JP2006342029A - Annealing furnace used for heat-treating metal fluoride single crystal and annealing method of metal fluoride single crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an annealing furnace and an annealing method, which are suitable for obtaining a metal fluoride single crystal having low double refraction and useful as an optical member of a semiconductor production apparatus, especially a large-sized metal fluoride single crystal having a diameter of >200 mm. <P>SOLUTION: The annealing furnace comprises a heat-insulating vessel (2) for covering the whole side periphery and the upper and lower parts of the fluoride metal single crystals (1) and a heater (4) arranged at the outside of the heat-insulating vessel (2). The heat-insulating vessel (2) is preferably constituted of a heat-insulating material having a coefficient of thermal conductivity in the thickness direction of ≤2 W/m×K<SP>-1</SP>. In such an annealing furnace, the temperature distribution in the heat-insulating vessel can be made narrow. Thereby, strain can be efficiently released even for a single crystal in which strain is hardly released and a large-sized single crystal having a small double refraction can be obtained by annealing a metal fluoride single crystal (e.g., calcium fluoride) by using the annealing furnace. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an annealing furnace and an annealing method for heat-treating a metal fluoride single crystal. More specifically, the present invention relates to an annealing furnace and an annealing method suitable for obtaining a metal fluoride single crystal having a small birefringence that is useful as an optical member of a semiconductor manufacturing 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).

  However, even with the single crystal pulling method, which is a method with little distortion, there are cases in which residual stress and distortion are still too large depending on the optical application described above. If there is a large residual stress or strain in the single crystal, the birefringence due to these will also increase, and problems may arise if the single crystal is used for applications having extremely strict optical properties, particularly projection system lenses. is there. Therefore, when the strain is too large for the intended use, after the obtained single crystal is processed into a disk shape, the residual stress and strain are removed by further performing a heat treatment called annealing.

  In the annealing, as the temperature distribution inside the single crystal to be annealed is made as flat as possible, a better result is obtained, and several techniques for obtaining such thermal uniformity have been proposed. For example, a method in which two or more heaters capable of independent temperature control are provided on the outer side of a container containing a single crystal, and reflectors are arranged above and below (for example, refer to Patent Document 3). A method in which a container containing crystals is housed in a further airtight container, and heaters capable of temperature control are arranged independently on the outer side, bottom and top of the airtight container (for example, Patent Document 4). See), the container containing the metal fluoride single crystal is further housed in an airtight container that can be evacuated, and the length of the heater disposed on the outer side of the airtight container is equal to or greater than the thickness of the single crystal to be heat-treated. A method (see, for example, Patent Document 5), a method in which a heater is disposed on the outer side of an airtight container containing a single crystal, and a heat insulating member is disposed in the vertical direction in which heat easily escapes (for example, see Patent Document 6). ) Disk-shaped single crystal Housed in airtight container, a method for shortening the heat transfer path to the center by disposing the main heater the top and bottom of the container (e.g., see Patent Document 7) it has been proposed. Further, as a method of relieving internal stress by annealing, when placing a single crystal in a crucible in a heat treatment furnace, heating the single crystal with external force applied to compensate the internal stress distribution (for example, Patent Document 8) has also been proposed.

  Usually, annealing of a metal fluoride single crystal is performed in a vacuum or in an inert gas or a fluorine-based gas. Therefore, in each of the above techniques, a container containing a metal fluoride single crystal is usually a more airtight container. And a method of heating with a heater from the outside of the airtight container is employed. This hermetic container has high heat conductivity such as high density carbon, graphite, boron nitride, silicon carbide, etc. so that the heating efficiency by the heater is good and the container body has a uniform temperature distribution quickly. It is made of a material (see, for example, Patent Documents 7 and 9).

JP 2004-182588 A JP 2005-029455 A JP-A-10-231194 Japanese Patent Laid-Open No. 10-265300 JP 2000-026198 A JP 2000-034193 A JP 2004-131368 A JP 2000-281492 A Japanese Patent Laying-Open No. 2003-081700 Paragraph 0031

  However, even if annealing is performed by the method as proposed above, it is still difficult to flatten the temperature distribution inside the single crystal, and a single-crystal fluoride metal single crystal having a small birefringence from which distortion has been sufficiently removed is not necessarily obtained. It was not always possible to obtain it, and an improvement in yield was desired. This problem becomes more serious as the metal fluoride single crystal becomes larger, because temperature distribution is more likely to occur in the crystal, and a large-diameter single crystal that is highly useful as an optical material for the next-generation photolithography technology, for example, 200 mmφ It was difficult to obtain the above single crystal and small birefringence sufficient for the application. In such applications, the birefringence is desired to be 1 nm / cm or less, particularly 0.6 nm / cm or less, on average.

  In particular, when the (100) crystal has the same degree of strain, the measured birefringence is several times larger than that of the (111) crystal. Therefore, in order to obtain a (100) single crystal having a birefringence as low as that of the (111) single crystal, it is necessary to remove strain more highly. However, with the conventional method, it has been extremely difficult to obtain a (100) single crystal from which distortion has been removed to such an extent that it can be used for optical material applications as described above.

  In view of the above problems, the inventors of the present invention, when annealing a metal fluoride single crystal in an annealing (heat treatment) furnace, the temperature distribution in the metal fluoride single crystal disposed in the furnace is made as flat as possible. The method that can be done was studied earnestly. As a result, surprisingly, contrary to the prior art, by further disposing a member having low thermal conductivity between the storage container storing the metal fluoride single crystal and the heater, The temperature portion in the crystal was averaged, and therefore, after annealing, it was found that a metal fluoride single crystal having extremely low birefringence can be obtained efficiently, and further investigations were made. As a result, the present invention was completed.

  That is, the present invention relates to an annealing furnace used for heat-treating a metal fluoride single crystal, comprising a heat insulating container that covers the entire circumference and upper and lower sides of the metal fluoride single crystal and a heater disposed outside the heat insulating container. It is.

  Another invention includes a storage container capable of storing the metal fluoride single crystal in multiple stages, a heat insulating container covering the entire circumference and top and bottom sides of the storage container, and a heater disposed outside the heat insulating container. An annealing furnace used for simultaneously heat-treating a plurality of metal fluoride single crystals.

Another invention is the annealing furnace in which the heat insulation container is made of a material having a thermal conductivity of 2 W · m ⁻¹ · K ⁻¹ or less, and the other invention is a side heat insulation in the heat insulation container. It is the said annealing furnace whose heat transfer capability of the thickness direction of material is 100 W * m ^<-2> * K ^{< -1>} or less.

  Yet another invention is an annealing method for a metal fluoride single crystal, wherein the entire circumference and upper and lower sides of the metal fluoride single crystal are covered with a heat insulating material and heated from the outside of the heat insulating material with a heater.

  According to the present invention, when annealing a metal fluoride single crystal, heating can be performed in a state where the temperature distribution in the metal fluoride single crystal is extremely flattened. Therefore, even when the crystal is large, the internal stress and strain of the crystal can be surely removed. As a result, the birefringence is sufficiently small, and it is highly useful as an optical material for next-generation photolithography technology. A large-diameter single crystal can be obtained with a high yield.

  The annealing furnace of the present invention is a heat treatment furnace for heat-treating a metal fluoride single crystal to remove its distortion and residual stress.

  The metal fluoride single crystal may be any known metal fluoride single crystal. Specific examples of the metal fluoride single crystal include lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, magnesium fluoride, calcium fluoride, and fluoride. Barium, strontium fluoride, aluminum fluoride, lithium barium fluoride, potassium magnesium fluoride, lithium aluminum fluoride, calcium strontium fluoride, magnesium magnesium fluoride, lithium strontium fluoride, cesium calcium fluoride, lithium calcium calcium fluoride Single crystals of lithium strontium aluminum fluoride, lanthanoid fluorides, and the like.

  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 single crystal annealing of alkaline earth metal fluorides, It is more preferable to apply to alkaline earth metals, and the effects of the present invention are particularly remarkable when calcium fluoride is the target.

  The method for obtaining a metal fluoride single crystal as described above (before annealing) is not particularly limited, and the crystal pulling method (Czochralski method), crucible descent method (Bridgeman method), zone melting method (zone melting method) In addition, a single crystal obtained by any known production method such as a floating zone melting method (floating zone method) may be used. For example, as a crystal pulling method, JP-A-2005-29455, JP-A-2004-231502, JP-A-2004-182588, JP-A-2004-182588, JP-A-2003-183096, JP-A-2003-119095, JP-A-2002-60299, JP-A-2002-23495, etc. The manufacturing method and apparatus of these are mentioned. Examples of the crucible lowering method include manufacturing methods and apparatuses described in JP-A-9-227293, JP-A-9-315894, JP-A-2004-262742, and the like.

  A typical crystal pulling method will be briefly described. First, a fluororesin in which most of impurities such as oxides and moisture are removed by heating and melting in the presence of a scavenger such as zinc fluoride, lead fluoride or carbon tetrafluoride. The metal halide raw material is put into a crucible in a single crystal pulling furnace.

  Prior to melting, the metal fluoride raw material charged into the crucible is preferably subjected to a heat treatment under reduced pressure 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.

  The temperature at which the single crystal is pulled is determined according to the target metal fluoride. For example, at the measurement temperature of the bottom of the crucible, in the case of calcium fluoride, it is 1440 ° C. or higher, preferably 1440-1520 ° C. In the case of barium fluoride, it is preferably carried out at a temperature of 1300 to 1400 ° C. Moreover, it is preferable that the temperature increase rate to this temperature is 10-500 degreeC / hour.

  In order to eliminate the influence of residual moisture, the removal of water and the pulling up by the heating are preferably performed in the presence of a scavenger. Scavengers include solid scavengers such as zinc fluoride, lead fluoride, and polytetrafluoroethylene that are charged together with the raw metal fluoride, as well as carbon tetrafluoride, carbon trifluoride, and hexafluoride introduced into the chamber as an atmosphere. A gas scavenger such as ethane fluoride is used. It is preferable to use a solid scavenger, and the amount used is preferably 0.005 to 5 parts by weight with respect to 100 parts by weight of the raw metal fluoride.

  The seed crystal used for the pulling method is a single crystal of metal fluoride, and the growth surface of the seed crystal depends on the main growth surface of the crystal of the as-grown single crystal to be produced, depending on the [111] plane, [100] What is necessary is just to select from a surface etc. suitably. During the growth of the single crystal, these seed crystals are preferably rotated about the pulling axis, and the rotation speed is preferably 2 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. As a temperature-fall rate, 0.1-3 degree-C / min is preferable.

  The metal fluoride single crystal to be annealed by the annealing furnace of the present invention may be an ingot taken out of the furnace, but in order to anneal more efficiently, the ingot is cut into an appropriate size to form a disk. 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.

  The annealing furnace of the present invention was provided with a heat insulating container covering the entire circumference and upper and lower sides of the metal fluoride single crystal, and a heater disposed outside the heat insulating container, obtained as in the above example, Alternatively, a metal fluoride single crystal having strain and residual stress obtained by another method is annealed and used to reduce the strain and residual stress.

  Such an annealing furnace of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic longitudinal sectional view showing a typical embodiment of the annealing furnace of the present invention, and FIG. 2 is a schematic cross sectional view.

  As shown in FIGS. 1 and 2, the metal fluoride single crystal (1) is stored in a heat insulating container (2) that covers the entire circumference and upper and lower sides of the metal fluoride single crystal. In the figure, the metal fluoride single crystal is further stored in the storage container (3) so that a plurality of metal fluoride single crystals can be annealed simultaneously.

  As the heat insulating material (raw material) constituting the heat insulating container (2) of the present invention, any known heat insulating material may be used as long as it is a heat insulating material that can be used at the temperature applied during annealing. However, since annealing of a metal fluoride single crystal is usually performed at a maximum temperature of 700 ° C. or higher, and in many cases 1000 ° C. or higher, it is common to use an inorganic heat insulating material. Examples of inorganic heat insulating materials include carbon, silicate, glass, and various ceramics. From the viewpoint of high temperature heat resistance, heat insulating properties, strength, contamination prevention of metal fluoride single crystals, etc. A material is preferred. Specific examples of carbon-based heat insulating materials include pitch-based graphite molded heat insulating materials, fiber-based graphite formed heat insulating materials, carbon felt-based heat insulating materials, porous carbon-based heat insulating materials, and the like.

In the heat insulating container (2) in the present invention, in order to obtain the effect of the present invention more favorably, the heat transfer capability in the thickness direction of the side heat insulating material is 100 W · m ⁻² · K ⁻¹ or less. Is preferable, and it is preferable that it is 1-50W * m ^<-2 > * K ^{< -1} >. In the present invention, the heat transfer capability in the thickness direction is a value obtained by dividing the thermal conductivity (W · m ⁻¹ · K ⁻¹ ) at 1000 ° C. (high temperature side) in the thickness direction by the thickness (m). Say.

  The heat transfer capacity decreases as the thermal conductivity of the member constituting the side heat insulating material decreases and the thickness increases. However, if the thickness is too thick, the size of the annealing furnace increases and usually the weight increases. Become. On the other hand, when the thickness is too thin, there is a problem in strength and often breaks easily. Therefore, the thickness of the side heat insulating material in the heat insulating container (2) is preferably in the range of 5 to 200 mm, and more preferably in the range of 10 to 100 mm.

The heat conductivity of a normal heat insulating material is 5 W · m ⁻¹ · K ⁻¹ or less. Preferably, the heat conductivity is 2 W · m ⁻¹ · K ⁻¹ or less, more preferably 1 W · m ⁻¹ · K ⁻¹ or less, particularly preferably 0.1 to 0.5 W ·. By using a heat insulating material of m ⁻¹ · K ⁻¹ , it becomes easy to obtain a heat insulating container having an appropriate thickness and a preferable range of heat transfer capability. For example, if a heat insulating material having a thermal conductivity of 2 W · m ⁻¹ · K ⁻¹ is used, if the thickness is 20 mm or more, the heat transfer capability is 100 W · m ⁻² · K ⁻¹ or less, and if the thickness is 40 mm or more, heat is applied. The moving ability can be 50 W · m ⁻² · K ⁻¹ or less. Similarly, when a heat insulating material having a thermal conductivity of 0.5 W · m ⁻¹ · K ⁻¹ is used, if the thickness is 5 mm or more, the heat transfer capability is 100 W · m ⁻² · K ⁻¹ or less. In the present invention, the thermal conductivity is a value when the temperature on the high temperature side is 1000 ° C. In addition, the thermal conductivity of fiber-based heat insulating materials is often different depending on the fiber lamination direction, but the thermal conductivity is the thermal conductivity in the direction of the thickness direction of the heat insulating container.

Moreover, the heat insulating material of the upper-lower position of a heat insulation container (2) is also 2 W * m < ^-1 > * K <^-1> or less heat insulating material like the side part heat insulating material, Preferably it is 1 W * m <^-1> * K <^-1> or less. It is preferable to be formed of a heat insulating material, more preferably 0.1 to 0.5 W · m ⁻¹ · K ⁻¹ . As will be described later, when the heater is disposed only on the side of the heat insulating container (2), it is preferable that the heat transfer capability is lower than that of the side heat insulating material, and more preferably 1 of the side heat insulating material. / 2 to about 1/10. The heat transfer capacity of the upper and lower parts is reduced by increasing the thickness of the upper and lower heat insulating materials, adopting heat insulating materials with lower thermal conductivity than the side heat insulating materials, or combining them. However, it is preferable to use a heat insulating material that is the same material as the side heat insulating material so that the thickness thereof is about 2 to 10 times in that the heat insulating container can be easily manufactured.

  In the annealing furnace according to the present invention, a heater (4) for heating is disposed outside the heat insulating container (2). In other words, the heat insulating container (2) exists between the heater (4) and the metal fluoride single crystal (1), and thus the temperature distribution in the metal fluoride single crystal when heated is the conventional temperature distribution. Compared to the annealing furnace, it is easier to reduce distortion and to obtain a material having a small birefringence. This effect is particularly useful when annealing a large single crystal in which temperature distribution is likely to occur, particularly a single crystal having a diameter of 200 mm or more or a thickness of 50 mm or more.

  The heater is not particularly limited as long as it can be heated to a temperature at which annealing is performed, and a heater such as a resistance heating type or an induction heating type is used. Moreover, the arrangement | positioning which combined those may be sufficient as the position on the side of the heat insulation container (2), upper and lower sides. When annealing is performed by storing a plurality of metal fluoride single crystals (1) in multiple stages, from the viewpoint of simplifying the structure and reducing the occurrence of temperature distribution, as shown in FIG. It is preferable to arrange a heater (4) having a length substantially the same as the height of the heat insulating container or a longer side (4). Furthermore, in order to further reduce the occurrence of the temperature distribution, it is also preferable to arrange the heater so that a part of the heater also wraps under the heat insulating container.

  When arranged on the side of the heat insulating container, as shown in FIG. 2, a plurality of heaters (4) are provided so as to surround the heat insulating container (2) so that the heat insulating container is heated as much as possible. It is preferable to arrange. In FIG. 2, eight heaters (4) are arranged, but the number is not limited to eight. The larger the number of heaters, the more uniformly and uniformly it can be heated, but on the other hand, the structure of the annealing furnace becomes complicated and the cost for manufacturing and maintenance becomes expensive. Therefore, when arrange | positioning a some heater, it is preferable that the number is 6-48, and it is more preferable to set it as 8-36. Further, the heater is not limited to a rod-like heater, and may have any shape such as a plate. When arranging a plurality of heaters, it is preferable to arrange them at a rotationally symmetric position.

  In FIG. 1, the storage container (3) having a five-stage structure is described so that a plurality of metal fluorides can be annealed simultaneously. However, the number of stages may be appropriately increased or decreased as necessary. In addition, an annealing furnace having a structure in which a plurality of metal fluoride single crystals are not stored in multiple stages in the vertical direction but arranged in a horizontal position may be used, or a heat resistant fiber or the like may be used in the furnace without using a storage container. It may be a form that hangs on. Furthermore, when only one metal fluoride single crystal to be annealed is used, it is possible to employ a method in which the storage container is not used but a direct arrangement in the heat insulating container. From the viewpoints of ease of structure, installation area, and the like, it is preferable to form a storage container that can store metal fluoride single crystals in multiple stages (preferably 3 to 15 stages).

  Also, use a container having a hole in the bottom of the storage container (3 ′) as shown in FIG. 3a, or between the storage container (3) and the metal fluoride single crystal (1) as shown in FIG. 3b. It is also a preferable aspect to arrange some small spacers (5) having a shape such as a prism or cylinder. By using the storage container having such a structure, friction between the storage container and the metal fluoride single crystal due to thermal expansion / contraction of the metal fluoride single crystal due to a temperature change during annealing is reduced, and more complex. There is a tendency to obtain a metal fluoride single crystal with low refraction.

  The storage container (3) may be made of any material as long as it is a heat-insulating material that can be used at a temperature applied during annealing, and its thermal conductivity is not particularly limited, and constitutes the heat-insulating container. It may be formed of a material having a low thermal conductivity like the heat insulating material, or may be formed of a material having a high thermal conductivity. Since many materials having low thermal conductivity are porous, they tend to be inferior in strength, and in order to avoid this, the material is usually formed of a material having relatively high thermal conductivity.

  Further, in order to avoid contamination of the metal fluoride single crystal (1) during annealing, the material constituting the storage container (3) is preferably a carbon-based material. Considering heat resistance, strength, and possibility of contamination of the metal fluoride single crystal, the most preferable is a storage container made of high-purity high-density graphite.

  As shown in FIG. 1, the annealing furnace of the present invention is usually provided with a chamber (6) covering the whole, and further, between the chamber (6) and the heater (4), an annealing is performed. It is usual to arrange a heat insulating material (7) having a heat insulating property such that the temperature of the chamber does not become dangerous (note that these are omitted in FIG. 2). As the heat insulating material (7) and the chamber (6), a heat insulating material and a chamber used in a known annealing furnace can be used without any particular limitation. For example, as the heat insulating material (7), a heat insulating material similar to the material constituting the heat insulating container can be used.

  In order to further reduce the temperature distribution, it is preferable to rotate the metal fluoride single crystal during annealing. The rotation is generally performed so that the central axis of the metal fluoride single crystal (1) is the axis. As a typical mode for performing such rotation while making the structure of the annealing furnace as simple as possible, a support rotation shaft (8) as shown in FIG. The method of making the heat insulation container (2) rotatable is mentioned. The support rotating shaft (8) penetrates the heat insulating material (7) and the chamber (6) disposed between the chamber and the heater, and drives a drive mechanism (not shown) for rotating the heat insulating container (2). ). The material of the rotating support shaft (8) is not particularly limited, but is preferably made of a heat insulating material in order to further reduce the temperature distribution.

  When annealing a metal fluoride single crystal using the annealing furnace of the present invention, it is usually performed in a high vacuum, an inert atmosphere such as argon, or a scavenger atmosphere such as carbon tetrafluoride or hydrogen fluoride. is there. The means for supplying and discharging the atmospheric gas is not particularly limited, and a structure in which the metal fluoride single crystal is in sufficient contact with the atmospheric gas may be employed. To illustrate a typical embodiment of the gas flow in the annealing furnace of the present invention, when a heater is disposed on the side of the heat insulating container as shown in FIG. 1 and the metal fluoride single crystal is rotated, A gas introduction hole (9) is provided in the lower part of the heat insulating container (2), and a gas discharge hole (10) is provided in the upper part. The gas introduction hole (9) passes through the inside of the support rotary shaft (8) and is shown in the figure. What is necessary is just to connect to external piping and gas supply sources (gas cylinder etc.) which are not. The gas supplied from the gas introduction hole (9) passes through the gas discharge hole (10), and is further discharged out of the furnace from a discharge portion (not shown) provided in the chamber. Moreover, although not shown in figure, a gas distribution hole is also provided in a storage container (3).

  Note that the heat insulating material constituting the heat insulating container (2) is often porous and hardly airtight, and it is not always necessary to open a circulation hole for supplying or discharging the atmospheric gas. For such introduction and discharge, it is preferable to provide the gas circulation holes as described above.

  As described above, in the present invention, the object of annealing is a metal fluoride single crystal. What is necessary is just to set suitably the temperature conditions in the case of annealing according to the kind of metal fluoride used as object. Although the strain tends to be removed in a shorter time as the annealing is performed at a higher temperature, the annealing needs to be performed at a temperature lower than the melting point of the object. For example, when annealing calcium fluoride, since the melting point of calcium fluoride is about 1420 ° C., the temperature is lower than this temperature, preferably 1000 to 1400 ° C. (maximum temperature of crystal), particularly 1100 to 1350 ° C.

  In order to obtain a better strain removal effect and to obtain a single crystal having a small birefringence, it is preferable that the temperature decrease rate is low. On the other hand, if the speed is too slow, the time required for annealing becomes longer and the productivity is lowered. The influence of the rate of temperature drop on the strain removal effect becomes more pronounced as the temperature becomes higher. Therefore, it is preferable that the temperature drop is performed slowly in a relatively high temperature region and the temperature decrease rate is increased in a low temperature region. Also, when raising the temperature, it is preferable to relatively slow the temperature raising rate in the high temperature region.

  As an example, when annealing calcium fluoride, it is 5 to 200 ° C./hr from 1000 to 1400 ° C. (more preferably, 10 to 200 ° C./hr up to 500 to 900 ° C., and 1 to 5 ° C./hr thereafter) The temperature is raised at about 0, held at that temperature for about 0 to 120 minutes, then lowered to about 500 to 900 ° C. at about 0.1 to 5 ° C./hr, then further lowered to about 1 to 5 ° C./minute to room temperature. An annealing temperature pattern may be mentioned.

  Thus, the metal fluoride single crystal annealed in the annealing furnace of the present invention may be finished to a predetermined shape, for example, a lens by performing grinding, polishing or the like by a known method as necessary. The metal fluoride single crystal annealed in the annealing furnace of the present invention has extremely low birefringence and is particularly useful for such optical material applications.

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

  In addition, the metal fluoride single crystal used for the present Example and the comparative example manufactured the as-grown single crystal of calcium fluoride by the method (Czochralski method) described in Unexamined-Japanese-Patent No. 2005-029455, After slicing, it is further ground to a predetermined size to form a disk.

  The disk-shaped single crystal was polished on a surface of # 2000 and placed in an automatic birefringence distribution measuring apparatus (EXICOR 450AT; light source 633 nm, manufactured by Hinds instruments, Inc.), and the birefringence was measured using this. The least mean square value and the maximum value were obtained. This birefringence measurement was performed before and after annealing on the same disk.

Examples 1 and 2
As shown in FIG. 1, an annealing furnace having a heat insulating container and a multistage storage container inside, and having 24 heaters arranged on the side of the heat insulating container so as to surround the heat insulating container was manufactured. . Here, the heat insulating container is formed using pitch-based graphite as the heat insulating material, and the thermal conductivity at 1000 ° C. in the thickness direction of the side heat insulating material in the heat insulating container is 0.3 W · m ^−1. -K- ¹ . Moreover, the thickness of the side heat insulating material in this heat insulation container was 20 mm, the thickness of the upper heat insulating material was 60 mm, and the thickness of the lower heat insulating material was 100 mm. Accordingly, the heat transfer capability of the side heat insulating material is 15 W · m ⁻² · K ⁻¹ .

  In addition, the storage container has five stages, and each metal fluoride single crystal storage space has an inner diameter of 310 mm and a height of 80 mm. The storage container is made of high-density graphite having a thickness of 15 mm. The storage space below the upper storage space (floor) from the lower storage space so that gas can flow between each storage space and outside the storage container. Eight through holes penetrating above (ceiling) are provided.

  (100) Calcium fluoride single crystal (Example 1) having a diameter of about 200 mm and a thickness of 20 mm is placed in the uppermost storage space of the storage container in the annealing furnace having the structure, and the diameter of the third stage is about 145 mm and the thickness. A 40 mm (100) calcium fluoride single crystal (Example 2) was placed.

While reducing the pressure in the furnace to about 10 ⁻⁴ Pa, the temperature was raised to 200 ° C. at 10 ° C./hr and held for 30 minutes, and the furnace was baked. Then, when the temperature was raised at 10 ° C./hr and reached 600 ° C., carbon tetrafluoride gas was introduced at a rate of 2.5 L / min until the inside of the furnace became normal pressure. Subsequently, the temperature was raised to 1150 ° C. at 10 ° C./hr and held for 30 minutes, then 1 ° C./hr to 700 ° C., 4 ° C./hr at 700 to 300 ° C., and then lowered to near room temperature at 10 ° C./hr. .

  Table 1 shows the result of comparing the birefringence of the calcium fluoride single crystal thus annealed before and after annealing.

Comparative Examples 1 and 2
In the annealing furnace used in Example 1, instead of the heat insulating container made of pitch-based graphite, the wall thickness is 10 mm and the thermal conductivity in the thickness direction is about 50 W · m ⁻¹ · K ⁻¹ (at 1000 ° C.). A container made of high-density graphite was installed. The heat transfer capability in the side direction of the container is 5000 W · m ⁻² · K ⁻¹ .

  A (100) calcium fluoride single crystal (Comparative Example 1) having a diameter of about 200 mm and a thickness of 20 mm is provided on the uppermost stage of the storage container, and a (100) calcium fluoride single crystal having a diameter of about 140 mm and a thickness of 40 mm is provided on the third stage. (Comparative Example 2) was placed and annealed under the same conditions as in Example 1. Table 1 shows the results of measuring the birefringence of the calcium fluoride single crystal before and after annealing.

Examples 3 and 4
Using the same annealing furnace as in Examples 1 and 2, the (111) calcium fluoride single crystal (Example 3) having a diameter of about 270 mm and a thickness of 50 mm was placed on the second stage from the top of the container, and the diameter was about 4th stage. A (100) calcium fluoride single crystal (Example 4) having a thickness of 140 mm and a thickness of 20 mm was placed, and annealing was performed under the same conditions as in Example 1. Table 2 shows the results of measuring the strain (birefringence) of the calcium fluoride single crystal before and after annealing.

Examples 5-8
Using the same annealing furnace as in Example 1, the calcium fluoride single crystals shown in Table 3 were arranged in order from the top of the storage container up to the fourth stage, and annealing was performed under the same conditions as in Example 1. Table 3 shows the results of measuring the birefringence of the calcium fluoride single crystal before and after annealing. In addition, it is Example 5, 6, 7 and 8 from the top in the order stored.

Example 9
Using the same annealing furnace as in Example 1, a (100) calcium fluoride single crystal having a diameter of about 190 mm and a thickness of 29 mm was arranged on the fifth stage from the top of the storage container. The temperature was raised to 1350 ° C. and held for 30 minutes in the same manner as in Example 1 except that the temperature raising rate was 40 ° C./hr and the maximum temperature was 1350 ° C. Then, the temperature was lowered to 600 ° C. at 4 ° C./hr. Thereafter, the temperature was lowered to about room temperature at 15 ° C./hr. Annealing was performed. Table 4 shows the results of measuring the birefringence of the calcium fluoride single crystal before and after annealing.

Example 10
Using the same annealing furnace as in Example 1, a (100) calcium fluoride single crystal having a diameter of about 140 mm and a thickness of 27 mm was arranged on the fifth stage from the top of the storage container. The temperature was raised to 1000 ° C. and held for 30 minutes in the same manner as in Example 1 except that the temperature raising rate was 40 ° C./hr and the maximum temperature was 1000 ° C. Then, 0.5 ° C./hr up to 900 ° C. Then, the temperature was lowered to 800 ° C. at 0.7 ° C./hr, to 500 ° C. at 1 ° C./hr, to 300 ° C. at 2 ° C./hr, and then at 5 ° C./hr to near room temperature. Annealing was performed. Table 4 shows the results of measuring the birefringence of the calcium fluoride single crystal before and after annealing.

It is the longitudinal cross-sectional schematic diagram which showed the typical one aspect | mode of the annealing furnace of this invention. It is the cross-sectional schematic diagram which showed the typical one aspect | mode of the annealing furnace of this invention. It is a schematic diagram which shows the typical aspect of the storage container used when storing a metal fluoride single crystal in the annealing furnace of this invention.

Explanation of symbols

1: Metal fluoride single crystal 2: Heat insulating container 3, 3 ′: Storage container 4: Heater 5: Spacer 6: Chamber 7: Heat insulating material 8: Support rotating shaft 9: Gas introduction hole 10: Gas discharge hole

Claims (5)

  An annealing furnace used for heat-treating a metal fluoride single crystal, comprising a heat insulating container that covers the entire circumference and upper and lower sides of the metal fluoride single crystal, and a heater disposed outside the heat insulating container.   A plurality of metal fluorides, comprising: a storage container capable of storing a metal fluoride single crystal in multiple stages; a heat insulating container covering the entire circumference and upper and lower sides of the storage container; and a heater disposed outside the heat insulating container. An annealing furnace used to heat treat single crystals simultaneously. The annealing furnace according to claim 1 or 2, wherein the heat insulating container is made of a material having a thermal conductivity of 2 W · m ^-1 · K ^-1 or less. The annealing furnace according to any one of claims 1 to 3, wherein the heat transfer capability in the thickness direction of the side heat insulating material in the heat insulating container is 100 W · m ^-2 · K ^-1 or less. An annealing method for a metal fluoride single crystal, wherein the entire circumference and upper and lower sides of the metal fluoride single crystal are covered with a heat insulating material and heated by a heater from the outside of the heat insulating material.

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