JP4463730B2 - Method for producing metal fluoride single crystal - Google Patents

Description

  The present invention relates to a method for producing a metal fluoride single crystal. More specifically, the present invention relates to a method for producing a metal fluoride single crystal having an improved light transmittance in a vacuum ultraviolet region of 120 to 200 nm, which is useful as an optical member of a semiconductor production apparatus.

Single crystals of metal fluorides such as calcium fluoride and barium fluoride have high transmittance over a wide wavelength band, low dispersion and excellent chemical stability. Demand is growing as optical materials such as various devices using a laser, lenses for cameras, CVD devices, and window materials. In particular, a calcium fluoride single crystal is a light source window material, a light source lens, an ArF laser (193 nm) or an F ₂ laser (157 nm), which is being developed as a next-generation short wavelength light source in the photolithography technology. Expectation is expected as a projection system lens.

  Conventionally, such a single crystal of a metal fluoride has been manufactured by a method in which a metal fluoride as a raw material is once melted at a high temperature to obtain a single crystal by growing a crystal from the melt. Typical methods for producing a single crystal by such melt growth include a crucible descent method (commonly referred to as the Bridgeman method) and a single crystal pulling method (commonly referred to as the Czochralski method). . The crucible lowering method is a method for growing a single crystal in a crucible by cooling a molten liquid of a single crystal production raw material in the crucible while gradually lowering the entire crucible. On the other hand, in the single crystal pulling method, a seed crystal consisting of a target single crystal is brought into contact with the melt surface of the single crystal production raw material in the crucible, and then the seed crystal is gradually pulled from the heating area of the crucible. In this method, a single crystal is grown under the seed crystal by cooling. The single crystal produced by this method is obtained by the crucible descent method in which only the seed crystal part is fixed and the other part is not in contact with the crucible or the like, so that the obtained single crystal is always in contact with the crucible inner wall. There is an advantage that distortion is smaller than that of a single crystal. In addition, unlike the crucible descent method, in which it is difficult to produce a single crystal with a direction other than the <111> orientation, which is the preferred growth orientation, the single crystal pulling method has an arbitrary crystal orientation by selecting the crystal orientation of the seed crystal. Can be obtained. When trying to grow a larger single crystal, in the crucible descent method, impurities often enter from the part in contact with the inner wall of the crucible and become a polycrystal partially as a nucleus. Is an excellent method (see, for example, Patent Documents 1 and 2).

  On the other hand, when producing a metal fluoride single crystal by such melt growth, it is important to remove various impurities contained in the metal fluoride raw material in advance in order to obtain a good quality single crystal. It is. Therefore, prior to crystal growth, a metal fluoride raw material and a scavenger are mixed, heated and melted to remove impurities and purify. Further, the scavenger may be used in a crystal growth process or an annealing process (see, for example, Patent Documents 3 to 5).

JP 2004-182588 A JP 2005-029455 A JP 09-227293 A JP 09-315893 A JP 2004-262754 A

  However, in the as-grown single crystal of metal fluoride produced by the melt growth method, those having good light transmittance at 120 to 200 nm in the vacuum ultraviolet region (hereinafter simply referred to as VUV transmittance) are stably used. In particular, the as-grown single crystal of metal fluoride produced by the crystal pulling method cannot be produced, and has a sufficient performance as an optical material when using light with a short wavelength as described above. It was low.

The cause of the low VUV transmittance is not clear, but the presence of impurities taken in during the crystal growth process is considered. In the crucible descent method, crystal growth is usually performed under a high vacuum of 10 ⁻⁵ torr or less, whereas in the single crystal pulling method, when such a high vacuum is used, the amount of volatilization from the melt surface in the crucible This may cause various troubles, and is generally performed near normal pressure. For this reason, it is presumed that the single crystal pulling method is more likely to cause impurities to remain than the crucible descent method, and therefore, it is likely to generate a low VUV transmittance.

  The present inventors conducted various studies on methods for stably producing a metal fluoride single crystal excellent in VUV transmittance that can be used as an optical material for the vacuum ultraviolet region. In the crystal pulling method, since a space for crystal pulling is required, the opening of the crucible is wide, and impurities existing in the portion other than the inside of the crucible in the single crystal growth furnace (adsorption, etc.) This crucible is considered to be volatilized and diffused by heating at times, and that some of it is likely to be mixed into the melt in the crucible, and thus the VUV transmittance of the resulting metal fluoride single crystal is often reduced. The present inventors completed the present invention as a result of intensive studies on a method for efficiently removing impurities existing in portions other than the inside.

That is, according to the present invention, in a method for producing a metal fluoride single crystal by melt growth, prior to introducing a metal fluoride raw material into a single crystal growth furnace, the growth furnace has an electronegativity of the metal fluoride. A state in which the inside of the growth furnace is not in contact with the outside air after performing a heating step in the presence of a fluoride that is larger than the metal element constituting the element and smaller than oxygen and capable of reacting with water The method for producing a metal fluoride single crystal is characterized in that a metal fluoride raw material is introduced into the growth furnace while maintaining

Further another aspect of the present invention, at least a portion of the manufacturing method of the metal fluoride monocrystal carried out under reduced pressure of the heating step.

  ADVANTAGE OF THE INVENTION According to this invention, the metal fluoride single crystal with favorable VUV transmittance | permeability can be manufactured easily and stably with a sufficient yield.

  In particular, the method of the present invention produces a metal fluoride single crystal by a single crystal pulling method that tends to have a large diameter and low internal stress, crystal distortion, etc., but tends to have a low VUV transmittance. When applied to the above, it becomes possible to stably produce a metal fluoride single crystal having extremely excellent physical properties as an optical material for the next-generation photolithography technology.

  Also in the crucible lowering method, the production method of the present invention exhibits a particularly high effect when producing a metal fluoride single crystal having a high vapor pressure at a high temperature and it is difficult to increase the degree of vacuum during crystal growth. .

  The present invention can be applied to any method for producing a metal fluoride single crystal that can be crystallized by a melt growth method. Specific examples of the metal fluoride include lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, magnesium fluoride, calcium fluoride, barium fluoride, strontium fluoride, aluminum fluoride, and barium fluoride. Lithium, potassium magnesium fluoride, lithium aluminum fluoride, calcium strontium fluoride, potassium magnesium fluoride, lithium strontium fluoride, cesium calcium fluoride, lithium calcium calcium fluoride, lithium strontium aluminum fluoride, lanthanoid fluoride, etc. Can be mentioned.

  Among the above metal fluorides, fluorides such as calcium fluoride, magnesium fluoride, barium fluoride, and strontium fluoride, which are often used as optical materials for lithography at a short wavelength, which are highly demanded for the effects obtained by the present invention. It is preferably applied to the production of alkaline earth metal fluorides, barium lithium fluoride, lithium calcium calcium aluminum, etc., more preferably applied to alkaline earth metal fluorides, especially calcium fluoride Then, the effect of the present invention is particularly remarkable.

  In the production method of the present invention, the metal fluoride is once melted by heating, and a single crystal of the metal fluoride is obtained by a melt growth method in which a single crystal is obtained from the melt. Examples of the melt growth method include a crystal pulling method (Czochralski method), a crucible descent method (Bridgeman method), a zone melting method (zone melting method), and a floating zone melting method (floating zone method). The crystal pulling method is particularly preferable in that the effect of the present invention is remarkably obtained. Hereinafter, the present invention will be described in more detail mainly using a crystal pulling method as an example.

  FIG. 1 shows a schematic diagram of a single crystal growth furnace (often called a crystal pulling apparatus) having a typical structure used in the crystal pulling method.

  In the single crystal growth furnace shown in FIG. 1, in the chamber (1), a molten liquid (10) of a metal fluoride raw material is accommodated on a cradle (3) supported by a support shaft (2). A crucible (4) is placed on the crucible (4), and a heater (5) is provided around the crucible (4). Further, a heat insulating material wall (6) surrounds the heater (5). A bottom heat insulating material (17) is also provided below the crucible (4). It is also preferable that the support shaft (2) has a rotatable structure so that the crucible can be rotated.

  Usually, the height of the upper end of the heater (5) is preferably approximately the same as or slightly higher than the height of the upper end of the crucible (4). The heat insulating material wall (6) may surround the crucible (4) from the lower end to the upper end. From the viewpoint of slowly cooling the pulled single crystal, It is preferable that the metal fluoride single crystal (11) is surrounded by a space.

  Further, an isolation wall (15) may be provided between the heater (5) and the outer wall of the crucible (4) for the purpose of uniformizing radiant heat from the heater. In order to prevent the heat of the heater (5) from escaping upward, the upper end of the isolation wall (15) is made higher than the upper end of the heater (5), and the upper end and the heat insulating material wall It is preferable that a lid member (14) for closing the gap between the isolation wall (15) and the heat insulating material wall (6) is placed between (9) and the gap is closed.

  On the other hand, on the center axis of the crucible (4), a rotatable single crystal pulling rod (9) having a holder (8) for a seed crystal (7) attached at the tip is suspended. The seed crystal (7) is pulled up after the lower end surface is brought into contact with the molten metal fluoride raw material (10) in the crucible (4), and a single crystal (11) is grown downward. The lower end of the support shaft (2) penetrates the bottom wall of the chamber (1) and extends outside the chamber. Although not shown, the crucible is moved up and down after contacting the cooler. It is connected to a mechanism for rotating it accordingly. It is also preferable to provide a viewing window (12) for visually confirming the crystal growth state. Further, in order to control the atmosphere in the single crystal growth furnace, it is preferable to provide gas introduction / exhaust holes (not shown). Among the single crystal growth furnaces having such a basic structure, when producing a metal fluoride single crystal (11) by the method of the present invention, for example, an apparatus described in JP-A No. 2004-182587 is a single crystal. The uniformity of the temperature distribution in the pulling region is good, and the ingot can be manufactured satisfactorily without cracks, which is preferable.

  Furthermore, when applying the manufacturing method of the present invention, the present inventors have already proposed as Japanese Patent Application No. 2004-370658, the crucible (4) having a double structure comprising an outer crucible (4a) and an inner crucible (4b). A crucible (4 ′) is particularly preferable. If the crucible having such a structure is employed, single crystal growth can be performed from the start to the end of pulling while keeping the melt depth of the metal fluoride raw material constant. This can be carried out satisfactorily even if the single crystal to be pulled is large, in particular, the diameter of the straight body part is 150 mm or more and the length of the straight body part is 100 mm or more. Therefore, even in the case of producing such a large metal fluoride single crystal body, the entire period of pulling up and the pulling up period of the straight body part are in a state where the melt in which the formation of scatterers is highly suppressed is shallow, It can be carried out stably, and it becomes easy to pull up a metal fluoride single crystal in which the abundance of the scatterer is extremely small.

  The dual structure crucible (4 ') will be described in more detail with reference to FIG. As shown in FIG. 2, the crucible (4 ′) has a double structure composed of an outer crucible (4a) and an inner crucible (4b), and the double structure crucible (4 ′) has an outer crucible (4a). The storage depth of the inner crucible (4b) can be continuously changed. This double structure crucible (4 ') is provided with at least one communication hole (18) in the wall of the inner crucible (4a), etc., and is provided in both the outer crucible (4a) and the inner crucible (4b). The vacant space is partly connected. For this reason, in the crucible having the above structure, when the melt (10) of the metal fluoride raw material accommodated in the inner crucible (4b) decreases as the single crystal is grown, the outer crucible is selected according to the amount of decrease. The storage depth of the inner crucible (4b) with respect to (4a) can be increased, and the melt (10) can be replenished from the outer crucible (4a) into the inner crucible (4b). As a result, in this pulling apparatus, it is possible to grow a single crystal while keeping the depth of the molten metal fluoride raw material (10) in the inner crucible (4b) from the start to the end of the pulling. And the depth of the melt (10) can be maintained in a shallow state in which formation of a scatterer described later can be highly suppressed during the entire pulling-up period.

  When the production method of the present invention is carried out using a single crystal growth furnace (crystal growth furnace) as described above, before the metal fluoride raw material is put into the crucible (4) of the crystal growth furnace, the growth furnace The fluoride of an element having an electronegativity greater than that of the metal element constituting the metal fluoride and less than oxygen and capable of reacting with water (hereinafter referred to as a cleaning fluoride) A step of heating in the presence is provided.

  In the production of a metal fluoride single crystal, if an oxide of the metal is present, this oxide is taken into the crystal, causing problems such as a decrease in VUV transmittance of the obtained single crystal. In order to eliminate the influence of this oxide, as described above, there is a method of removing the oxide using a scavenger in the stage of the metal fluoride raw material or in the crystal growth process. Further, in order to remove impurities in the growth furnace in advance, the furnace is often baked under reduced pressure before the metal fluoride raw material is charged. However, even if such treatment is performed, a satisfactory result is not always obtained, and a single crystal having a low VUV transmittance is often obtained. This tendency is particularly remarkable in the crystal pulling method.

  When producing a single crystal by the melt growth method, it is necessary to continue heating to several hundred to several hundreds of degrees for several days to several tens of days, depending on the size. In order to maintain a high temperature over a long period of time, the moisture adsorbed on the heat insulating material and the furnace wall (physical adsorption, chemical adsorption) is slowly diffused in the growth furnace and mixed into the melt. It is presumed that it reacts with metal fluoride to produce an oxide, which reduces the VUV transmittance. In particular, when a carbon-based material is used as the heat insulating material, water adsorption is strong and cannot be completely removed in a time of several hours to several tens of hours even when performing the above-described baking. .

  In the present invention, by providing the step of heating the growth furnace in the presence of the cleaning fluoride, impurities that are difficult to remove by the conventional method as described above, particularly water, are efficiently removed, and thus the VUV transmittance is increased. It is estimated that a good metal fluoride single crystal can be produced with good reproducibility.

  In the present invention, the cleaning fluoride is a fluoride of an element having an electronegativity greater than that of the metal element constituting the metal fluoride to be manufactured. Therefore, it is possible to efficiently remove moisture in the growth furnace in a short time. On the contrary, if the impurities are not removed even in the presence of the cleaning fluoride, the possibility of becoming a problem in the production of a single crystal of metal fluoride having a lower reactivity is extremely low. In order to remove water which is the largest impurity, the cleaning fluoride is a fluoride of an element having an electronegativity smaller than that of oxygen. Electronegativity is known for each element. For example, page 40 of “Revised chemical structure, properties and reactions” (published on January 1, 1981) by Toshinari and 4 other authors. Are listed in Tables 3 and 6. If the electronegativity (in parentheses) of typical elements is exemplified, Li (1.0), Mg (1.2), Ca (1.0), Sr (1.0), Ba (0.9) ), Cu (1.9), Ag (1.9), Zn (1.6), Pb (1.8), C (2.5), N (3.0), O (3.5) , F (4.0).

  It must also react with water, but with the exception of hydrogen fluoride, any fluoride that satisfies the above conditions will react with water by heating to a high temperature of several hundred to 2,000 degrees C. To form hydrogen fluoride.

  Further, since it is a fluoride, it volatilizes and diffuses when heated in the growth furnace, and easily comes into contact with impurities such as water adsorbed on the heat insulating material and the furnace wall.

  Specific examples of fluorides that can be used as cleaning fluorides in the present invention include gaseous substances such as carbon tetrafluoride, hexafluoroethane, boron trifluoride, and nitrogen trifluoride, zinc fluoride, fluoride. Examples thereof include solid fluorides such as lead fluoride, silver fluoride, copper fluoride, cobalt fluoride, manganese fluoride, and cadmium fluoride, and ammonium fluoride. Also, liquid or solid fluorinated resins such as polytetrafluoroethylene do not react with water per se, but are decomposed by heating to a high temperature, and this decomposition product reacts with water. That is, the cleaning fluoride in the present invention may be generated in the growth furnace by chemical reaction, thermal decomposition, or the like, with its precursor charged in the growth furnace.

  From the viewpoint of preventing polycrystallization during crystal growth, the fluoride for cleaning is preferably on a solid, more preferably a metal fluoride, and further various colors such as coloring of the obtained single crystal and laser resistance. In consideration of physical properties, a fluoride of a metal belonging to Group 5-15 is preferable, a fluoride of a metal belonging to Groups 7-14 is more preferable, and a fluoride of a metal belonging to Groups 11-14 is most preferable.

  When the cleaning fluoride is a gas at room temperature, the gas cleaning fluoride may be introduced directly into the chamber from a gas introduction hole (not shown) provided in the chamber (1). When the cleaning fluoride is solid or liquid at room temperature, it is generally put in the crucible (4), but the purpose is to control the furnace temperature at which the cleaning fluoride volatilizes. Therefore, it may be arranged in another place in the crystal growth furnace. Further, the solid scavenger may be put into the crystal growth furnace in any shape even in the form of powder, granule, or lump.

  When these cleaning fluorides are present in the crystal growth furnace, the amount used thereof cannot be generally stated depending on the size of the internal space of the growth furnace, the type of metal fluoride raw material to be used, etc. It is 0.005-1 mol% with respect to the metal fluoride raw material (100 mol%) thrown in in a growth furnace.

  In the present invention, after the cleaning fluoride is present in the crystal growth furnace, the growth furnace is heated. After the crystal growth furnace is heated, the cleaning fluoride is introduced into the growth furnace. Heating is performed in a state where a part of the cleaning fluoride used is present in the crystal growth furnace, and the cleaning fluoride is present in the crystal growth furnace, such as a method of adding a cleaning fluoride. As long as the cleaning fluoride is solid or liquid at room temperature, the entire amount of cleaning fluoride used in the crystal growth furnace is charged and then heated. However, the operation is simple and the safety is high. On the other hand, when a gaseous cleaning fluoride is used, a method of gradually adding the cleaning fluoride while heating is preferable because the efficiency of the cleaning fluoride is good.

  As described above, these cleaning fluorides react with impurities such as water existing in the crystal growth furnace by heating. It is presumed that this reaction usually generates an oxide of elements other than fluorine and hydrogen fluoride constituting the cleaning fluoride. The rate of such a chemical reaction is generally faster and more efficient at higher temperatures. However, as one of the advantages of the present invention, the heating is performed before the metal fluoride raw material is put into the crystal growth furnace. It is possible to remove impurities in the crystal growth furnace with extremely high efficiency by applying a temperature so high that it cannot be applied after the metal fluoride raw material is added to the crystal.

  As described above, conventionally, there is a method of removing oxides in a metal fluoride raw material by coexisting an oxide remover called a scavenger when growing a crystal. Many of the scavengers are compounds that overlap with the cleaning fluoride in the present invention, and therefore are often fluorides that react with water, and have some effect of removing water during the crystal growth. However, the crucial difference between the above scavenger method and the production method of the present invention is that the cleaning fluoride is placed in the crystal furnace prior to the introduction of the metal fluoride raw material into the single crystal growth furnace. It is in the point to heat.

  In other words, the melt growth method is a method in which a single crystal is gradually grown from the melt using the temperature gradient generated in the crystal growth furnace. Therefore, it is necessary to control the temperature in the crystal growth furnace so that the temperature of the contact surface (crystal growth interface) with the single crystal becomes the crystallization temperature (melting point) of the metal fluoride. The crystal growth furnace cannot be heated to a high temperature as the temperature rises. Even before starting crystal growth, if a too high temperature is applied after the metal fluoride is put into the furnace, the ratio of the raw material from which the metal fluoride volatilizes in a large amount and becomes a single crystal decreases. Or the volatilized metal fluoride solidifies in a lower temperature region in the furnace, which falls into a crucible or the like and causes trouble in crystal growth.

  As described above, the removal effect of impurities in the growth furnace becomes more efficient as the temperature is higher. However, because of the above-mentioned limitations, the temperature that can be applied by the conventional method using scavengers during crystal growth is also limited. Furthermore, in the technique using a scavenger during crystal growth, it is necessary to use a compound that can be removed under crystal growth conditions so that the used scavenger is not taken into the crystal.

  In contrast, as described above, in the production method of the present invention, heating is performed before the metal fluoride raw material is put into the crystal growth furnace. Therefore, the upper limit of the temperature that can be heated depends on the performance of the crystal growth furnace to be used. It is not limited by the crystallization temperature of the target metal fluoride, and therefore impurities in the crystal growth furnace can be removed with extremely high efficiency. Furthermore, even fluorides that produce oxides and the like that cannot be removed at the crystallization temperature of the metal fluoride can be evacuated to a temperature higher than that temperature or reduced to a pressure lower than that possible during crystal growth, Alternatively, it is possible to remove both by combining both of them, or after cooling once and then performing wiping or the like, so that there is a high degree of freedom in selecting fluorides that can be used.

  In the present invention, the growth furnace is heated in the presence of a cleaning fluoride. The crystal growth furnace used for the melt growth method always has a temperature difference (temperature gradient) in the furnace in principle, and even in the crystal pulling furnace having the structure shown in FIG. In the space surrounded by the material (17) and the lid material (14) (hereinafter also referred to as heat insulation space), a temperature difference of about 300 to 600 ° C. is inevitably generated. In the crystal pulling method, the temperature of the crystal growth interface is controlled to a specific value, and if the temperature of the crystal growth interface is changed, the temperature in other places in the growth furnace is the same. Change direction. Unless otherwise specified below, when simply referred to as “temperature (when heating)”, a place corresponding to the crystal growth interface in the crystal pulling method (Note: since it is before the raw material is charged, the crystal growth actually Temperature) although no interface exists. Further, the production method of the present invention may be applied to other melt growth methods such as a crucible pulling-down method, but in this case as well, the temperature at a location corresponding to the crystal growth surface in the crucible is also referred to. .

  In the present invention, the temperature at the time of heating is not particularly limited, but in order to obtain the effects of the present invention more remarkably, the temperature is higher than the temperature applied during crystal growth of the metal fluoride to be produced. It is preferable to heat so that it may become, It is more preferable to set it as +50 degreeC or more, It is more preferable to set it as +70 degreeC or more, It is especially preferable to set it as +100 degreeC or more.

  For example, when the metal fluoride to be manufactured is calcium fluoride, the crystallization temperature of the calcium fluoride is about 1420 ° C. Therefore, the temperature when heating in the presence of the cleaning fluoride is also 1420. Or higher, more preferably 1470 ° C. or higher, particularly preferably 1490 ° C. or higher, and most preferably 1520 ° C. or higher.

  By setting the temperature at the location corresponding to the crystal growth interface to be higher than the temperature applied when performing crystal growth in this way, the temperature becomes higher than the temperature reached when performing crystal growth at other locations in the growth furnace. In addition, impurities such as water can be removed.

  On the other hand, if a too high temperature is applied, various materials constituting the crystal growth furnace may be thermally damaged. Although depending on the material of the member constituting the crystal growth furnace, the maximum temperature during heating is 2000 ° C. or less, more generally 1900 ° C. or less, and particularly 1800 ° C. or less.

  The longer the heating time is to a certain extent, the better the effect of the present invention can be obtained, but there is a tendency to gradually reach a peak, and if it is too long, the productivity is lowered. Moreover, there exists a tendency for heating time to be short, so that heating temperature is high. The heating time may be appropriately determined depending on the size of the internal space of the crystal growth furnace, the amount of impurities, the type and amount of the cleaning fluoride to be used, etc. It is preferable that the temperature applied during the crystal growth of the single crystal + 50 ° C. or higher (preferably + 70 ° C. or higher) is 5 to 720 minutes, more preferably 30 to 720 minutes, and still more preferably. 60 to 600 minutes.

  In addition, the temperature raising time and the temperature raising pattern during the heating are not particularly limited, and the temperature can be raised in any pattern, such as raising the temperature at once in a short time or gradually increasing the temperature in several steps. Good.

  Many cleaning fluorides react with oxygen at high temperatures. Therefore, the heating in the presence of the cleaning fluoride is preferably performed in an inert atmosphere, particularly preferably in an argon gas atmosphere. Furthermore, it is a preferable method to carry out in an inert atmosphere in order to protect the material constituting the crystal growth furnace.

  When the crystal growth furnace is heated in the presence of the cleaning fluoride as described above, the cleaning fluoride volatilizes and diffuses and comes into contact with the water adsorbed on the heat insulating material wall or the crucible wall. It is assumed that it is converted to oxide or hydrogen fluoride. Further, these oxides and hydrogen fluoride may further react with other substances present in the growth furnace, such as a heat insulating material constituting the crystal growth furnace.

  Of the reactants generated in this way, hydrogen fluoride may be used as a scavenger during crystal growth, and its presence does not pose a major problem, but oxides are removed and discharged from the growth furnace. It is preferable to do. In the crystal pulling furnace shown in FIG. 1, outside the heat insulating space surrounded by the heat insulating material wall (6), the bottom heat insulating material (17) and the lid material (14), that is, in these members and the chamber (1). It is sufficient to move to the enclosed space, but it is preferable to discharge most of the space outside the crystal growth furnace. It should be noted that if it is outside the heat insulating space, it may exist in the crystal growth furnace because this space is thermally shielded from the heater (4) by the heat insulating wall (6) or the like, This is because there is almost no possibility of approaching again in the heat insulation space (that is, in the vicinity of the crucible) by the heating of.

  In the present invention, when the oxide other than fluorine constituting the cleaning fluoride is in a gaseous state, such as carbon or nitrogen, an inert gas such as argon is flowed into the crystal furnace, or the pressure is reduced. Then, it can be easily removed from the crystallization furnace by a method such as evacuation.

  On the other hand, when the element other than fluorine constituting the cleaning fluoride is a metal such as zinc or lead, the resulting oxide is zinc oxide, lead oxide or the like. Even such metal oxides are volatile at high temperatures (for example, zinc oxide begins to sublimate from 1300 ° C. at 1 atm), so it is also removed by flowing an inert gas or reducing the pressure. it can. As described above, according to the method of the present invention, since the temperature during heating is not limited by the metal fluoride to be manufactured, the temperature rises to a temperature at which the generated oxide can be removed sufficiently and efficiently. It is possible to make it.

  Furthermore, when a carbon-based material is used for a heat insulating material, a crucible, or the like as a member constituting the crystal growth furnace, the carbon of the member may reduce the generated oxide to generate a (single) metal. is there. Since a single metal is much more volatile than an oxide, in this case, that is, when a carbon-based material is used for a member constituting the growth furnace, the element from the inside of the growth furnace is further increased. Removal is easy.

  When removing these metal oxides and / or simple metals, they can be removed by flowing an inert gas or reducing the pressure in the same manner as in the method of removing the gaseous oxide.

In particular, according to the method of evacuating under reduced pressure, a small amount of the reactant in the crystal growth furnace can be discharged and removed, and the effect is high. The degree of decompression when reducing the pressure is 20 Pa or less, preferably 1 Pa or less, and particularly preferably 1 × 10 ⁻² Pa or less.

  In the present invention, the cleaning fluoride is used to remove impurities such as water existing in the growth furnace, and the discharge is performed after the fluoride has sufficiently reacted with the impurities in the growth furnace. It is preferable. Therefore, if the entire process of heating in the presence of the cleaning fluoride is performed under reduced pressure, the cleaning fluoride is also discharged outside the growth furnace in a state where it has not yet reacted with the impurities in the furnace. Efficiency tends to decrease, such as the need for very large amounts of fluoride.

  In order to avoid the above problem, the heating of the growth furnace in the presence of the cleaning fluoride in the present invention is performed in the first half under non-depressurization, and the cleaning fluoride gas is sufficiently retained in the growth furnace. After reacting with impurities such as the above, it is preferable to discharge the produced reactants and volatile substances such as unreacted cleaning fluoride (hereinafter referred to as reactants) in the growth furnace under a reduced pressure as described above. Of course, if necessary, for example, when the cleaning fluoride is in a solid state, the inside of the growth furnace can be set to a negative pressure or a positive pressure for the purpose of controlling the evaporation rate.

  In the case of adopting a method in which the first half heating is performed under non-depressurization and the second half is depressurized, the time for performing the depressurization is not particularly limited as long as the above discharge is performed. Whether or not the discharge is completed can be confirmed by monitoring the pressure change in the growth furnace. Specifically, it may be exhausted under reduced pressure until the pressure becomes about the above-mentioned pressure. Moreover, it can discharge | emit more highly by making it higher temperature. Although depending on the size of the growth furnace and the performance of the vacuum pump, the pressure can generally be reached in about 5 to 600 minutes.

  For this reason, when employing a method in which heating is performed under non-depressurization and then the reactants are removed by depressurization, the temperature at which the reactants are removed should be higher than the temperature under non-depressurization. Is particularly preferable because the inside of the crystal growth furnace can be cleaned to a high degree.

  After removing most of the impurities from the inside of the crystal growth furnace in this way, particularly from the heat insulating space, the metal fluoride raw material is charged into the crucible as described above, and the raw material is melted. A single crystal may be grown from the melt.

In the present invention, when the metal fluoride is introduced into the crystal growth furnace, the inside of the growth furnace is maintained in a state where it does not come into contact with the outside air . That is, the heating in the presence of the cleaning fluoride is performed before introducing the metal fluoride raw material into the crystal growth furnace. For subsequent crystal growth, the growth furnace is opened and the It is necessary to introduce metal halide raw materials. Here, if the growth furnace is opened and brought into contact with the outside air, the water (humidity) contained in the outside air and other components harmful to crystal growth enter the furnace again and are adsorbed. The effect of heating in the presence of fluoride may be significantly impaired. Here, “outside air” is a component harmful to crystal growth as described above (usually only water. Unless it is in a special environment, harmful components other than water are included in the environmental atmosphere. The atmosphere in the clean room is the present invention when the crystal growth furnace is disposed in a clean room conditioned at a very low humidity. It does not correspond to “outside air”.

  The method of introducing the metal fluoride raw material into the growth furnace while maintaining the state in which the inside of the crystal growth furnace is not in contact with the outside air is not particularly limited, and a known method is appropriately selected and applied according to the structure of the growth furnace. do it. For example, as shown in a simplified diagram in FIG. 3, the opening portion of the crystal growth furnace heated in the presence of the cleaning fluoride by the above-described method is completely covered with an airtight sheet before opening. (A in FIG. 3). The airtight sheet is not particularly limited, but a resinous sheet such as polypropylene or polyethylene is preferable from the viewpoints of operability, cost, prevention of recontamination in the furnace, and the like. In the airtight sheet, a metal fluoride raw material to be put into the crystal growth furnace is put, and air is removed from the airtight sheet as much as possible, and further, an inert gas such as argon gas is used. It is preferable to substitute. Subsequently, the crystal growth furnace is opened. Since the opening is covered with the airtight sheet as described above, the inside of the furnace is not exposed to the outside air by this opening (b in FIG. 3). After introducing the metal fluoride raw material, the opening of the crystal growth furnace is closed again, the airtight sheet is removed, and then a single crystal is grown. In this method, the method of shutting off the inside of the crystal growth furnace from the outside air with an airtight sheet is exemplified, but other methods, for example, a crystal growth furnace provided with a metal fluoride raw material chamber capable of gas replacement inside is used. Is also possible.

  The metal fluoride raw material is charged preferably after the temperature of the crystal growth furnace is lowered to 50 ° C. or lower, preferably near room temperature, from the viewpoint of workability and the like.

  As the metal fluoride raw material to be introduced, a known metal fluoride raw material used when producing a single crystal by the melt growth method may be used. For example, natural minerals such as fluorite in calcium fluoride may be used, but it is preferable to use a chemically synthesized product in terms of purity. Furthermore, in order to obtain the effects of the present invention more remarkably, it is preferable to use a chemically synthesized product from which impurities such as moisture and oxide are removed as much as possible. Such impurity removal methods are known. For example, the pretreatment for removing moisture and the like is performed by heat-treating the metal fluoride raw material under reduced pressure by a vacuum pump, but it is difficult to sufficiently remove the moisture and the like inside the raw material simply by firing. Therefore, it is more preferable to melt the metal fluoride raw material in an atmosphere containing carbon tetrafluoride, carbon trifluoride, hexafluoroethane or the like as a gas scavenger following the heat treatment. Most preferably, carbon tetrafluoride is used as the gas scavenger. In the treatment, powder may be used as the metal fluoride raw material, but since the volume reduction when melted is severe, granular materials, preferably particles having a particle size of 60 μm or more, preferably 60 to 1000 μm. The product is preferably subjected to purification.

  The metal fluoride raw material subjected to such pretreatment may be used as it is as a raw material for single crystal growth by the pulling method, but it is preferable to solidify once by cooling and solid impurities present on the surface thereof. It is preferable to use after cutting and removing from the viewpoint of reducing polycrystallization.

  After introducing the metal fluoride raw material as described above into the crystal growth furnace as described above, according to a known method, for example, a method described in JP-A No. 2004-182588, JP-A No. 2005-29455, or the like. A single crystal of metal fluoride may be grown. Hereinafter, a method of growing a single crystal by such a single crystal pulling method will be briefly described.

  In the growth of a metal fluoride single crystal by the melt growth method, particularly the single crystal pulling method, the presence of moisture causes oxides to be taken into the single crystal and cause coloring and the like. The crystal growth furnace is heated in the presence of the cleaning fluoride prior to the introduction of the metal fluoride raw material described above, and the metal fluoride raw material to be charged may be the one purified as described above. There may be water adsorbed on the raw material or water mixed in for some reason during the operation of charging the metal fluoride raw material into the crystal growth furnace. Therefore, it is preferable to perform melting and growth of a single crystal after removing the moisture as much as possible after putting the metal fluoride raw material into the crystal growth furnace.

  That is, after the metal fluoride raw material is placed in the crucible and then melted, it is preferable to perform heat treatment under reduced pressure to remove the pretreated water adsorbed. Further, in order to remove moisture adsorbed to the charged metal fluoride raw material and to carry out a single crystal growth step to be described later in the presence of a scavenger, a scavenger may be simultaneously added to the raw material. The melting of the metal fluoride raw material and the growth of the single crystal are preferably performed in an atmosphere of an inert gas, and the inert gas is continuously supplied into the apparatus so that the scavenger and residual moisture, oxides, etc. The reaction product produced by the reaction can be discharged out of the apparatus.

  The pulling of the single crystal is performed by adjusting so that the crystal growth interface becomes the crystallization temperature. Although depending on the size and structure of the crucible and the amount of the melt, the temperature of the crucible bottom tends to be higher in the crystal growth furnace having the structure shown in FIG. Usually, the temperature of the crystal growth interface is obtained by heating to about + 150 ° C., for example, when the metal fluoride is calcium fluoride, the temperature is set to 1420 ° C. to 1520 ° C. The rate of temperature rise to the temperature is preferably 50 to 500 ° C./Hr. Before starting the pulling of the single crystal, the measurement temperature at the bottom of the crucible is maintained for about 30 to 180 minutes at a temperature 20 to 150 ° C. higher than the melting point of the metal fluoride raw material, and the solid impurities floating in the melt during that time Removal is also effective for suppressing polycrystallization.

  In the single crystal pulling method, after removing impurities as much as possible as described above, the seed crystal is brought into contact with the melt and gradually pulled to obtain a single crystal. As the seed crystal, a single crystal made of the same material as the metal fluoride to be grown is preferably used. The seed crystal growth surface can be arbitrarily selected. However, when a calcium fluoride seed crystal is used, the {111} plane, the {100} plane, the {110} plane, and their equivalent planes are preferably used. be able to.

  The seed crystal and the growing crystal are preferably rotated about the pulling axis, and the rotation speed is preferably 1 to 20 times / minute. In addition to the rotation of the seed crystal, the crucible may be rotated in the opposite direction at the same rotation speed. A suitable crystal pulling speed is 1 to 10 mm / hour.

  Such pulling of the single crystal is preferably performed in the presence of a scavenger in order to eliminate the influence of a minute amount of moisture remaining even after the crystal growth furnace and the pretreatment of the metal fluoride raw material. A known scavenger can be used as the scavenger, and a method of introducing a gas scavenger such as carbon tetrafluoride described in the pretreatment of the metal fluoride raw material into the atmosphere may be adopted. Therefore, a solid scavenger such as zinc fluoride or lead fluoride, preferably a method in which zinc fluoride is charged into a crucible together with a metal fluoride raw material is preferable. As for the usage-amount of a solid scavenger, 0.01-5 mass parts is preferable with respect to 100 mass parts of metal fluoride raw materials.

  In the case of using the double structure crucible as described above for the growth of the single crystal, the depth of the molten liquid (10) of the metal fluoride raw material accommodated in the inner crucible (4b) in the double structure crucible. Is preferably 0.65 times or less the diameter of the straight body portion of the as-grown single crystal to be pulled up. Then, the molten liquid (from the outer crucible (4b) to the inner crucible (4) is maintained so as to maintain the depth as many as possible from the start to the end of the pulling, preferably throughout the entire period. The replenishment of 7) may be performed.

  The depth of the crucible of the pulling apparatus used for the production of a conventional metal fluoride single crystal is usually about 3 to 5 times the diameter of the straight barrel of the as-grown single crystal, which is sufficient for the crucible. When the molten metal fluoride (10) is accommodated, the depth of the molten liquid is about twice as large as the diameter of the straight body, and this value is also at the end of pulling. Usually, a liquid amount exceeding 0.75 times the diameter of the straight body part remains. Thus, when the single crystal is pulled up in such a deep state of the melt, the influence of natural convection on the flow of the melt becomes large, and the flow is coupled with the forced convection due to the rotation of the single crystal or the crucible. It becomes complicated and the temperature distribution near the growth interface of the single crystal becomes unstable. When the temperature distribution in the vicinity of the growth interface is unstable, a large number of vacancies that cause scatterers are formed in the single crystal when the single crystal is grown. On the other hand, as described above, when the depth of the melt is reduced to a depth of 0.65 times or less of the diameter of the straight body of the as-grown single crystal, natural convection causing such vacancies is obtained. Is greatly weakened, and the number of scatterers present in the pulled single crystal can be significantly reduced.

  Here, the scatterer is an internal defect that is visually observed as a particle that scatters and shines light when observed under condensing illumination, and the maximum diameter of the particle is generally 100 μm. The following is observed, and usually 10 to 100 μm is observed. Moreover, most of the actual conditions are holes, and these generally have an angular shape such as an octahedron. These scatterers usually have substantially uniform planes in a specific orientation of the single crystal, and when irradiated with laser light, scattered light is observed only in a specific direction determined by the incident light and the orientation of the single crystal.

  The depth of the melt (10) of the metal fluoride raw material housed in the inner crucible (5) described above is from the viewpoint of more remarkably exerting the effect of suppressing the formation of scatterers inside the single crystal. More preferably, the depth is 0.55 times or less, more preferably 0.50 times or less the diameter of the straight body portion of the as-grown single crystal. In general, the depth of the melt of the metal fluoride raw material is preferably 15 cm or less, more preferably 12 cm or less. Also, in the crystal pulling process, from the viewpoint of preventing contact between the single crystal body and the crucible or between the single crystal body and a part of the raw material solidified at the bottom of the crucible, the melting of the metal fluoride raw material accommodated in the crucible The depth of the liquid is more preferably maintained at a depth of 0.1 times or more of the diameter of the straight body portion of the as-grown single crystal, and generally 3 cm or more.

  After the single crystal grows to a predetermined size, the temperature is lowered to a certain extent and the single crystal is taken out from the crystal growth furnace. The temperature-decreasing rate may be selected as appropriate. However, if it is cooled too rapidly when it is taken out, the resulting single crystal may be greatly distorted or, in extreme cases, damaged by thermal shock. The subsequent temperature drop is preferably 0.1 to 3 ° C./min.

  In the above, the method of growing a single crystal by the single crystal pulling method has been described. Of course, the manufacturing method of the present invention can also be applied to other melt growth methods such as a crucible descent method. A single crystal may be grown by a known method.

  The as-grown single crystal of metal fluoride obtained by the method as described above may be processed into a desired shape as an optical member or the like by cutting or polishing. In addition, the single crystal obtained by the crystal pulling method has a very small birefringence. If it is desired to further reduce this value, the temperature depends on the type of metal fluoride, for example, calcium fluoride. If it is a single crystal, it may be annealed at 900 to 1350 ° C. for about 1 to 48 hours.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated, this invention is not limited to these Examples.

  In addition, the evaluation method of each physical property is as follows.

  Measurement of vacuum ultraviolet light transmittance; surface polishing was performed until the surface roughness was 0.5 nm or less by RMS to prepare a sample having a thickness of 10 mm. This was measured using a VUV transmittance measuring device (manufactured by JASCO Corporation, KV-201; measured in a nitrogen atmosphere with an oxygen content of 0.2 ppm or less) in the range of 120 to 300 nm.

  Measurement of the number of scatterers; filled with a matching oil (oil adjusted to a refractive index comparable to the refractive index of the calcium fluoride single crystal) in an amount that allows the entire single crystal to be measured to be immersed in a glass water tank. The as-grown body was left inside. Next, irradiate white halogen lamp light from one direction, rotate the single crystal body, search for a position where scattered light from the scatterer can be observed while changing the viewpoint, and determine the number of scatterers present in the measurement target. It was measured visually.

Example 1
Zinc fluoride (Stella Chemifa Co., Ltd.) made of high-purity graphite crucible (outer diameter 30 cm, height 50 cm. The bottom wall surface is a mortar-shaped shape with an inclination angle of 15 degrees downward with respect to the horizontal plane) 40 g) was placed in a chamber in a single crystal growth furnace having a structure schematically shown in FIG. The single crystal growth furnace is a resistance heating method using a graphite heater, and a cylindrical heat insulating material (6) made of a graphite porous material is provided so as to surround the heater. The chamber (1) is provided with a vacuum exhaust line to which an oil rotary pump and an oil diffusion pump are connected.

Next, the inside of the chamber is evacuated, and when the inside of the chamber becomes 5 × 10 ⁻³ Pa or less, the vacuum exhaust line is shut off and high purity argon gas is supplied into the chamber, and the internal pressure is 1.05 atm. I made it. Thereafter, the heater was energized to start heating. The temperature to be controlled was measured by measuring the bottom temperature of the crucible with a platinum-platinum rhodium thermocouple and increasing the temperature to 1700 ° C. at about 200 ° C./Hr. As will be described later, the temperature at the bottom of the crucible when growing a calcium fluoride single crystal (melting point: 1420 ° C.) is 1550 ° C. (difference of 130 ° C.). The temperature of the portion corresponding to is estimated to be as low as about 130 ° C. (about 1570 ° C.). Further, the simulation calculation result based on the comprehensive heat transfer analysis also showed that the temperature corresponding to the crystal growth interface was 1570 ° C. when the temperature at the bottom of the crucible was 1700 ° C.

After the temperature at the bottom of the crucible reached 1700 ° C., the temperature was maintained for 180 minutes. While the temperature was raised to 1700 ° C. and maintained at that temperature, argon gas was not passed, but only the expanded furnace gas was exhausted, and the internal pressure of the chamber was maintained in the range of 1.0 to 1.1 atm. . Thereafter, the chamber was evacuated to 1 Pa or less by using an oil rotary pump and an oil diffusion pump. The pressure was maintained for an additional 180 minutes after reaching that pressure. The temperature at the time of holding under this reduced pressure was set to 1750 ° C. at the bottom of the crucible. The degree of vacuum after reaching 180 Pa after reaching 1 Pa or less was 2 × 10 ⁻² Pa.

  Next, after the heater output was reduced to zero and the temperature was lowered to around room temperature, the vacuum exhaust line was shut off, and high-purity argon gas was supplied into the chamber to bring the internal pressure to 1.05 atm.

  Next, the outer portion of the chamber opening is completely covered with an airtight polypropylene sheet by a method described in a simplified diagram as FIG. 3, and 40 kg of calcium fluoride raw material and zinc fluoride are enclosed in a space surrounded by the chamber and the airtight sheet. 4 g (manufactured by Stella Chemifa) was set. After sufficiently replacing the air in the space with Ar gas, the chamber was opened, and the calcium fluoride raw material and zinc fluoride were charged into the crucible in a state in which outside air was not mixed.

Next, the chamber opening was completely closed and the inside of the chamber was evacuated. When the pressure reached 5 × 10 ⁻³ Pa or less, the heater was energized while continuing the evacuation to start heating the raw material. The temperature was raised at about 200 ° C./Hr until the temperature at the bottom of the crucible reached 250 ° C. and held at this temperature for 6 hours. The degree of vacuum in the chamber after 6 hours was 2 × 10 ⁻³ Pa. After the above holding, the temperature rise is started again at about 200 ° C./Hr. When the temperature at the bottom of the crucible reaches 600 ° C., the vacuum exhaust line is shut off, high-purity argon is supplied into the chamber, and the internal pressure is 1 .05 atm. High purity argon was supplied at a flow rate of 1 L / min, and evacuation was performed so that the chamber internal pressure was maintained at 1.05 atm. Observation from a viewing window provided in the upper part of the chamber, the heater output was maintained for 60 minutes at a temperature of 1600 ° C. at the bottom of the crucible where the raw material was completely melted. At that time, a small amount of solid impurities appeared on the surface of the melted raw material, which was scraped and removed from the raw material.

  Next, the heater output was reduced and held at a temperature of 1550 ° C. at the bottom of the crucible for 30 minutes, and then the seed crystal of [100] was brought into contact with the melt surface to start growing. The seed crystal was rotated at 10 times / minute and pulled up at 5 mm / Hr. After the start of the growth, the crystal growth rate was measured by a load cell provided on the pulling shaft, and this was fed back to the heater output to perform automatic control so as to approach the preset crystal shape.

  After completion of crystal growth, the temperature was lowered to 300 ° C. at 50 ° C./Hr, and then the heater was turned off to lower the temperature to room temperature.

The obtained calcium fluoride single crystal had a maximum diameter of 17 cm and a weight of 18.3 kg. Further, the number of scatterers present in the straight body portion in the single crystal was about 0.06 / cm ³ . After slicing the crystal along a plane perpendicular to the growth direction, vacuum ultraviolet light transmittance of the sample was measured. The results are shown in Table 1 and FIG.

Example 2
When the furnace is heat-treated in the presence of a cleaning fluoride prior to feeding the calcium fluoride raw material into the growth furnace, lead fluoride (40 g) is used as the cleaning fluoride instead of zinc fluoride. A calcium fluoride single crystal was obtained in the same manner as in Example 1 except that was used.

  The obtained calcium fluoride crystals had a maximum diameter of 17 cm and a weight of 16.1 kg. A sample was prepared from this single crystal in the same manner as in Example 1, and the vacuum ultraviolet light transmittance was measured. The results are shown in Table 1 and FIG.

Example 3
A single crystal was grown in the same manner as in Example 1 except that the seed crystal used was changed to [111].

  The obtained calcium fluoride crystals had a maximum diameter of 17 cm and a weight of 19.5 kg. A sample was prepared from this single crystal in the same manner as in Example 1, and the vacuum ultraviolet light transmittance was measured. The results are shown in Table 1 and FIG.

Example 4
Prior to introducing the calcium fluoride raw material into the growth furnace, when the furnace is heated in the presence of a cleaning fluoride (zinc fluoride), the internal pressure of the chamber is reduced from 1.0 atm in an argon atmosphere. 1.1 Calcium fluoride single crystal in the same manner as in Example 1 except that the temperature at the time of holding at 1 atm is 1650 ° C. and the temperature at the time of holding by vacuuming to 1 Pa or less is 1690 ° C. Manufactured. In the simulation calculation result by the comprehensive heat transfer analysis, when the temperature at the bottom of the crucible was 1650 ° C., the temperature corresponding to the crystal growth interface was 1520 ° C.

  The obtained calcium fluoride crystals had a maximum diameter of 17 cm and a weight of 16.7 kg. A sample was prepared from this single crystal in the same manner as in Example 1, and the vacuum ultraviolet light transmittance was measured. The results are shown in Table 1 and FIG.

Example 5
Prior to introducing the calcium fluoride raw material into the growth furnace, when the furnace is heated in the presence of a cleaning fluoride (zinc fluoride), the internal pressure of the chamber is reduced from 1.0 atm in an argon atmosphere. 1.1 Calcium fluoride single crystal in the same manner as in Example 1 except that the temperature at which the pressure is maintained is 1610 ° C. and the temperature at which the pressure is maintained at 1 Pa or less is 1650 ° C. Manufactured. In the simulation calculation result by comprehensive heat transfer analysis, when the temperature at the bottom of the crucible was 1610 ° C., the temperature corresponding to the crystal growth interface was 1480 ° C.

  The obtained calcium fluoride crystals had a maximum diameter of 17 cm and a weight of 18.3 kg. A sample was prepared from this single crystal in the same manner as in Example 1, and the vacuum ultraviolet light transmittance was measured. The results are shown in Table 1.

Example 6
Calcium fluoride was obtained in the same manner as in Example 1 except that the crystal pulling rate was 3 mm / hr. The obtained calcium fluoride crystals had a maximum diameter of 23 cm and a weight of 16.4 kg. A sample was prepared from this single crystal in the same manner as in Example 1, and the vacuum ultraviolet light transmittance was measured. The results are shown in Table 1 and FIG.

Example 7
The crucible used was a double structure crucible whose structure is shown in FIG. The outer crucible (4a) had an inner diameter of 38 cm (outer diameter of 40 cm) and a height of 30 cm. The inner crucible (4b) accommodated in the outer crucible (4a) while being fixed to the lid material (16) of the chamber by a connecting member has an inner diameter of 25 cm (outer diameter of 26 cm) and a height of 14 cm. It was a thing. The bottom wall of the inner crucible (4a) had a V-shaped (mortar shape) vertical cross-sectional shape inclined at an inclination angle of 15 degrees downward with respect to the horizontal plane. A cylindrical communicating hole (18) having a diameter of 4 mm and a cylindrical communicating hole (18) each having a uniform diameter of 8 on the circumference at a position of a height of 25 mm was formed at the lower end thereof (these communicating holes). The total opening area was 0.2% with respect to the upper opening area of the inner crucible).

  Zinc fluoride, which is a fluoride for cleaning, is charged into the space formed by the outer crucible (4a) and the inner crucible (4b) in the double structure crucible, and the growth furnace is the same as in Example 1 except that Was heat-treated.

  Next, the same operation as in Example 1 was carried out except that the calcium impurity raw material and zinc fluoride were charged and heated to obtain a molten liquid until the temperature at the bottom of the crucible reached 1550 ° C. Went. Thereafter, the surface state of the melt contained in the inner crucible (4b) was observed from the observation window (12). As a result, it was confirmed that the solid impurities were floating. The storage depth of the inner crucible (4b) with respect to the crucible (4a) is reduced, and the entire amount of the single crystal raw material melt (10) stored in the inner crucible is once flowed out into the outer crucible, and then again, The support shaft (2) is raised to increase the storage depth of the inner crucible (4b) with respect to the outer crucible (4a), and the raw material calcium fluoride melt in the outer crucible (4a) is put into the inner crucible. The operation to supply to was carried out. After the above operation, the storage depth of the inner crucible (4b) relative to the outer crucible (4a) is such that the depth of the raw calcium fluoride melt (7) is 6 cm in the inner crucible (4b). That was it. Further, the surface state of the melt (10) accommodated in the inner crucible (4b) was observed again from the observation window (12), but at this time, solid impurities could not be confirmed.

  Next, the single crystal pulling rod (9) was suspended, the seed crystal (111) was brought into contact with the surface of the raw material calcium fluoride melt, and the pulling of the single crystal was started. The seed crystal was rotated at 6 times / minute, while the outer crucible (4) was also pulled up at a speed of 2 times / minute in the opposite direction. During the pulling up, the support shaft (2) was continuously raised so that the depth of the melt (7) in the inner crucible (5) was maintained at 6 cm. After completion of the pulling, the temperature was lowered to room temperature at a cooling rate of 15 ° C./Hr.

The obtained calcium fluoride crystals had a maximum diameter of 17 cm and a weight of 17.5 kg. A sample was prepared from this single crystal in the same manner as in Example 1, and the vacuum ultraviolet light transmittance was measured. The results are shown in Table 1. Note that the number of scatterers present in the straight body portion of the single crystal was about 0.002 particles / cm ³ .

Comparative Example 1
Prior to charging the calcium fluoride raw material into the growth furnace, the heat treatment is performed without using any cleaning fluoride instead of the process of heating the furnace in the presence of the cleaning fluoride (zinc fluoride). A calcium fluoride single crystal was produced in the same manner as in Example 1 except that.

In addition, the vacuum degree after evacuating to 1 kPa or less and holding for 180 minutes in the heat treatment before feeding the calcium fluoride raw material is 3 × 10 ⁻² Pa. The vacuum after holding for 2 hours was 2 × 10 ⁻³ Pa.

  The obtained calcium fluoride single crystal had a maximum diameter of 17 cm and a weight of 17.7 kg. A sample was prepared from this single crystal in the same manner as in Example 1, and the vacuum ultraviolet light transmittance was measured. The evaluation results are shown in Table 1 and FIG.

FIG. 1 is a schematic view showing a typical embodiment of a crystal growth furnace to which the production method of the present invention is applied. FIG. 2 is an enlarged view of a double structure crucible portion used in another embodiment of the crystal growth furnace to which the manufacturing method of the present invention is applied. FIG. 3 is a schematic view showing an embodiment for preventing the inside of the growth furnace from coming into contact with the outside air when the metal fluoride raw material is charged into the crystal growth furnace heated in the presence of the scavenger. FIG. 4 is a graph comparing the vacuum ultraviolet light transmittance when the heat treatment with the cleaning fluoride is performed and when it is not performed. FIG. 5 is a graph showing the vacuum ultraviolet light transmittance of a calcium fluoride single crystal produced by the production method of the present invention.

Explanation of symbols

1; chamber 2; support shaft 3; cradle 4; crucible 4 '; double structure crucible 4a; outer crucible 4b; inner crucible 5; heater 6; heat insulating material wall 7; seed crystal 8; Crystal pulling rod 10; Metal fluoride raw material melt 11; Metal fluoride single crystal ingot 12; Viewing window 13; Single crystal pulling rod insertion hole 14; Ceiling plate 15; Isolation wall 16; Lid member 17; 18; communication hole 19; bottom wall 20 of the inner crucible; side wall of the inner crucible

Claims (3)

In a method for producing a metal fluoride single crystal by melt growth, prior to introducing a metal fluoride raw material into a single crystal growth furnace, the growth furnace is made to have an electronegativity higher than that of a metal element constituting the metal fluoride. Larger and smaller than oxygen, and after the step of heating in the presence of a fluoride capable of reacting with water , the growth furnace is maintained in a state where it does not come into contact with outside air. The method for producing a metal fluoride single crystal, wherein a metal fluoride raw material is charged into a furnace .   At least one of the steps of heating the growth furnace in the presence of a fluoride having an electronegativity greater than that of the metal element constituting the metal fluoride and less than oxygen and capable of reacting with water. The method for producing a metal fluoride single crystal according to claim 1, wherein the part is performed under reduced pressure. During the step of heating the growth furnace in the presence of a fluoride having an electronegativity greater than that of the metal element constituting the metal fluoride and less than oxygen and capable of reacting with water 3. The method for producing a metal fluoride single crystal according to claim 1, wherein the highest temperature to be applied is a temperature higher by 50 ° C. or more than a temperature at which the metal fluoride single crystal is produced by melt growth. .

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