JP2006327837A - Fluorite single crystal manufacturing apparatus and method for manufacturing fluorite single crystal using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fluorite single crystal manufacturing apparatus which utilizes an impurity segregation effect at crystal growth to enable efficient, reliable manufacturing of a high-quality fluorite single crystal whose transmittance does not decrease, even when exposed to an excimer laser beam over a long period of time, and a method for manufacturing the fluorite single crystal using the same. <P>SOLUTION: The fluorite single crystal manufacturing apparatus employs a vertical Bridgman method and is equipped with a furnace main body 1 that forms a furnace chamber, an insulation member 14b located in a partitioning section 10b that vertically separates the furnace chamber into two chambers, i.e. a high-temperature furnace chamber 10a and a low-temperature furnace chamber 10c, a crucible 11 that can be moved by a drag-down bar 13 through the partitioning section 10b between the high-temperature furnace chamber 10a and the low-temperature furnace chamber 10c and heaters located in the high-temperature furnace chamber 10a and the low-temperature furnace chamber 10c. The ratio (crucible outside diameter R<SB>2</SB>/heater inside diameter R<SB>1</SB>) of the outside diameter R<SB>2</SB>of the crucible to the inside diameters R<SB>1</SB>of the heaters is ≥0.76. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an apparatus for producing a fluorite single crystal and a method for producing a fluorite single crystal using the apparatus.

In semiconductor manufacturing, the demand for higher integration of integrated circuits is increasing, and in recent years, the miniaturization of optical lithography technology for drawing an integrated circuit pattern on a wafer has progressed rapidly. In order to achieve high integration of integrated circuits, it is necessary to improve the resolving power of a projection lens used in an exposure apparatus (stepper). The resolving power of the projection lens depends on the exposure wavelength λ of light used for exposure and the NA of the projection lens. (Numerical aperture). When the exposure wavelength λ is the same, the angle of the diffracted light increases as the pattern becomes finer. Therefore, if the NA of the lens increases, the diffracted light cannot be captured. Also, the shorter the exposure wavelength λ, the smaller the angle of diffracted light in the same pattern, so the NA of the lens may be smaller. For such resolution and depth of focus, the following equations (1) and (2):
(Resolution) = k ₁ · λ / NA (1)
(Depth of focus) = k ₂ · λ / (NA) ² (2)
(In formulas (1) and (2), k ₁ and k ₂ are proportional constants, respectively)
It can be expressed as As is clear from these equations (1) and (2), in order to improve the resolution, the NA of the lens is increased (the lens is enlarged) or the exposure wavelength λ is shortened. It can be seen that shortening λ is more advantageous in terms of depth of focus.

  In such an exposure apparatus, the exposure wavelength λ has been shortened so far, and at present, the wavelength range of excimer lasers with shorter wavelengths from g-line (wavelength 436 nm) and i-line (wavelength 365 nm). It has moved to. In the optical system of such an exposure apparatus, optical glass can be used up to the wavelength range of i-line, but the wavelength of KrF excimer laser (wavelength 248 nm), ArF excimer laser (wavelength 193 nm), etc. In the area, it becomes difficult to use optical glass in the optical system of the exposure apparatus from the viewpoint of the transmittance. Very few optical materials can be used for photolithography applications using short wavelengths of 250 nm or less. Therefore, at present, two types of optical materials, fluorite (calcium fluoride) and quartz glass, are commonly used in the optical system of an exposure apparatus that uses the excimer laser wavelength range as a light source. Such fluorite has a high transmittance even in the wavelength region of the excimer laser, but there is a problem in that the transmittance decreases when light having a high photon energy such as an excimer laser is irradiated for a long time. For this reason, in recent years, there has been a strong demand for excimer resistance (property that transmittance does not decrease even when excimer laser light is irradiated for a long time) against fluorite.

Such fluorite single crystals (CaF ₂ ) are mainly manufactured by the vertical Bridgman method (referred to as the stock burger method or the crucible descent method). In such a vertical Bridgman method, the objective is to produce a single crystal of fluorite that is used in the optical system of an exposure apparatus that uses light in the wavelength region of an excimer laser as exposure light (growth of the single crystal). In this case, pulverized natural fluorite or synthetic fluorite cannot be used as a raw material because it is absorbed in the ultraviolet or vacuum ultraviolet region. For this reason, it is effective to advance the purification of such raw materials. At present, it is common to use a mixture of calcium fluoride high-purity powder raw material produced by chemical synthesis and a scavenger. is there. Also, as such a raw material, when powdered calcium fluoride is used, the loss when the powder is directly melted is severe due to the bulk specific gravity. In some cases, a polycrystalline bulk obtained through a pretreatment step such as solidification after pulverization or cullet obtained by pulverizing the bulk is used. In the vertical Bridgman method, after the crucible is filled with the raw material, the crucible is placed in a so-called Bridgman type single crystal production (growing) apparatus to produce a single crystal.

  FIG. 1 shows a schematic diagram of a typical embodiment of such a conventional Bridgman type single crystal manufacturing apparatus. The single crystal manufacturing apparatus shown in FIG. 1 includes a furnace body 1 composed of a vacuum chamber, and a crucible 11 filled with a raw material is filled in a hollow part 10 in the furnace covered with a heat insulating material 2 of the furnace body 1. 12 is installed. The crucible support 12 is connected to a pull-down bar 13, and the pull-down bar 13 is connected to lifting means (not shown). Furthermore, in the high temperature side furnace chamber 10a above the single crystal growth zone 14 set in the axial direction of the hollow portion 10 in the furnace, a high temperature heater 14a is arranged on the outer periphery of the side surface of the crucible, and the low temperature side furnace below. A low temperature heater 14c is arranged in the chamber 10c. Further, a heat insulating member 14b is installed in the partition portion 10b that partitions the high temperature side furnace chamber 10a and the low temperature side furnace chamber 10c. The furnace body 1 of the single crystal manufacturing apparatus having such a configuration is connected to a vacuum exhaust system (not shown).

  In the single crystal manufacturing (growing) method using the conventional single crystal manufacturing apparatus having the configuration shown in FIG. 1, first, the crucible 11 filled with the raw material is partitioned in the hollow portion 10 in the furnace via the pulling rod 13. Raise to a predetermined position above the portion 10b. Next, a predetermined temperature is set in each of the high-temperature heating unit 14a and the low-temperature heating unit 14c to create a temperature gradient in the vertical direction. Thus, in the single crystal growth zone 14 in which the temperature gradient is set in the vertical direction in this way. The crucible 11 is lowered while controlling the lowering speed of the crucible 11 to be a predetermined speed.

  The temperature distribution in the single crystal growth zone 14 is set to be higher than the melting temperature of the grown crystal in the upper high temperature side furnace chamber 10a shown in FIG. 1 and gradually lower toward the lower temperature side furnace room 10c. It has a steep temperature gradient. Then, by passing the crucible 11 through the single crystal growth zone 14 having the temperature distribution set in this way, the crystal growth of the raw material in the crucible 11 once melted in the upper high temperature region is started, and the steep temperature is increased. In the low temperature region of the gradient, it is gradually cooled and solidified and crystallized to produce a single crystal. In addition, it is common to manufacture a single crystal in the furnace in a high vacuum with the above-mentioned vacuum exhaust system.

  When a single crystal is grown using such a conventional single crystal manufacturing apparatus, generally, the lower portion of the crucible 11 shown in FIG. A seed crystal with a plane orientation designated to promote single crystal growth is placed on the substrate, and at least a part of the seed crystal is not melted during the production of the single crystal at a predetermined height position in the low temperature region in the furnace. The crucible 11 is arranged so that the lower part of the crucible 11 comes. By adopting such an arrangement, a uniform single crystal matching the crystal plane orientation of the seed crystal can be obtained.

As a conventional single crystal manufacturing apparatus using such a fluorite crucible descent method (vertical Bridgman method), for example, in Japanese Patent Application Laid-Open No. 4-349198, there is an apparatus in which a ceiling heater is added to the upper part of a furnace chamber. Japanese Patent Laid-Open No. 7-277869 discloses an apparatus in which a heat insulating part having a predetermined length is provided between a high temperature part and a low temperature part of a heating mechanism. However, in such a conventional single crystal manufacturing apparatus, the ratio of the outer diameter of the crucible to the inner diameter of the heater (crucible outer diameter / heater The inner diameter) was required to be less than 0.76. Such a conventional single crystal manufacturing apparatus utilizes the effect of segregation of impurities during crystal growth, and does not decrease the transmittance even when irradiated with excimer laser light for a long time. In terms of producing single crystals of stone, it has not been sufficient yet.
JP-A-4-349198 JP 7-277869 A

  The present invention has been made in view of the above-mentioned problems of the prior art, and utilizes the impurity segregation effect at the time of crystal growth, so that the high-quality fluorescent light whose transmittance does not decrease even when irradiated with excimer laser light for a long time. An object of the present invention is to provide a fluorite single crystal production apparatus capable of efficiently and reliably producing a single crystal of stone, and a method for producing a fluorite single crystal using the same.

  As a result of intensive studies to achieve the above object, the present inventors have found that the ratio of the outer diameter of the crucible to the inner diameter of the heater (outside of the crucible) with respect to the relative geometric arrangement of the crucible and the side heater. By setting the effective temperature gradient in the crucible to a steep value by setting the diameter / heater inner diameter) to a predetermined value or more, the transmissivity can be obtained even if the excimer laser light is irradiated for a long time by utilizing the impurity segregation effect during crystal growth. The inventors have found that it is possible to efficiently and reliably produce a high-quality fluorite single crystal that does not deteriorate, and the present invention has been completed.

That is, the fluorite single crystal production apparatus of the present invention is disposed in a furnace body that forms a furnace chamber, and a partition that vertically separates the furnace chamber into a high temperature side furnace chamber and a low temperature side furnace chamber. A heat insulating member, a crucible installed so as to be movable through the partition between the high temperature side furnace chamber and the low temperature side furnace chamber by a pull-down rod, the high temperature side furnace chamber, and the low temperature An apparatus for producing a single crystal of fluorite used in the vertical Bridgman method, comprising a heater disposed in each of the side furnace chambers,
The ratio of the outer diameter of the crucible to the inner diameter of the heater (crucible outer diameter / heater inner diameter) is 0.76 or more.

  In the fluorite single crystal production apparatus of the present invention, the ratio between the inner diameter of the crucible and the inner diameter of the heater (the inner diameter of the crucible / the inner diameter of the heater) is preferably 0.70 or more.

  Furthermore, in the fluorite single crystal production apparatus of the present invention, the ratio between the vertical length of the partition portion and the inner diameter of the crucible (vertical length of the partition portion / inner diameter of the crucible) is 1/7. It is preferable to be in the range of ˜1 / 10.

  The method for producing a fluorite single crystal of the present invention is characterized in that a single crystal of fluorite is obtained using the above-described apparatus for producing a single crystal of fluorite of the present invention.

  By using the fluorite single crystal manufacturing apparatus of the present invention, a high-quality fluorite single crystal whose transmittance does not decrease even when irradiated with excimer laser light for a long time is utilized by utilizing the impurity segregation effect during crystal growth. The reason why efficient and reliable production is possible is not always clear, but the present inventors infer as follows.

  The fluorite used in the optical component of an exposure apparatus that uses light in the excimer laser wavelength range as exposure light has no internal transmission loss or very low internal transmission loss for excimer laser light (excimer resistance) is important. Therefore, a fluorite (member) used for an illumination system portion where the energy of laser light is particularly large is required to have a much higher performance in terms of optical design than an optical member for a projection system lens with a low energy. In order to reduce such internal transmittance loss, it is necessary to eliminate impurities that induce light absorption from the crystal as much as possible. Is essential. Further, during crystal growth, it is necessary to reduce the impurity concentration in the crystal by utilizing the impurity segregation effect during crystal growth.

Such segregation of impurity elements during crystal growth is determined by the magnitude of the effective segregation coefficient Keff. That is, the smaller the Keff value, the smaller the residual concentration of impurities in the crystal. When the crystal growth rate is V, the effective segregation coefficient Keff at the crystal growth rate V is expressed by the following equation (3):
(Keff) = [K + (1-K) exp (−V · δ / D)] ⁻¹ (3)
(In formula (3), K represents the segregation coefficient of impurities at the interface, δ represents the thickness of the semi-molten layer, and D represents the diffusion coefficient of impurities in the melt.)
It is represented by From the above equation (3), it can be seen that the value of Keff decreases as the growth rate V decreases and decreases as the thickness δ of the semi-molten layer decreases.

  FIG. 2 shows a conceptual diagram of crystal growth of a fluorite single crystal inside the crucible. As shown in FIG. 2, a solid-liquid interface 23 exists between the crystal 21 and the melt 22 in the crucible during crystal growth, and a semi-molten layer 24 exists on the solid-liquid interface 23.

In the production of a fluorite single crystal, the thickness δ of the semi-molten layer 24 decreases as the difference between the solid-liquid interface temperature Tm ₁ and the melt temperature Tm ₂ increases. Therefore, in order to reduce the thickness δ of the semi-molten layer, it is important to create a steep temperature gradient in the single crystal growth zone during crystal growth. In order to create such a steep temperature gradient, in the conventional single crystal manufacturing apparatus shown in FIG. 1 and the single crystal manufacturing apparatus described in Patent Documents 1 and 2, as described above, the heat insulating portion is used. This temperature gradient was attempted to be realized by arranging a high temperature heater and a low temperature heater. However, the effective temperature gradient in the crucible cannot be set so steep that it is directly linked to the value of the effective segregation coefficient Keff only by determining the temperature settings of the high temperature heater and the low temperature heater. Therefore, a single crystal of fluorite obtained using a conventional single crystal manufacturing apparatus has excimer resistance that satisfies the performance required in an exposure apparatus that uses light in the wavelength region of an excimer laser as exposure light. It wasn't.

  Therefore, the present inventors set the effective temperature gradient in the crucible steeply and use the impurity segregation effect during crystal growth, that is, the relative geometric arrangement of the crucible and the side heater, that is, the crucible and the heater. Focused on the distance between. That is, the positional relationship between the crucible and the heater disposed on the side surface of the crucible greatly reflects the amount of heat transferred to the inside of the crucible, which is accompanied by the formation of a solid-liquid interface, the formation of a semi-molten layer, and the crystallization of the melt. In order to affect the segregation of impurities, the positional relationship between the crucible and the heater arranged on the side of the crucible is “excimer resistance of the obtained fluorite single crystal ingot (transmittance even when irradiated with excimer laser light for a long time. The present inventors thought that the superiority or inferiority of “the property that does not decrease”). In particular, when producing a single crystal of a large-diameter fluorite, a steep effective temperature gradient near the solid-liquid interface in the crucible is realized by heating a heater arranged on the side of the crucible. The relative geometrical arrangement with the heater arranged on the side of the crucible is important.

Therefore, the positional relationship between the heater disposed in the crucible side and the crucible was further investigated in terms of form factor f ₁₂ of the inter-emissivity coefficient F _.epsilon.1 and the crucible side and the heater surface. Here, in the fluorite single crystal manufacturing apparatus, the heat is transferred only between the crucible and the heater because the chamber is kept in vacuum. The relationship between the side surface of the crucible and the side surface heater can be assumed to be a 2 gray surface forming a concentric cylindrical surface. Furthermore, the area of the crucible side as A _1, the area of the side heater arranged around the periphery of the crucible and A _2. Radiance factor F _.epsilon.1 and form factor f ₁₂ between the crucible side and the heater surface the following formula (4):
_{F ε1 = f 12 = [1} / ε 1 + (A 1 / A 2) × (1 / ε 2 -1)] -1 (4)
(Here, ε ₁ indicates the emissivity of the side surface of the crucible, and ε ₂ indicates the emissivity of the heater surface disposed around the outer periphery of the crucible.)
It is represented by From the above formula (4), the surface area ratio A ₁ / A ₂ is closer to ₁ as the distance between the two faces facing each other is closer depending on the installation distance between the crucible side surface and the heater surface whose geometric arrangement is a concentric cylindrical surface. On the other hand, the surface area ratio A ₁ / A ₂ takes a value close to 0 as the distance between the two facing surfaces increases. Thus, the surface area ratio A ₁ / A ₂ can range from 0 to ₁ , and the form factor f ₁₂ between the crucible side surface and the heater surface also varies depending on the installation distance between the crucible side surface and the heater surface. . And when the same material is selected for the heater and the crucible, and the processed surface state is composed of similarly finished members, the emissivity of the above equation (4) is ε ₁ = ε ₂ , so the form factor f ₁₂ is determined only by the surface area ratio A ₁ / A _{2 of the} two faces facing each other. Here, if the heater surface and the crucible side surface have a cylindrical shape arranged with the same central axis, the surface area ratio of these two surfaces is defined by the ratio of the diameters of the two surfaces. From such a viewpoint, as a result of intensive studies, the present inventors have found a relationship between the diameter of the heater and the diameter of the crucible that can sufficiently obtain the segregation effect of impurities.

  According to the present invention, by utilizing the effect of segregation of impurities during crystal growth, it is possible to efficiently and reliably produce a high-quality fluorite single crystal whose transmittance does not decrease even when irradiated with excimer laser light for a long time. It is possible to provide a single crystal production apparatus for fluorite that makes it possible and a method for producing a single crystal of fluorite using the same.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the following description and drawings, the same or corresponding elements are denoted by the same reference numerals, and duplicate descriptions are omitted.

  FIG. 3 is a schematic diagram showing the configuration of an embodiment suitable as a single crystal production apparatus for fluorite of the present invention.

The fluorite single crystal production apparatus shown in FIG. 3 includes a furnace body 1 made of a vacuum bell jar. The furnace body 1 includes a heat insulating material 2 covering the furnace chamber on the inner wall of the furnace body 1, and a partition portion 10b that divides the furnace chamber into two chambers in a vertical direction into a high temperature side furnace chamber 10a and a low temperature side furnace chamber 10c. The high-temperature heater 14a disposed in the vertical wall of the high-temperature side furnace chamber 10a, and the low-temperature heater 14c disposed in the vertical wall of the low-temperature side furnace chamber 10c. (In the case of “heater” hereinafter, both the high-temperature heater 14a and the low-temperature heater 14c are shown.) Furthermore, the crucible installed on the crucible support base 12 is provided in the furnace main body 1 so that the high temperature side furnace chamber 10a and the low temperature side furnace chamber 10c can be moved by a pull-down rod 13 at a substantially central portion in the furnace main body 1. 11 is accommodated. And the pulling-down rod 13 is connected to the raising / lowering means which can adjust the pulling-down speed which is not shown in figure. The furnace body 1 is connected to a vacuum exhaust system (not shown). In FIG. 3, R ₁ indicates the inner diameter of the heater, R ₂ indicates the outer diameter of the crucible, R ₃ indicates the inner diameter of the crucible, and L indicates the length of the partition portion in the vertical direction.

In such a fluorite single crystal production apparatus, the ratio (R ₂ / R ₁ ) between the outer diameter R _{2 of the} crucible and the inner diameter R _{1 of the} heater is 0.76 or more. When the value of R ₂ / R ₁ is less than 0.76, the effective temperature gradient in the crucible cannot be set sharply, and impurities are difficult to segregate. Therefore, when such a fluorite single crystal production apparatus having an R ₂ / R ₁ value of less than 0.76 is used, a high-quality fluorite single crystal having excellent excimer resistance can be obtained. Can not. Further, the value of R ₂ / R ₁ is preferably 0.76 to 0.92. If the value of R ₂ / R ₁ exceeds the upper limit, the design of the single crystal manufacturing apparatus tends to be difficult due to problems such as contact between the heater and the crucible during manufacturing of the single crystal. If it is less than 1, it tends to be impossible to obtain a high-quality fluorite single crystal having more excellent excimer resistance.

The ratio (R ₃ / R ₁ ) of the inner diameter R _{3 of the} crucible and the inner diameter R _{1 of the} heater is preferably 0.70 or more. Further, the value of R ₃ / R ₁ is more preferably 0.70 to 0.87. If the value of R ₃ / R ₁ is less than the lower limit, the impurity concentration in the crystal is reduced, such as a decrease in transmittance of the fluorite single crystal after long-time irradiation with an excimer laser, that is, a decrease in excimer laser resistance. The segregation effect tends not to be obtained sufficiently, and on the other hand, if the upper limit is exceeded, the crucible becomes too thin, and the crucible strength during actual handling such as when stored in a single crystal manufacturing apparatus Tend to run out.

Furthermore, the ratio (L / R ₃ ) between the vertical length L of the partition portion and the inner diameter R _{3 of the} crucible is preferably in the range of 1/7 to 1/10, and 1/8 to 1 / More preferably, it is in the range of 10. If the value of L / R ₃ is less than the lower limit, the influence of the mutual heat flow of the heaters 14a and 14c disposed above and below the partition portion is likely to be given to the crucible, and the temperature gradient is steeply realistic. Since the excimer laser resistance of the obtained fluorite single crystal is reduced, the segregation effect for reducing the impurity concentration in the crystal tends not to be sufficiently obtained. If the upper limit is exceeded, the temperature gradient cannot be set steeply due to the increase in the partition portion length L, and it tends to be difficult to obtain a segregation effect for reducing the impurity concentration in the crystal.

Incidentally, the inner diameter R ₁ of the heater is not particularly limited, in view of the reasons such as design of the size and manufacturing apparatus of fluorite single crystal to be produced is preferably generally about 140～550Mm. Also, the outer diameter R ₂ of the crucible and the inner diameter R ₃ of the crucible are not particularly limited, and from the same viewpoint, the outer diameter R ₂ of the crucible is generally preferably about 110 to 500 mm, and the inner diameter R of the crucible is preferable. _In general, the size of ₃ is preferably about 100 to 470 mm.

Next, the raw material 30 used in this embodiment will be described. The raw material 30 may be a mixed powder of a high-purity calcium fluoride raw material powder and a scavenger, but the calcium fluoride single crystal is obtained by increasing the filling rate when filling the crucible 11 and purifying the raw material. In order to improve the internal quality, it is preferable to use a polycrystal obtained by pretreatment and a cullet obtained by pulverizing the polycrystal. As such a pretreatment method, first, a pretreatment crucible in which a high-purity calcium fluoride powder raw material and a scavenger are mixed and filled in a pretreatment electric heating furnace connected to an evacuation system is used. It is installed and brought into a vacuum state of 10 ⁻⁴ to 10 ⁻⁵ Pa before heating the inside of the furnace. Next, the temperature inside the apparatus is gradually raised while continuing the vacuum evacuation. For example, when lead fluoride (PbF ₂ ) is used as a scavenger, the temperature is temporarily held at 800 ° C. to 900 ° C. to sufficiently advance the scavenge reaction. . Thereafter, the temperature in the apparatus is further raised to a temperature of 1370 ° C. to 1450 ° C. above the melting point of the raw material. Then, after passing the step of volatilizing excess scavengers and reaction products and melting the raw material, the temperature is gradually lowered to solidify the melt. The polycrystal can be obtained by performing such pretreatment. The scavenger is not particularly limited, and examples thereof include Teflon (registered trademark), lead fluoride, silver fluoride, cobalt fluoride, and manganese fluoride. The amount of scavenger added is determined by the particle size and volume of the raw material powder, the type of scavenger, etc., but is about 0.1 mol% to 5.0 mol% with respect to the raw material calcium fluoride powder. It is preferable, and it is more preferable that it is about 0.5 mol%-3.0 mol%.

  In the present embodiment, the raw material 30 is a polycrystal obtained by performing the pretreatment as described above.

  Hereinafter, a method for producing a fluorite single crystal using the fluorite single crystal production apparatus shown in FIG. 3 suitable for the method for producing a fluorite single crystal of the present invention will be described.

As a method for producing such a calcium fluoride single crystal, a so-called “Bridgeman method” (stock burger method, crucible descent method) is used. First, the crucible 11 filled with the raw material 30 is placed in the manufacturing apparatus, the inside of the manufacturing apparatus is maintained in a vacuum atmosphere lower than 10 ⁻³ Pa (more preferably 10 ⁻⁴ Pa), and the temperature in the manufacturing apparatus is set. The raw material 30 in the crucible 11 is melted by gradually raising the temperature to the melting point of calcium fluoride or higher (1370 ° C. to 1450 ° C.). Next, by lowering the growth crucible at a speed of about 0.1 mm / hr to 5 mm / hr while controlling the heater power for heating in the furnace, the melt is gradually crystallized from the lower part of the crucible. Grow. In this way, a single crystal of fluorite can be obtained. After crystallization, annealing and gradual cooling are performed to such an extent that the fluorite single crystal is not broken.

  The fluorite single crystal thus obtained is a high-quality fluorite single crystal whose transmittance does not decrease even when irradiated with excimer laser light for a long time. Therefore, the single crystal of fluorite obtained as described above can be suitably used as an optical member used in an exposure apparatus that uses short-wavelength light such as excimer laser light as exposure light. Used for lenses, mirrors, and the like of exposure apparatuses that use light having a wavelength of 250 nm or less as exposure light.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

(Examples 1-7 and Comparative Examples 1-5)
1. Preparation of raw material First, after mixing a high-purity calcium fluoride raw material powder (calcium fluoride having a purity of 99.0% or more) with a scavenger (PbF ₂ ) at a ratio of 1.0 mol%, a mixed powder was obtained. The mixed powder was filled in a pretreatment furnace. Next, the furnace temperature was raised to 850 ° C. and held for 8 hours to advance the scavenge reaction. Next, the furnace temperature is further raised to 1400 ° C., which is equal to or higher than the melting point of calcium fluoride to melt the calcium fluoride to form a melt, and then the temperature is gradually lowered to solidify the melt. To obtain polycrystals of fluorite.

2. Production of Fluorite Single Crystal A fluorite single crystal was produced using the fluorite single crystal production apparatus shown in FIG. That is, the fluorite polycrystal thus obtained was used as a raw material 30, the raw material 30 was filled in the crucible 11, and the crucible 11 was placed on the crucible support base 12 in the single crystal manufacturing apparatus. At that time, a seed crystal whose crystal plane orientation is set to {111} is stored in the lowermost part of the crucible 11, and the height position of the part storing the seed crystal coincides with the height position of the upper end of the partition portion 10b. Thus, the height position of the crucible 11 was determined.

Next, after making the inside of the furnace main body 1 consisting of a vacuum bell jar a vacuum atmosphere lower than 10 ⁻⁴ Pa, the temperature of the high temperature side furnace chamber 10 a was gradually raised to 1410 ° C. Thereafter, while controlling the high-temperature heater 14a and the low-temperature heater 40b for heating in the furnace, the crucible 11 is gradually pulled down at a speed of 1.0 mm / hr to crystallize the melt in the crucible 11. Thus, a single crystal ingot of fluorite was obtained. After crystallization, annealing and slow cooling were performed to such an extent that the fluorite single crystal ingot was not broken, and the fluorite single crystal ingot was taken out of the furnace.

In Examples and Comparative Examples, respectively heater (high temperature heater 14a and the low temperature portion heater 40b than it is that of the same diameter) by changing the value of the inner diameter R ₁ of fluorite single crystal as described above Manufactured. The ratio α (R ₂ / R ₁ ) between the outer diameter R ₂ of the crucible 11 and the inner diameter R _{1 of the} heater in each example and each comparative example, and the ratio α between the inner diameter R _{3 of the} crucible 11 and the inner diameter R _{1 of the} heater '(R ₃ / R ₁ ) and the ratio β (L / R ₃ ) between the vertical length L of the partition and the inner diameter R _{3 of the} crucible are shown in Table 1, respectively. Note that the outer diameter ratio between the heater (the high-temperature heater 14a and the low-temperature heater 40b) and the heat insulating material 2 disposed on the outer periphery of the heater was constant in all combinations.

<Measurement of transmittance>
The transmittance of the single crystal ingot of fluorite obtained in each example and each comparative example was measured. In the measurement, first, the upper part of the single crystal ingot of fluorite obtained in each example and each comparative example was cut and processed to prepare a test piece for transmittance measurement having a diameter of 30 mm and a thickness of 10 mm. did. Next, ArF excimer laser light having a wavelength of 193.4 nm was irradiated to such a test piece, and after measuring the internal initial transmittance (hereinafter referred to as “internal initial transmittance”) of each test piece, The internal transmittance after irradiation with 1 × 10 ⁵ pulses of ArF excimer laser under the condition of 50 mJ (hereinafter referred to as “internal transmittance after irradiation”) was measured. The measurement results of the internal initial transmittance and the internal transmittance after irradiation are shown in Table 1, and a graph showing the relationship between the α and the internal transmittance after irradiation is shown in FIG.

As is clear from the results shown in Table 1, the single crystal ingot of fluorite obtained in Examples 1 to 7 in which the ratio α between the outer diameter of the crucible and the inner diameter of the side heater is 0.76 or more, Even when excimer laser light is irradiated for a long time, the decrease in the transmittance is very small, and the internal transmittance can be achieved 99.7% / cm even after the ArF excimer laser is irradiated with 1 × 10 ⁵ pulses at 50 mJ. Was confirmed.

  As described above, according to the present invention, by utilizing the impurity segregation effect during crystal growth, a high-quality fluorite single crystal whose transmittance does not decrease even when irradiated with excimer laser light for a long time can be efficiently obtained. Also, it is possible to provide a fluorite single crystal production apparatus that can be reliably produced, and a fluorite single crystal production method using the same.

  Therefore, the fluorite single crystal production apparatus of the present invention is used to produce high-quality fluorite used in lenses, mirrors, etc. used in the optical system of an exposure apparatus that uses light in the wavelength region of an excimer laser as exposure light. It is particularly useful.

It is a schematic diagram which shows the Bridgman-type conventional single crystal manufacturing apparatus. It is a graph which shows the conceptual diagram which shows the solid-liquid interface vicinity in a crucible. It is a schematic diagram which shows the structure of one Embodiment suitable as a single crystal manufacturing apparatus of the fluorite of this invention. It is a graph of the relationship between the ratio α (R ₂ / R ₁ ) between the outer diameter R ₂ of the crucible 11 and the inner diameter R _{1 of the} heater and the transmittance after irradiation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Furnace main body, 2 ... Heat insulation material, 10 ... Hollow part in a furnace, 10a ... High temperature side furnace chamber, 10b ... Heat insulation part, 10c ... Low temperature side furnace chamber, 11 ... Crucible, 12 ... Crucible support stand, 13 ... Pull-down rod 14 ... single crystal growth zone, 14a ... high temperature heater, 14b ... heat insulation member, 14c ... low temperature heater, 21 ... crystal, 22 ... melt, 23 ... solid-liquid interface, 24 ... semi-molten layer, 30 ... raw material, R ₁ ... inner diameter of the heater, R ₂ ... outer diameter of the crucible, R ₃ ... inner diameter of the crucible, L ... length of the partition portion in the vertical direction.

Claims (4)

A furnace body that forms a furnace chamber, a heat insulating member disposed in a partition that vertically separates the furnace chamber into two chambers, a high temperature side furnace chamber and a low temperature side furnace chamber; A vertical crucible installed so as to be movable between the chamber and the low-temperature side furnace chamber through the partition, and a heater disposed in each of the high-temperature side furnace chamber and the low-temperature side furnace chamber An apparatus for producing a single crystal of fluorite used in the Bridgman method,
An apparatus for producing a single crystal of fluorite, wherein the ratio of the outer diameter of the crucible to the inner diameter of the heater (crucible outer diameter / heater inner diameter) is 0.76 or more.   2. The apparatus for producing a single crystal of fluorite according to claim 1, wherein the ratio of the inner diameter of the crucible to the inner diameter of the heater (the inner diameter of the crucible / the inner diameter of the heater) is 0.70 or more.   The ratio of the vertical length of the partition portion to the inner diameter of the crucible (the length of the partition portion in the vertical direction / the inner diameter of the crucible) is in the range of 1/7 to 1/10. Or the apparatus for producing a single crystal of fluorite according to 2.   A method for producing a fluorite single crystal, wherein the fluorite single crystal is obtained using the fluorite single crystal production apparatus according to claim 1.

Images