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

The progress, current state, and prognosis for CaF2 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 CaF2 grown using a natural fluorite, investigations of the effect of PbF2 and ZnF2 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% ZnF2 additive, is shown to be highly recommendable for the production of ultraviolet and vacuum ultraviolet grades of CaF2 and Ca1−xSrxF2 crystals. For visible and infrared grades PbF2 appears to be a better additive than ZnF2. The technique for growth of Ca1−xSrxF2 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.