Calcium fluoride crystal (CaF2) is an excellent optical crystal material, with a transmission range covering ultraviolet, visible, and infrared bands. It has a very high transmission rate in the range of 0.13-10 μ m, and also has good thermal mechanical properties, stable physical and chemical properties, and strong resistance to radiation damage. In recent years, with the gradual development of lithography technology towards shortwave light sources, excimer laser sources represented by ultraviolet light such as 193 nm and 248 nm have gradually become the mainstream light source of lithography technology. Calcium fluoride crystals are believed to replace quartz crystals as the main optical components of lithography machine light sources. Therefore, researchers have focused their research on calcium fluoride crystals mainly on growing larger diameter single crystals, reducing stress birefringence through annealing, and improving optical uniformity.
UM OPTICS has long been committed to the growth and processing of fluoride optical crystals, as well as research on the performance of related components. It has broken through the bottleneck of the growth process of calcium fluoride crystals and achieved the growth of 8-inch<111>and<100>crystal oriented calcium fluoride single crystals using the Bridgman method.
The 8-inch calcium fluoride single crystal obtained by the research team has a complete appearance and no macroscopic defects such as cracking and scattering. After directional cutting, a transparent cylindrical crystal blank with a diameter of 40 mm x 6 mm is obtained. The blank sample is subjected to secondary annealing treatment and then ground and polished to obtain the final sample. The series of samples were tested for UV visible transmittance, optical uniformity, and stress birefringence. The results showed that the crystal transmittance reached 90% at a wavelength of 200 nm, the average stress birefringence was less than 0.5 nm/cm, and the optical uniformity reached 2.63 × 10-6. The above results reflect that the calcium fluoride element annealed in three temperature zones has excellent stress birefringence and optical uniformity, and can be used as a window material for excimer lasers.
