Study of the oxygen incorporation during growth of large CaF 2-crystals

Abstract

Single crystalline calcium fluoride (CaF 2) is designated as optical material in systems for photolithographic processing of silicon ICs at wavelengths in the deep ultraviolet. The prerequisites for this application are a high transmission and a high resistance with respect to radiation damage under high-intensity laser irradiation. Pure CaF 2 has excellent transmission properties without absorption bands over a wide wavelength range from UV to IR. Selective absorption bands exist in CaF 2 only if the crystal contains impurities. Oxygen is considered as a major impurity in CaF 2. In the present contribution incorporation of oxygen during crystal growth and its influence on the optical properties of CaF 2 is investigated systematically by a specially designed crystal growth facility and processing. A special gas supply system is used to introduce oxygen directly into the CaF 2-melt during crystal growth by a Bridgman-type technique. The concentration of the oxygen in the growth atmosphere and therewith in the crystal was varied over a wide range during growth. The oxygen concentration in the as-grown CaF 2-crystal was determined quantitatively by a special characterization technique called elastic recoil detection (ERD). Concentrations between 5 and 144 ppm were detected. Investigations of the optical properties of the oxygen-doped CaF 2-crystal revealed that the oxygen causes an increase of the absorption in the UV range with a maximum at 197 nm. The relation between the oxygen concentration in the crystal and the absorption coefficient at 197 nm was found to be linear. This calibration relation can now be used in general for the quantitative determination of the oxygen concentration in CaF 2-crystals by absorption measurements at 197 nm. Furthermore, the influence of oxygen on the radiation hardness was examined for X-ray and excimer F 2-laser (157 nm) irradiation.