JP2012012258A - Method for producing metal fluoride single crystal whose laser resistance is improved - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a metal fluoride single crystal which has high laser resistance by utilizing the merit of a gas scavenger (scavenger gas) and is useful as a stepper's optical system lens.SOLUTION: In a method for producing a metal fluoride single crystal which grows metal fluoride single crystals such as calcium fluoride and magnesium fluoride by a method for pulling a single crystal which uses a metal fluoride single crystal growing furnace having a heat insulating material wall 8 made from carbon, the metal fluoride single crystal 13 is made to grow by making the inside of the metal fluoride single crystal growing furnace to be an inert gas atmosphere of such as argon and helium, and supplying a scavenger gas such as tetrafluoromethane into the melt 12 of the metal fluoride or spraying to the liquid surface of this melt 12, while specifically introducing an inert gas into the furnace from the bottom in the growing furnace.

Description

  The present invention relates to a method for producing a metal fluoride single crystal used for an optical material suitable for a semiconductor lithography optical lens, particularly a light source lens.

  Single crystals such as calcium fluoride and magnesium fluoride have high transmittance, low refractive index and low dispersion over a wide wavelength band from vacuum ultraviolet to infrared, and excellent chemical stability. Therefore, it is used for window materials, lenses, prisms and the like as optical materials in a wide area. In particular, window materials for devices such as steppers (reduction projection type exposure apparatus) using ArF laser (193 nm) and F2 laser (157 nm) light sources, which are being developed as next-generation short-wavelength light sources in photolithography technology. It is expected to be used as a light source system lens, illumination system lens, and projection system lens.

A lens used in such a high-definition stepper is required to have high imaging performance (resolution, depth of focus). For this reason, materials used for lenses are required to have high optical characteristics such as low residual stress / strain (birefringence distribution), high light transmittance, high laser resistance, and few lattice defects.
When the residual stress / strain is large, the birefringence increases due to this, and it becomes unsuitable for an application requiring extremely strict optical characteristics such as a projection system lens of a stepper. A large number of lattice defects leads to a decrease in transmittance due to light scattering, a decrease in contrast, the occurrence of flare and ghosts, greatly reducing the properties of the material, and is also suitable for optical materials that require advanced optical properties. Disappear. In recent years, the energy intensity of lasers has increased, and accordingly, there has been an increasing demand for prevention of deterioration of transparency (laser resistance) due to repeated laser irradiation of lenses.

  As a cause of damaging the optical characteristics of the metal fluoride single crystal, it is considered that oxygen, water, or metal impurities attached to various raw materials or various members in the single crystal growth furnace or due to leakage are present. . These impurities are said to cause a decrease in light transmittance in the vacuum ultraviolet region and generation of a color center after laser irradiation.

  In order to remove such impurities, a scavenger is used in the production process of a metal fluoride single crystal, and various devices have been made. For example, prior to melt growth, a technique in which a metal fluoride raw material is brought into contact with a fluorine-containing gas to replace impurities adsorbed on the raw material surface with fluorine (Patent Document 1), a pretreatment process of the metal fluoride raw material, and this pretreatment In the process of crystal growth using raw materials, a technique using a copper fluoride or silver fluoride having a relatively low boiling point as a scavenger (Patent Document 2), in the raw material purification, crystal growth and annealing processes, a solid scavenger and a gas scavenger ( A technique (Patent Document 3) using a combination of scavenger gas has also been developed.

  When a solid metal scavenger is used, it is mixed into the melt and the scavenger-derived metal remains in the metal fluoride single crystal of the product, resulting in a decrease in light transmittance in the vacuum ultraviolet region and a color center after laser irradiation. There is a tendency to make it. The occurrence of the color center causes a change in optical physical properties such as a refractive index, resulting in a disadvantage that the performance of the optical lens used for forming the laser beam changes.

  On the other hand, the gas scavenger has an advantage that it is difficult to be taken into the metal fluoride single crystal. However, gas scavengers made of fluorinated hydrocarbon gases such as carbon tetrafluoride react with the carbon members that make up the heater and insulation at high temperatures. As a result, these members are damaged and the heater output and insulation performance are reduced. The production of the metal fluoride single crystal may be hindered.

JP 2004-315255 A JP2001-19586 JP2003-221297

  The inventors of the present application pursued a method for producing a metal fluoride single crystal having high laser resistance by taking advantage of the above-mentioned scavenger gas, to supply the scavenger gas in the growth furnace, and to the atmosphere in the growth furnace. As a result of careful examination, the present invention has been completed.

That is, according to the present invention,
A crucible containing metal fluoride is disposed inside the metal fluoride single crystal growth furnace having a carbon heat insulating material wall, and the metal fluoride is heated to form a metal fluoride melt, By immersing the seed crystal held at the lower end of the pulling shaft in the melt of the metal fluoride and growing the metal fluoride single crystal below the seed crystal, pulling up the pulling shaft, In the method for producing a metal fluoride single crystal for growing a metal single crystal,
An inert gas atmosphere is formed in the metal fluoride single crystal growth furnace, and a scavenger gas is supplied into the melt of the metal fluoride or sprayed onto the melt surface to grow the metal fluoride single crystal. A method for producing a metal fluoride single crystal is provided.
In the invention of the manufacturing method,
1) introducing an inert gas into the furnace from the bottom of the metal fluoride single crystal growth furnace;
2) The crucible is a double crucible composed of an inner crucible and an outer crucible, and the scavenger gas is supplied into the metal fluoride melt in the outer crucible.
3) The crucible is a double crucible composed of an inner crucible and an outer crucible, and a scavenger gas is blown onto the melt surface of the metal fluoride in the inner crucible.
4) The scavenger gas is a fluorinated hydrocarbon gas,
5) The inert gas is preferably argon.

In addition, according to the present invention,
A crucible containing metal fluoride is disposed inside the metal fluoride single crystal growth furnace having a carbon heat insulating material wall, and the metal fluoride is heated to form a metal fluoride melt, By immersing the seed crystal held at the lower end of the pulling shaft in the melt of the metal fluoride and growing the metal fluoride single crystal below the seed crystal, pulling up the pulling shaft, In the method for producing a metal fluoride single crystal for growing a metal single crystal,
The scavenger gas composed of the fluorine-based gas is supplied into the melt of the metal fluoride or sprayed to the liquid surface of the melt, inside the space constituted by the heat insulating material wall, and from the crucible bottom. Provided is a method for producing a metal fluoride single crystal, wherein a metal fluoride single crystal is grown while supplying an inert gas from a lower position.

  According to the production method of the present invention, a large-diameter metal fluoride single crystal having high laser resistance and little damage to the heater and heat insulating material of the growth furnace can be stably produced by the single crystal pulling method.

It is a schematic diagram of a single crystal pulling apparatus for introducing gas using a conventional double crucible. It is a schematic diagram of the single crystal pulling apparatus of the present invention using a double crucible and a resistance heating type heater. It is a schematic diagram of a single crystal pulling apparatus of the present invention using a double crucible and a high frequency induction heating type heater. It is a LIA spectrum figure of Examples 1-3 and Comparative Examples 1 and 2. It is a LIA spectrum figure of Examples 4-5 and comparative example 3.

[Metal fluoride single crystal]
The production method of the present invention can be applied to a known metal fluoride single crystal that can be produced by a single crystal pulling method. Examples of the metal fluoride include a metal fluoride made of a single metal such as calcium fluoride, magnesium fluoride, strontium fluoride, barium fluoride, lithium fluoride, aluminum fluoride, cerium fluoride, and lanthanum fluoride; BaLiF ₃ , Metal fluoride such as KMgF ₃ , LiCaAlF ₆ , LiSrAlF _{6, and the} like. Among these, it is suitably used for the production of a single crystal of alkaline earth metal fluoride, especially calcium fluoride, in that the effect of the present invention is remarkably exhibited.

[Scavenger gas]
The scavenger gas used in the present invention (referred to as a gaseous scavenger at normal temperature and normal pressure) is appropriately selected from known scavenger gases according to the metal fluoride to be produced.
Specific examples include fluorinated hydrocarbon gases such as carbon tetrafluoride, carbon trifluoride, and hexafluoroethane, or fluorine-based gases such as fluorine (F ₂ ) and carbonyl fluoride. A fluorinated hydrocarbon gas is preferable from the viewpoint of ease of handling and the like, and carbon tetrafluoride is particularly preferable in that a high-purity hydrocarbon gas can be obtained relatively inexpensively.
In the present invention, the scavenger gas supply method is characterized in that 1) the crystal fluoride is supplied directly into the melt of metal fluoride (hereinafter referred to as “supply method in melt”), or 2) the metal fluoride is supplied. Spray directly onto the melt surface (hereinafter referred to as “liquid surface spray method”)
Is. The apparatus structure and the supply method will be specifically described below.

The manufacturing apparatus (crystal growth furnace) to be used can use a well-known manufacturing apparatus except the site | part regarding the scavenger gas introduction pipe mentioned later. Preferably, the manufacturing apparatus employs a double structure crucible composed of an inner crucible and an outer crucible disclosed in Japanese Patent Application Laid-Open Nos. 2007-106662 and 2009-102194.
FIG. 2 shows a schematic diagram of a single crystal growth furnace by a single crystal pulling method when a resistance heater is used as a heating device. When a resistance heater is used, the heater is generally installed inside the heat insulating material wall. FIG. 3 shows a single crystal growth furnace using a single crystal pulling method when a high frequency induction heating type heater is used. In this case, the high-frequency coil corresponding to the heating device is generally installed outside the heat insulating material wall. The manufacturing method of the present invention can be applied to either such resistance heating type or high frequency induction heating type heaters.

The embodiment shown in FIG. 2 shows a method of directly supplying the scavenger gas into the former melt of metal fluoride (supply method in melt). The scavenger gas introduction pipe (21) in the melt supply method penetrates the heat insulating material wall (8) and the lid material (18) from the side surface of the chamber (4), and is stored in the outer crucible (1) ( 12) It is installed at a position where the tip enters. This is an example, and the introduction pipe may be penetrated from the ceiling surface and the lower surface of the chamber and / or the heat insulating material wall, and may be appropriately set in accordance with the structure of the operating part of the crystal growth furnace.
The material of the introduction tube must be one that does not deform or deteriorate at the temperature of the melt, and it is preferable to use a carbon-based material such as graphite carbon or a high melting point metal such as platinum or molybdenum. A carbon-based material is particularly preferable because it is relatively inexpensive.
The diameter of the introduction pipe may be appropriately set according to the supply amount of the scavenger gas, but generally the cross-sectional area of the tip is about 0.5 to ⁷ cm ² .
The insertion depth of the introduction tube is not particularly limited and can be appropriately selected from just below the liquid level to near the bottom of the crucible, but when using a method of gradually raising the outer crucible using a double crucible as described below, It is preferable that the position be higher than the lowermost end of the inner crucible.
The introduction rate of the scavenger gas in the melt supply method is preferably 2 to 15 mL / min, more preferably 5 to 10 mL, from the viewpoint of effectively expressing the reactivity with the impurities and performing stable growth. A range of / min is employed.

On the other hand, the mode shown in FIG. 3 shows a method of spraying scavenger gas directly onto the liquid surface of the latter metal fluoride melt (liquid surface spraying method). The tip (outlet) of the scavenger gas introduction pipe (22) in the liquid level spraying method is about 1 to 5 cm above the melt level in the inner crucible (2), particularly about 2 to 4 cm. It is preferable that the scavenger gas is sprayed on the lower liquid surface. If the distance between the air outlet and the liquid surface is too close, it tends to be a disturbing factor for the stability of the liquid surface during crystal growth, and if it is too far, the effect of the present invention may not be sufficiently obtained.
In the case of the liquid level spraying method, the structure of the introduction pipe, the gas introduction rate, etc. are implemented according to the melt supply method, but the reaction efficiency with impurities is slightly reduced, so the gas introduction rate is set high. It is preferable to do this. Preferably the range of 5-100 mL / min, More preferably, the range of 7-20 mL / min is employ | adopted.

The melt supply method and the liquid level spraying method may be carried out independently, but it is preferable to carry out both methods simultaneously from the viewpoint of further improving the laser resistance.
Further, for the purpose of facilitating adjustment of the scavenger gas supply rate, the scavenger gas diluted with an inert gas can be used. In this case, since the effect of this invention will fall relatively when it dilutes too much, Preferably it is set to 2 times dilution or less, More preferably, it shall be 1.5 times dilution or less.

By the way, in the method for producing a metal fluoride single crystal by the single crystal pulling method, a production method using the above-described double crucible has been developed in order to grow large crystals of good quality by reducing the natural convection of the melt. ing. Also in this invention, the said double crucible is employ | adopted suitably by the following points.
The double crucible includes an outer crucible (1) and an inner crucible (2) having a communication hole (3) through which the melt (12) can move. When directly supplying the scavenger gas into the melt, as shown in FIG. 2, if the scavenger gas is introduced into the melt in the outer crucible (1) through the scavenger gas introduction pipe, the inner crucible wall becomes a buffer wall, Changes in the temperature distribution of the melt accompanying the introduction and the occurrence of convection of the melt can be prevented, and as a result, high-quality single crystals can be produced stably. In addition, when scavenger gas is sprayed on the liquid surface, as shown in FIG. 3, if the gas is sprayed on the melt surface in the inner crucible (2), the gas does not flow toward the higher temperature heater by the inner crucible wall. Moreover, it is possible to efficiently react with the melt.
In addition, since the shape of the double crucible, the number of communication holes, the diameter, and the position thereof are described in detail in JP 2009-40630, JP 2006-199577, etc., the double crucible in the present invention is It is produced and used according to these.

[Inert gas]
In the present invention, all or a part of the heat insulating material wall constituting the single crystal growth furnace is made of a carbon member. Since this carbon member reacts with the scavenger gas and causes the above-described problems, it is important to make the inside of the growth furnace an inert gas atmosphere during single crystal growth.
As the inert gas to be used, known argon gas or helium gas can be used without any limitation. In order to make the inside of the furnace an inert gas atmosphere, the atmosphere inside the furnace is preferably replaced with an inert gas before introducing the scavenger gas described above. Thereafter, an inert gas is introduced into the furnace in parallel with the introduction of the scavenger gas in order to keep the change in the atmosphere in the furnace due to the introduction of the scavenger gas small. The amount of gas introduced is appropriately determined depending on the furnace volume, growth temperature, etc., but is usually selected from the range of 0.1 to 2 L / min.
From the viewpoint of protecting the carbon insulating wall from scavenger gas damage, the inert gas is preferably introduced directly into the space formed by the insulating wall. Further, since the specific gravity of the scavenger gas is generally heavy and tends to accumulate at the bottom of the furnace, the inert gas is introduced below the bottom of the crucible, particularly upward from the bottom of the chamber as shown in FIG. As shown, it is preferable to supply an inert gas from the inlet (25) provided in the support shaft (5).

[Single crystal growth]
In the present invention, a seed crystal held at the lower end of a pulling shaft is immersed in a fluoride metal melt in a crucible, and a single crystal is produced by pulling up the pulling shaft while growing the single crystal below the seed crystal. This is applied to a known single crystal pulling method.
The details of the single crystal pulling method, that is, the structure and members of the growth apparatus, the pulling conditions and method, etc. are disclosed in JP 2005-29455 A, JP 2004-231502 A, and JP 2004-2004. Specifically described in JP-A-182588, JP-A-2004-182587, JP-A-2003-183096, JP-A-2003-119095, JP-A-2002-60299, and JP-A-2002-23495. The details of the present invention are implemented according to these.
However, as described above, all or a part of the heat insulating material wall constituting the single crystal growth furnace of the present invention is made of a carbon member. Examples of the carbon member include pitch-based graphite materials, fiber-based graphite materials, carbon felt-based materials, porous carbon-based materials, and the like. Among these, it is particularly preferable to use a pitch-based graphite material because a desired heat radiation capability can be achieved, and the material is excellent in resistance to a harsh environment during pulling and excellent in mechanical strength.
If the heat insulating wall (8) is excellent in heat insulating properties as a whole wall, not only the wall material made of the above-mentioned single material, but also a structure in which a plurality of plate-like carbon members are laminated, A structure in which a plurality of plate-like carbon members are laminated with a gas phase interposed therebetween may be employed. The thickness of the heat insulating material is not particularly limited, but is generally 3 to 10 cm.
Further, as a material for the crucible (outer crucible and inner crucible), high purity graphite carbon is preferably used.
As the heater, a resistance heating type heater or a high frequency heating heater can be appropriately selected and used. When the resistance heating heater is employed, it is preferably a carbon resistance heating heater from the viewpoint of being inert at high temperatures. In this case, the carbon heater is inevitably installed inside the heat insulating material wall and exposed to an extremely high temperature environment. Therefore, the effect of protecting the heater and the crucible can also be obtained as an effect of the present invention.
On the other hand, when a high-frequency heater (coil) is employed, it can be arranged outside the heat insulating material wall, and if arranged in this way, it does not come into contact with the scavenger gas at a high temperature and deteriorate quickly.

Hereinafter, a method for producing a metal fluoride single crystal using the above-described double structure crucible will be briefly described.
First, a metal fluoride raw material in which most of oxygen, moisture and oxides are removed by heating and melting in the presence of a known scavenger such as zinc fluoride, lead fluoride and / or carbon tetrafluoride in a growth furnace Put in the outer crucible (1). Next, the metal fluoride raw material charged into the crucible is subjected to heat treatment under reduced pressure prior to melting to further remove adsorbed moisture. After sufficiently heating and removing the adsorbed moisture, the metal fluoride raw material is melted and the single crystal is pulled up from the melt.
Specifically, as an operation for removing adsorbed moisture from the metal fluoride raw material charged in the crucible by the above heat treatment, the temperature is 200 ° C. or higher and a temperature lower than the temperature at which the action as the scavenger gas to be used is started prior to melting. It is preferable to evacuate. Subsequently, the scavenger gas was introduced into the crystal growth furnace, and the temperature was further raised to start the action as the scavenger gas, and the temperature was lower than the melting point of the raw metal fluoride, and could not be removed by evacuation alone. Remove moisture. Further, after passing through the process of evacuating the inside of the crystal growth furnace again, the metal fluoride raw material is melted (final melting) in order to contact the seed crystal, and the single crystal is pulled from the melt.

Here, in order to make the inside of the furnace an inert gas atmosphere as described above, it is preferable that the inside of the furnace is once filled with only the inert gas after the vacuum evacuation before pulling up the single crystal from the melt. The introduction of the inert gas may be started before the final melting of the metal fluoride raw material, or may be performed after the completion of the final melting and before the contact of the seed crystal (start of growth). The pressure when the furnace is filled with only an inert gas can be selected from the range of atmospheric pressure (101 kPa) to 20 kPa.
The supply of the scavenger gas is preferably started after the inside of the furnace is filled (replaced) with the inert gas in this way. In practicing the present invention, it is preferable to exhaust the gas corresponding to the amount of gas introduced. The evacuation can be performed from any location such as the top, side or bottom of the chamber.

The temperature at the time of pulling up the single crystal is determined according to the target metal fluoride. For example, at the measurement temperature at the bottom of the crucible, in the case of calcium fluoride, it is 1440 ° C. or higher, preferably 1440-1520 ° C. Preferably it is carried out at temperature. Moreover, it is preferable that the temperature increase rate to this temperature is 10-500 degreeC / hour.
The seed crystal used for the single crystal pulling method is a single crystal of metal fluoride, and the growth surface of the seed crystal is the [111] plane, [100], depending on the main growth plane of the as-grown single crystal to be produced. It may be selected appropriately from the viewpoint of the surface.
During the growth of the single crystal, these seed crystals are preferably rotated around 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 at the same rotational speed in the direction opposite to the rotation direction of the seed crystal. After pulling up a single crystal of a desired size in this way, the temperature is lowered to a temperature at which it can be taken out from the furnace. If the temperature is lowered too rapidly, the resulting single crystal may be greatly strained or damaged by thermal shock in extreme cases. Therefore, the rate of temperature reduction after crystal growth is preferably 0.1 to 3 ° C./min. It is.

[Post-processing]
The as-grown single crystal obtained by the above method is usually annealed in order to remove the strain existing inside. In the annealing, after growing as described above, it may be an ingot as it is taken out from the furnace, but in order to anneal more efficiently, the ingot is cut into an appropriate size into a disk shape, It is preferable to anneal this. It is also preferable to polish and clean the cut surface after the cutting and before annealing. Of course, other than the disk shape, it may be annealed after processing into a required shape.

EXAMPLES Hereinafter, although an Example demonstrates this invention further, this invention is not limited to these Examples at all. In addition, not all combinations of features described in the embodiments are essential to the solution means of the present invention.
Each evaluation method of light transmittance in the vacuum ultraviolet region (hereinafter simply referred to as VUV transmittance) and laser-induced absorption (hereinafter simply referred to as LIA) serving as an index of laser resistance is as follows.
(1) Measurement of VUV transmittance The surface was polished until the surface roughness became 0.5 nm or less by RMS, and a sample having a thickness of 10 mm or 25 mm was produced. This was ultrasonically cleaned in acetone for 2 minutes, dried, and then used for 15 minutes at an output of 7 mW / cm ² using an ultraviolet ozone cleaning apparatus (UV-208 manufactured by Technovision) using a low-pressure mercury lamp as a light source. UV cleaning was performed. Then, the transmittance | permeability was measured in the range of 120-210 nm for the wash | cleaned sample using the VUV transmittance | permeability measuring device (JASCO Corporation KV-201; measured in nitrogen atmosphere with an oxygen content of 0.2 ppm or less). .
(2) Measurement of LIA About the same sample which performed the said VUV transmittance | permeability measurement, after performing the said ultraviolet ozone washing process again, laser irradiation was performed using the ultraviolet visible spectrophotometer (Shimadzu Corporation UV-1800). The previous transmittance was measured in the range of 200-800 nm. Subsequently, using an ArF excimer laser light source device (manufactured by Coherent: LPX Pro 220), a laser having an energy density of 30 mJ / cm ² was irradiated with 100,000 pulses at a pulse repetition frequency of 100 Hz, and 200 to 800 nm after laser irradiation. The transmittance in the range was measured using the apparatus. The LIA spectrum is obtained by converting the difference in transmittance before and after laser irradiation into absorbance. The smaller the LIA spectrum, the smaller the change in transmittance before and after laser irradiation, and the better the laser durability.

Example 1
A calcium fluoride single crystal was manufactured using a pulling apparatus for manufacturing a single crystal having the structure shown in FIG. In this pulling apparatus for producing a single crystal, the high-purity graphite outer crucible (1) installed in the chamber (4) had an inner diameter of 50 cm (an outer diameter of 52 cm) and a height of 24 cm. . The inner crucible (2) fixed to the lid material (18) in the outer crucible (1) had an inner diameter of 31 cm (outer diameter of 32 cm) and a height of 23 cm.
The bottom wall of the inner crucible (2) had a V-shaped (mortar shape) vertical cross-sectional shape with an inclination angle of 30 degrees downward with respect to the horizontal plane. One cylindrical communication hole (3) having a diameter of 6 mm at the lower end thereof, and a cylindrical communication hole (3) having a diameter of 4 mm at equal intervals on the circumference at a height of 13 mm above the one. There are four cylindrical communication holes (3) having a diameter of 2.5 mm at regular intervals on the circumference at a height of 15 mm above the four. The heat insulating material wall (8) is a pitch-based graphite heat insulating material and has a heat dissipation capacity in the thickness direction of 9 W / m ² · K, while the ceiling plate (16) is made of graphite and has a thickness direction. The heat radiating capacity was 5000 W / m ² · K.

In the outer crucible (1), a total of 40 kg of raw material calcium fluoride lump subjected to sufficient purification treatment and moisture removal treatment in the presence of carbon tetrafluoride was charged. And after exhausting the inside of a chamber (4) with an oil rotary pump and an oil diffusion pump until it became 1x10 < ^-3 > Pa or less, heating was started. After raising the temperature to 320 ° C., the temperature was maintained for 16 hours. Thereafter, the temperature was raised to 500 ° C. and kept at this temperature for 16 hours. Then, after supplying 20 kPa (absolute pressure) of carbon tetrafluoride and 80 kPa (absolute pressure) of argon gas from the gas introduction pipe (22), the temperature was raised to 1000 ° C. The exhaust electromagnetic valve was automatically opened and closed to maintain the furnace pressure at atmospheric pressure. After holding at this temperature for 8 hours, the oil was evacuated to 1 × 10 ⁻³ Pa or less with an oil rotary pump and an oil diffusion pump, and held for 8 hours. After completion of the evacuation, argon gas was supplied from the gas introduction pipe (22) to atmospheric pressure, the temperature was raised to 1520 ° C., and the raw material calcium fluoride was melted. Also in this case, the furnace pressure was maintained at atmospheric pressure as described above.
After the raw material is completely melted and the raw calcium fluoride melt (12) is accommodated in the outer crucible (1) and the inner crucible (2), the raw material is held for 1 hour, and then from the viewing window (14). The surface state of the melt (12) accommodated in the inner crucible (2) was confirmed. As a result, solid impurities were confirmed to float, and the support shaft (5) was lowered to move the inner crucible (2) The total amount of the melt (12) accommodated in the was discharged into the outer crucible (1). Thereafter, the support shaft (5) was raised again, and an operation of supplying the melt (12) in the outer crucible (1) into the inner crucible (2) was performed. The surface state of the melt (12) accommodated in the inner crucible (2) was confirmed again from the observation window (14), but at this time, solid impurities could not be confirmed. In addition, the depth of the melt (12) of the metal fluoride raw material accommodated in the inner crucible (2) was 8 cm.

Next, after lowering the temperature to 1480 ° C. and holding it for 1 hour, the single crystal pulling rod (11) is suspended, and the lower end surface (single crystal growth surface) whose crystal plane of the seed crystal (9) is (111) is formed. The surface of the raw material calcium fluoride melt (12) was brought into contact, and the pulling of the single crystal was started. Simultaneously with the start of pulling, argon gas is supplied at 2 L / min from a gas introduction pipe (23) for supplying an inert gas, and tetrafluoride is supplied from a gas introduction pipe (21) for supplying a scavenger gas directly into the melt. Carbon tetrafluoride gas was allowed to flow at 10 mL / min from a gas introduction pipe (22) for blowing carbon gas at 5 mL / min and scavenger gas on the melt surface. The gas was introduced until the pulling was completed and cooling was started. The exhaust electromagnetic valve was automatically opened and closed, and the furnace pressure was maintained at atmospheric pressure. The seed crystal (9) was pulled up while being rotated at 3 times / minute. The pulling speed was 2 mm / hr. During the pulling up, the support shaft (5) was continuously raised so that the depth of the melt (12) in the inner crucible (2) was maintained at 8 cm. After completion of the pulling, the temperature was lowered to room temperature at a cooling rate of 1 ° C./min.
By the above operation, a (111) as-grown single crystal of calcium fluoride having a diameter of the straight body portion of about 8 cm and a weight of 3.0 kg was obtained. After slicing the crystal along a plane perpendicular to the growth direction, the surface was polished until the surface roughness became 0.5 mm or less by RMS to prepare a sample having a thickness of 25 mm, and VUV transmittance and LIA were measured. The spectrum of LIA is shown in FIG. Table 1 shows the transmittance at 193 nm and the integrated value of the LIA spectrum.

Example 2
A calcium fluoride single crystal was manufactured using a pulling apparatus for manufacturing a single crystal having the structure shown in FIG.
In the outer crucible (1), a total of 40 kg of raw material calcium fluoride lump subjected to sufficient purification treatment and moisture removal treatment in the presence of carbon tetrafluoride was charged. The operation from the start of the feed to the start of pulling was the same as in Example 1. Simultaneously with the start of pulling, argon gas is supplied at 2 L / min from a gas introduction pipe (23) for supplying an inert gas, and tetrafluoride is supplied from a gas introduction pipe (21) for supplying a scavenger gas directly into the melt. Carbon gas was flowed at 5 mL / min. The gas was introduced until the pulling was completed and cooling was started. The furnace pressure, the rotational speed of the seed crystal (9), the pulling rate, and the cooling rate after the pulling were the same as in Example 1.
By the above operation, a (111) as-grown single crystal of calcium fluoride having a straight body diameter of about 12 cm and a weight of 6.7 kg was obtained. After slicing the crystal along a plane perpendicular to the growth direction, the surface was polished until the surface roughness became 0.5 mm or less by RMS to prepare a sample having a thickness of 25 mm, and VUV transmittance and LIA were measured. The spectrum of LIA is shown in FIG. Table 1 shows the transmittance at 193 nm and the integrated value of the LIA spectrum.

Example 3
A calcium fluoride single crystal was manufactured using a pulling apparatus for manufacturing a single crystal having the structure shown in FIG.
In the outer crucible (1), a total of 40 kg of raw material calcium fluoride lump subjected to sufficient purification treatment and moisture removal treatment in the presence of carbon tetrafluoride was charged. The operation from the start of the feed to the start of pulling was the same as in Example 1. Simultaneously with the start of pulling, 2 L / min of argon gas is supplied from the gas introducing pipe (23) for supplying an inert gas, and carbon tetrafluoride gas is supplied from the gas introducing pipe (22) for spraying scavenger gas to the melt surface. Of 10 mL / min. The gas was introduced until the pulling was completed and cooling was started. The furnace pressure, the rotational speed of the seed crystal (9), the pulling rate, and the cooling rate after the pulling were the same as in Example 1.
By the above operation, a (111) as-grown single crystal of calcium fluoride having a straight body diameter of about 8 cm and a weight of 2.2 kg was obtained. After slicing the crystal along a plane perpendicular to the growth direction, the surface was polished until the surface roughness became 0.5 mm or less by RMS to prepare a sample having a thickness of 10 mm, and VUV transmittance and LIA were measured. The spectrum of LIA is shown in FIG. Table 1 shows the transmittance at 193 nm and the integrated value of the LIA spectrum.

Example 4
A calcium fluoride single crystal was manufactured using a pulling apparatus for manufacturing a single crystal having the structure shown in FIG.
In the outer crucible (1), a total of 40 kg of raw material calcium fluoride lump subjected to sufficient purification treatment and moisture removal treatment in the presence of carbon tetrafluoride was charged. And after exhausting the inside of a chamber (4) with an oil rotary pump and an oil diffusion pump until it became 1x10 < ^-3 > Pa or less, heating was started. After raising the temperature to 320 ° C., the temperature was maintained for 16 hours. Thereafter, the temperature was raised to 500 ° C. and kept at this temperature for 16 hours. Subsequently, the temperature was raised to 1000 ° C. and held at this temperature for 12 hours. After completion of the evacuation, argon gas was supplied from the gas introduction pipe (22) to atmospheric pressure, the temperature was raised to 1520 ° C., and the raw material calcium fluoride was melted. The operation from the melting to the start of the pulling was the same as in Example 1. Simultaneously with the start of pulling, argon gas is supplied at 2 L / min from a gas introduction pipe (23) for supplying an inert gas, and tetrafluoride is supplied from a gas introduction pipe (21) for supplying a scavenger gas directly into the melt. Carbon tetrafluoride gas was allowed to flow at 10 mL / min from a gas introduction pipe (22) for blowing carbon gas at 5 mL / min and scavenger gas on the melt surface. The gas was introduced until the pulling was completed and cooling was started. The furnace pressure, the rotational speed of the seed crystal (9), the pulling rate, and the cooling rate after the pulling were the same as in Example 1.
By the above operation, a (111) as-grown single crystal of calcium fluoride having a straight body diameter of about 8 cm and a weight of 2.1 kg was obtained. After slicing the crystal along a plane perpendicular to the growth direction, the surface was polished until the surface roughness became 0.5 mm or less by RMS to prepare a sample having a thickness of 25 mm, and VUV transmittance and LIA were measured. The spectrum of LIA is shown in FIG. Table 1 shows the transmittance at 193 nm and the integrated value of the LIA spectrum.

Example 5
A calcium fluoride single crystal was manufactured using a pulling apparatus for manufacturing a single crystal having the structure shown in FIG. In this pulling apparatus for producing a single crystal, the high-purity graphite outer crucible (1) installed in the chamber (4) had an inner diameter of 23 cm (outer diameter of 24 cm) and a height of 25 cm. . The inner crucible (2) fixed to the lid material (18) in the outer crucible (1) had an inner diameter of 20 cm (outer diameter of 21 cm) and a height of 24 cm.
The bottom wall of the inner crucible (2) had a V-shaped (mortar shape) vertical cross-sectional shape with an inclination angle of 30 degrees downward with respect to the horizontal plane. One cylindrical communication hole (3) having a diameter of 8 mm was formed at the lower end thereof. Except for the above, members such as a heat insulating wall and a ceiling board having the same performance as in Example 1 were used.
Into the outer crucible (1), a total of 9 kg of raw material calcium fluoride lump subjected to sufficient purification treatment and moisture removal treatment in the presence of carbon tetrafluoride was charged. And after exhausting the inside of a chamber (4) with an oil rotary pump and an oil diffusion pump until it became 1x10 < ^-3 > Pa or less, heating was started. After raising the temperature to 250 ° C., the temperature was maintained for 15 hours. Thereafter, carbon tetrafluoride 10 kPa (absolute pressure) and argon gas 40 kPa (absolute pressure) were supplied from a gas inlet (25) attached to the support shaft (5), and then the temperature was raised to 1050 ° C. After holding at this temperature for 5 hours, the oil was evacuated to 1 × 10 ⁻³ Pa or less with an oil rotary pump and an oil diffusion pump, and held for 8 hours. After the evacuation, argon gas 40 kPa (absolute pressure) was supplied from a gas inlet (25) attached to the support shaft (5), and the temperature was raised to 1520 ° C. to melt the raw material calcium fluoride.

The operation from the melting to the start of the pulling was the same as in Example 1. Simultaneously with the start of pulling, carbon tetrafluoride is introduced from a gas introduction pipe (23) for blowing argon gas at 100 mL / min and a scavenger gas from the gas introduction port (25) attached to the support shaft (5) to the melt surface. Gas was allowed to flow at 100 mL / min. The gas was introduced until the pulling was completed and cooling was started. In addition, the electromagnetic valve for exhaust was automatically opened and closed, and the furnace pressure was maintained at 40 kPa (absolute pressure). The seed crystal (9) was pulled up while being rotated at 3 times / minute. The pulling speed was 4 mm / hr. During the pulling, the support shaft (5) was continuously raised so that the depth of the melt (12) in the inner crucible (2) was maintained at 5 cm. After completion of the pulling, the temperature was lowered to room temperature at a cooling rate of 1 ° C./min.
By the above operation, a (111) as-grown single crystal of calcium fluoride having a straight body diameter of about 8 cm and a weight of 3.4 kg was obtained. After slicing the crystal along a plane perpendicular to the growth direction, the surface was polished until the surface roughness became 0.5 mm or less by RMS to prepare a sample having a thickness of 10 mm, and VUV transmittance and LIA were measured. The spectrum of LIA is shown in FIG. Table 1 shows the transmittance at 193 nm and the integrated value of the LIA spectrum.

Comparative Example 1
A calcium fluoride single crystal was manufactured using a conventional pulling apparatus for manufacturing a single crystal having the structure shown in FIG. The same members as in Example 1 were used except for the gas introduction pipe (20).
In the outer crucible (1), a total of 40 kg of raw material calcium fluoride lump subjected to sufficient purification treatment and moisture removal treatment in the presence of carbon tetrafluoride was charged. Except for introducing argon gas and carbon tetrafluoride gas using the conventional gas introduction pipe (20), the operation from the start of the raw material until the start of the pulling was the same as in Example 1. In this case, scavenger gas and argon gas were not introduced after the start of pulling. The rotational speed, pulling speed, and cooling rate after the pulling of the seed crystal (9) were the same as those in Example 1.
By the above operation, a (111) as-grown single crystal of calcium fluoride having a straight body diameter of about 12 cm and a weight of 5.3 kg was obtained. After slicing the crystal along a plane perpendicular to the growth direction, the surface was polished until the surface roughness became 0.5 mm or less by RMS to prepare a sample having a thickness of 25 mm, and VUV transmittance and LIA were measured. The spectrum of LIA is shown in FIG. Table 1 shows the transmittance at 193 nm and the integrated value of the LIA spectrum.

Comparative Example 2
A calcium fluoride single crystal was manufactured using a conventional pulling apparatus for manufacturing a single crystal having the structure shown in FIG.
In the outer crucible (1), a total of 40 kg of raw material calcium fluoride lump subjected to sufficient purification treatment and moisture removal treatment in the presence of carbon tetrafluoride was charged. Argon gas 35 kPa (absolute pressure) and carbon tetrafluoride 5 kPa (absolute pressure) are supplied from the gas introduction pipe (20), the temperature is raised to 1520 ° C., and the raw material calcium fluoride is melted. The operation until the start of pulling was the same as that in Comparative Example 1. Also in this case, scavenger gas and argon gas were not introduced after the start of pulling. The rotational speed, pulling speed, and cooling rate after the pulling of the seed crystal (9) were the same as those in Example 1.
By the above operation, a (111) as-grown single crystal of calcium fluoride having a straight body portion with a diameter of about 8 cm and a weight of 3.8 kg was obtained. After slicing the crystal along a plane perpendicular to the growth direction, the surface was polished until the surface roughness became 0.5 mm or less by RMS to prepare a sample having a thickness of 25 mm, and VUV transmittance and LIA were measured. The spectrum of LIA is shown in FIG. Table 1 shows the transmittance at 193 nm and the integrated value of the LIA spectrum.

Comparative Example 3
A calcium fluoride single crystal was manufactured using a pulling apparatus for manufacturing a single crystal having the structure shown in FIG.
Into the outer crucible (1), a total of 9 kg of raw material calcium fluoride lump subjected to sufficient purification treatment and moisture removal treatment in the presence of carbon tetrafluoride was charged. The operation from when the raw material was charged until when the pulling was started was the same as in Example 5. Also in this case, as in Comparative Example 1, scavenger gas and argon gas were not introduced after the start of pulling. Further, the rotation speed, pulling speed, and cooling speed after the pulling of the seed crystal (9) were the same as those in Example 5.
By the above operation, a (111) as-grown single crystal of calcium fluoride having a straight body diameter of about 8 cm and a weight of 3.3 kg was obtained. After slicing the crystal along a plane perpendicular to the growth direction, the surface was polished until the surface roughness became 0.5 mm or less by RMS to prepare a sample having a thickness of 25 mm, and VUV transmittance and LIA were measured. The spectrum of LIA is shown in FIG. Table 1 shows the transmittance at 193 nm and the integrated value of the LIA spectrum.

By comparing the transmittance at 193 nm and the integral value of the LIA spectrum obtained in Examples 1 to 5 and Comparative Examples 1 to 3, the following can be confirmed, and the production method of the present invention has high optical characteristics. It turns out that it is suitable for the manufacturing method of the metal fluoride single crystal which has.
Transmittance:
With respect to the VUV transmittance, there was no significant difference between the examples and the comparative examples, and both had good VUV transmittance.
LIA:
According to the present invention, that is, scavenger gas is supplied into the melt of metal fluoride or sprayed on the surface of the melt, and the inside of the single crystal growth furnace is made an inert gas atmosphere with argon gas or the like. The integrated value could be greatly reduced. This reduction in value indicates that the generation of color centers after laser irradiation could be suppressed. According to the present invention, it is presumed that impurities causing the color center can be more thoroughly removed. The integrated values of the LIA spectra of Example 1 and Example 4 in which both the scavenger gas supply method in the melt and the liquid level spraying method are performed simultaneously are particularly low and the effect is large.

DESCRIPTION OF SYMBOLS 1; Outer crucible 2; Inner crucible 3; Communication hole 4; Chamber 5; Support shaft 6; Base 7; Resistive heater 8; Insulation material wall 9; Seed crystal body 10; Holder 11; Single crystal pulling rod 12; Metal fluoride raw material melt 13; Metal fluoride single crystal ingot 14; Viewing window 15; Single crystal pulling rod insertion hole 16; Ceiling plate 17; Isolation wall 18; Lid material 19; Inert gas and scavenger gas introduction pipe 21; gas introduction pipe 22 for supplying scavenger gas directly into the melt; gas introduction pipe 23 for spraying scavenger gas to the melt surface; for supplying inert gas Gas inlet tube 24; high-frequency induction heater 25; inlet for supplying inert gas

Claims (7)

A crucible containing metal fluoride is disposed inside the metal fluoride single crystal growth furnace having a carbon heat insulating material wall, and the metal fluoride is heated to form a metal fluoride melt, By immersing the seed crystal held at the lower end of the pulling shaft in the melt of the metal fluoride and growing the metal fluoride single crystal below the seed crystal, pulling up the pulling shaft, In the method for producing a metal fluoride single crystal for growing a metal single crystal,
An inert gas atmosphere is formed in the metal fluoride single crystal growth furnace, and a scavenger gas is supplied into the melt of the metal fluoride or sprayed onto the melt surface to grow the metal fluoride single crystal. The manufacturing method of the metal fluoride single crystal characterized by the above-mentioned.   2. The method for producing a metal fluoride single crystal according to claim 1, wherein the inert gas is introduced into the furnace from the bottom of the metal fluoride single crystal growth furnace.   The metal fluoride single crystal according to claim 1 or 2, wherein the crucible is a double crucible composed of an inner crucible and an outer crucible, and a scavenger gas is supplied into the melt of the metal fluoride in the outer crucible. Manufacturing method.   3. The metal fluoride single crystal according to claim 1, wherein the crucible is a double crucible comprising an inner crucible and an outer crucible, and a scavenger gas is blown onto the melt surface of the metal fluoride in the inner crucible. Manufacturing method.   The method for producing a metal fluoride single crystal according to claim 1, wherein the scavenger gas is a fluorinated hydrocarbon gas.   The method for producing a metal fluoride single crystal according to claim 1, wherein the inert gas is argon. A crucible containing metal fluoride is disposed inside the metal fluoride single crystal growth furnace having a carbon heat insulating material wall, and the metal fluoride is heated to form a metal fluoride melt, By immersing the seed crystal held at the lower end of the pulling shaft in the melt of the metal fluoride and growing the metal fluoride single crystal below the seed crystal, pulling up the pulling shaft, In the method for producing a metal fluoride single crystal for growing a metal single crystal,
The scavenger gas composed of the fluorine-based gas is supplied into the melt of the metal fluoride or sprayed to the liquid surface of the melt, inside the space constituted by the heat insulating material wall, and from the crucible bottom. A method for producing a metal fluoride single crystal, comprising growing a metal fluoride single crystal while supplying an inert gas from a lower position.