Having special partial dispersion characteristics (: anomalous partial dispersion; Abbe number: 95) in addition to having extremely small chromatic dispersion, low refractive index and dispersion ratio compared to generic optical glasses, crystals of fluorite (CaF2 crystals) are used broadly in apochromatic lenses (apochromats), window plates of infrared analyzers, excimer lasers and the like, TV camera lenses and microscope lenses, lenses of semiconductor lithography (including steppers, scanners and the like) devices, which are devices for transferring microscopic patterns onto wafers, and the like.
Among them, regarding steppers (reduction projection-type exposure devices), which assume the miniaturization process in semiconductor lithography devices, shortening of the light source wavelength has been proceeding in order to raise the resolving power, and steppers were developed, which use for the light source an excimer laser serving as a high output laser oscillating in the ultraviolet region. Concomitantly to this, fluorite (CaF2, calcium fluoride) has been drawing attention as a lens material suitable thereto. A fluorite is characterized by the transmittance being high for light beams in a wavelength region called vacuum ultraviolet region, such as from a KrF laser (wavelength: 248 nm) or an ArF laser (wavelength: 193 nm), among the excimer laser beams.
However, although it is necessary for these lens materials used in such high precision steppers to be homogeneous fluorites, in which there are little dislocations and sub-boundary structures, preparing such fluorites is not straightforward.
This species of fluorites is generally produced by growing a CaF2 crystal by a single crystal growth method such as the Bridgman method and then heat-treating (also referred to as annealing) the obtained CaF2 crystal. Heat-treating the CaF2 crystal obtained in the crystal growth step allows the residual stress introduced during crystal growth to be eliminated, allowing residual distortions (strain birefringence) within the CaF2 crystal to be reduced. Accordingly, the heat-treatment step is particularly important in order to produce a fluorite provided with optical characteristics of such high degree as described above.
Thus, when fluorite production techniques are examined by focusing on the heat-treatment step, the heat-treatment method in prior art is in general a method whereby a CaF2 crystal obtained in the crystal growth step is disposed inside the heat-treatment oven, the temperature inside the oven is raised to a predetermined heat-treatment temperature, after the heat-treatment temperature has been reached, the temperature is controlled precisely to maintain the heat-treatment temperature for a given time in order to reduce/suppress as much as possible the temperature distribution (temperature difference) within the crystal, and then slowly cooled.
For instance, a method is described in Patent Document 1, in which a fluorite single crystal is retained for a given time at a first temperature (1,080° C.) in the range of 1,020° C. to 1,150° C., then, lowering in temperature from the first temperature (1,080° C.) to a second temperature (700° C. or lower) at a cooling rate of 1.0 (° C./h) or less; a method is described in Patent Document 2, in which a fluoride single crystal that has been crystal-extended is introduced into an annealing oven and heated for 20 hours or more by heating a crucible to 900 to 1,000° C.; and a method is described in Patent Document 3, in which the interior of a heat-treatment oven is turned into a vacuum atmosphere, retained at 1,000° C. for 24 hours for heat-treatment and then cooled.
In addition, a method is described in Patent Document 5, in which a single crystal base material is placed inside an ampule in an enveloped state with an external envelope fixture for heat-treatment comprising a material of the same species as the single crystal base material to carry out heat-treatment by heating from the exterior of the ampule thereby heating by containing with a material of the same species the outer periphery of the single crystal base material thereby suppressing a material alteration of the outer periphery of the single crystal base material.
Described in Patent Document 6 as a heat-treatment apparatus of fluorite single crystal is a heat-treatment apparatus of a constitution in which a temperature uniformizing member comprising a fluorite or the like having identical physical properties to the fluorite to be heat-treated is disposed in the periphery of the fluorite single crystal.
Furthermore, in Patent Document 7, a method is proposed as a heat-treatment method for a crystal-extended fluoride single crystal, in which the temperature inside the oven is raised to be within a range of 1,000 to 1,350° C., then, a rise-drop cycle in which a temperature drop and a temperature rise are carried out alternately in a temperature region of 900 to 1,350° C. is performed at least twice, followed by cooling, along with a method being described, in which, as an example of heat-treatment oven, a plurality of carbon vessels 54 are disposed inside an airtight oven 51 so as to be stacked vertically, and a CaF2 crystal substrate 55 is housed inside each of these carbon vessels 54, as shown in FIG. 19.