluoride is of a cylindrical shape having a straight body portion of a diameter of not smaller than 10 cm, has an internal transmittance of at least 85%/cm at 120 nm and at least 98%/cm at 193 nm and has, desirably, an induced absorption of not larger than 0.0030 absorption/cm at 255 nm and, particularly desirably, not larger than 0.0010 absorption/cm. at 255 nm immediately after the irradiation with 2 million shorts of an ArF excimer laser of an energy density of 30 mJ/cm2 and 2000 Hz. The invention further provides an optical element for optical lithography comprising the single crystal and an optical member for vacuum ultraviolet ray transmission comprising the single crystal.