Patent ID: 7033433

Claim:
A method of manufacturing a fluoride crystal comprising: heating a fluoride starting material in the melting zone of a growth furnace having a melting zone and a cooling zone to a temperature equal to or greater than its melting temperature to form a melt; growing a fluoride crystal from the fluoride melt by cooling the melt from the melting temperature to a first temperature below the melting temperature by lowering the melt from the melting zone into the cooling zone and controlling the temperature of the two zones so that the temperature difference between the two zones is less than 50° C. during crystal formation; and annealing the crystal in the cooling zone by cooling the crystal from the first temperature to a final temperature at a substantially constant cooling rate of 3° C./hour or less; wherein the fluoride starting material is selected from the group consisting of calcium fluoride, barium fluoride, magnesium fluoride and strontium fluoride, including mixtures thereof, and wherein during cooling from the melt temperature to the first temperature a decreasing fast cooling profile is applied to the melting zone and an increasingly slow cooling profile is applied to the growth/annealing zone to diminish the temperature difference between the two zones.