Control of oxygen contamination during the growth of optical calcium fluoride and calcium strontium fluoride crystals

摘要

The progress, current state, and prognosis for CaF_2 as an optical material with wide applicability are reviewed by outlining the basic issues: technology, demand, supply, and metrology. To achieve perfect quality for all application grades of CaF_2 grown using a natural fluorite, investigations of the effect of PbF_2 and ZnF_2 scavengers for removal of oxygen contaminants from the starting material, representing chemically purified fluorspar, either granular or as dense precursors is reported. The chemical and crystal-chemical reactions occurring during the crystallization due to the presence of oxygen contaminants, scavengers, and rare-earth impurities are discussed based on data for the composition of the residual atmosphere in the reactor chamber. The water vapors have been shown to have a major impact in deteriorating the optical properties of the grown crystals. The use of precursors, together with 0.1n% ZnF_2 additive, is shown to be highly recommendable for the production of ultraviolet and vacuum ultraviolet grades of CaF_2 and Ca_(1-x)Sr_xF_2 crystals. For visible and infrared grades PbF_2 appears to be a better additive than ZnF_2. The technique for growth of Ca_(1-x)Sr_xF_2 in an argon flow with a 5 nines inlet purity has proved to be suitable for producing high-quality crystals, whose optical properties satisfy the demands of micro-photolithography provided that precise control over the total pressure inside the crucible is implemented.