JP2006321690A - Fluoride single crystal, its growing method, and lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fluoride single crystal having a diameter of not less than 50 mm in the cross section perpendicular to the axis of the crystal growth direction and capable of obtaining a sufficient resolution performance when used as a lens material for a stepper, and to provide a growing method of such a fluoride single crystal capable of easily and reliably growing it and a lens composed of such a fluoride single crystal. <P>SOLUTION: The fluoride single crystal has a nearly circular shape having a diameter of not less than 50 mm in the cross section perpendicular to the axis A of the crystal growth direction, satisfies the formula: 0.1≤B<SB>2</SB>/B<SB>1</SB>≤100 when the birefringence of the central part C in the cross section is B<SB>1</SB>and the birefringence of the outer periphery part O in the cross section is B<SB>2</SB>, and is obtained by a growing method that comprises a growing step for growing a single crystal in a crystal growth furnace having a temperature gradient in the crystal growth direction, a reheating step for heating the single crystal to an even temperature lower than its melting point in the crystal growth furnace, and a cooling step for cooling the single crystal. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fluoride single crystal used in an optical apparatus, an optical system for optical lithography, and the like, a growth method thereof, and a lens made of the fluoride single crystal.

  Fluoride single crystals are used in optical systems and optical systems for optical lithography, and are further used in optical systems of semiconductor lithography steppers (exposure devices) that require high optical performance. In particular, calcium fluoride single crystal is widely used as a lens material for a stepper because it has excellent optical performance such as light transmittance and durability against laser.

  However, in recent years, in order to further increase the density and integration of semiconductors, it has been required to improve the performance of steppers for semiconductor lithography, and the wavelength of lasers used as light sources can be shortened and used as lens materials. There is a need for further improvement in the optical performance of fluoride single crystals. In order to meet such demands, various attempts have been made to improve optical performance in single crystals as optical materials such as calcium fluoride (for example, see Patent Documents 1 to 3).

  As a method for growing a calcium fluoride single crystal used as such a lens material, a method of growing a single crystal along the crystal plane of a seed crystal by melting and cooling calcium fluoride is known. (For example, refer to Patent Document 4).

JP-A-8-107060 JP 2002-37697 A JP 2001-335398 A Japanese Patent Laid-Open No. 10-265296

  However, in the above conventional fluoride single crystal, when the diameter of the cross section perpendicular to the crystal growth direction axis becomes large (for example, 50 mm or more), a residual stress distribution is formed in the crystal and birefringence occurs. When a single crystal is used as a lens material for a stepper, there is a problem that sufficient resolution performance cannot be obtained.

  The present invention has been made in view of the above-described problems of the prior art, and has a diameter of a cross section perpendicular to the crystal growth direction axis of 50 mm or more and sufficient resolution performance when used as a lens material for a stepper. It is an object to provide a fluoride single crystal that can be obtained, a method for growing a fluoride single crystal capable of easily and reliably growing such a fluoride single crystal, and a lens made of such a fluoride single crystal. And

In order to achieve the above object, the present invention provides a single crystal having a substantially circular shape with a diameter of 50 mm or more in the cross section perpendicular to the crystal growth direction axis, and the birefringence at the center in the cross section is B _1. , when the birefringence of the outer peripheral portion and _{B 2,} the value of B 2 / _{B 1} satisfies the following formula:
0.1 ≦ B ₂ / B ₁ ≦ 100
A fluoride single crystal characterized by satisfying the above conditions is provided.

According to such a fluoride single crystal, sufficient resolution performance can be obtained when used as a lens material for a stepper. Here, when the value of B ₂ / B ₁ is less than 0.1, the image is distorted due to the difference in image formation method between the central portion and the outer peripheral portion, so that sufficient resolution performance is obtained. I can't get it. On the other hand, even when the value of B ₂ / B ₁ exceeds 100, in a lens application, the image is distorted due to the difference in image formation between the central portion and the outer peripheral portion, so that sufficient resolution performance cannot be obtained. . When a fluoride single crystal is used as a lens material for a stepper, in order to obtain a sufficient resolution performance, it is desired that a difference in birefringence is small between the central portion and the outer peripheral portion, and B ₂ / B a value of ₁ is preferably as close to 1.

  Further, the fluoride single crystal of the present invention comprises a growth step of growing a single crystal in a crystal growth furnace having a temperature gradient in the crystal growth direction, and the single crystal is brought to a uniform temperature below its melting point in the crystal growth furnace. It is preferably obtained by a growing method including a reheating step for heating and a cooling step for cooling the single crystal.

  Here, for example, a fluoride single crystal obtained by a conventional growth method as described in Patent Document 4, for example, when the diameter of the cross section perpendicular to the crystal growth direction axis becomes larger than 50 mm, for example, There has been a problem that the birefringence of the outer peripheral portion of the cross section becomes extremely larger than the birefringence of the central portion.

On the other hand, the fluoride single crystal of the present invention obtained by the growth method described above has a cross section perpendicular to the crystal growth direction axis, in which the occurrence of residual stress and strain in the crystal is suppressed in the reheating step. Thus, a single crystal having a small difference in birefringence between the central portion and the outer peripheral portion is obtained. Such a fluoride single crystal satisfies the above formula in terms of the value of B ₂ / B ₁ and can provide sufficient resolution performance when used as a lens material for a stepper.

  In addition, the fluoride single crystal of the present invention may be in an as-grown state obtained after the cooling step, and an annealing step is performed to anneal the single crystal after the cooling step. It may be obtained.

  Here, the as-grown state means a grown state as it is, and means a state in which processing such as cutting, grinding, polishing, or annealing after the cooling step is not performed.

  The present invention also includes a growth step for growing a single crystal in a crystal growth furnace having a temperature gradient in the crystal growth direction, and a reheating step for heating the single crystal to a uniform temperature below its melting point in the crystal growth furnace. And a cooling step for cooling the single crystal. A method for growing a fluoride single crystal is provided.

According to such a growth method, the occurrence of residual stress and strain in the crystal is suppressed, and the fluoride single crystal of the present invention in which the value of B ₂ / B ₁ satisfies the above formula is easily and reliably produced. can do.

  The present invention further provides a lens comprising the above-described fluoride single crystal of the present invention.

  Such a lens can obtain sufficient resolution performance when used in, for example, a stepper for semiconductor lithography.

  ADVANTAGE OF THE INVENTION According to this invention, the diameter of the cross section perpendicular | vertical to a crystal growth direction axis | shaft is 50 mm or more, Comprising: When using as a lens material for steppers, the fluoride single crystal which can obtain sufficient resolution performance can be provided. . Moreover, the growth method of the fluoride single crystal which can grow such a fluoride single crystal easily and reliably can be provided. Furthermore, it is possible to provide a lens made of such a fluoride single crystal and capable of obtaining sufficient resolution performance when used in a stepper.

  Hereinafter, the present invention will be described in detail with reference to the preferred embodiments with reference to the drawings. In the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the positional relationship such as up, down, left and right is based on the positional relationship shown in the drawings unless otherwise specified. Further, the dimensional ratios in the drawings are not limited to the illustrated ratios.

  FIG. 1 (a) is a perspective view schematically showing one embodiment of the fluoride single crystal of the present invention, and FIG. 1 (b) is a crystal growth direction axis of the fluoride single crystal of FIG. 1 (a). It is a schematic diagram of a cross section perpendicular | vertical to.

  As shown in FIG. 1 (a), the fluoride single crystal 10 of the present invention has, for example, a cylindrical shape, and has a substantially circular cross section perpendicular to the crystal growth direction axis A and a diameter d of the cross section. 50 mm or more.

Specific examples of the fluoride single crystal in the present invention include calcium fluoride (CaF ₂ ) single crystal, magnesium fluoride (MgF ₂ ) single crystal, and barium fluoride (BaF ₂ ) single crystal. A calcium fluoride (CaF ₂ ) single crystal is preferred.

  The crystal growth direction axis A means a direction axis in which the fluoride single crystal 10 is grown when the fluoride single crystal 10 is grown. When the fluoride single crystal 10 has a cylindrical shape as shown in FIG. 1A, the longitudinal axis is usually the crystal growth direction axis A.

  The diameter d is preferably 100 mm or more, more preferably 200 mm or more. A fluoride single crystal is more useful for obtaining a large-diameter stepper lens as the diameter d is larger. In a stepper, as the large-diameter lens is used, higher density and higher integration of a semiconductor can be achieved.

The fluoride single crystal 10 of the present invention has a birefringence of B _{1 at} the center C and a birefringence of the outer periphery O in a cross section perpendicular to the crystal growth direction axis A (see FIG. 1B). when was a _{B 2,} the value of B 2 / _{B 1} satisfies the following formula:
0.1 ≦ B ₂ / B ₁ ≦ 100
The above conditions are satisfied.

Here, the central portion C means a peripheral portion of the center P of the cross section where the radius of the cross section is r and the length of r / 10 is a radius, and the outer peripheral portion O is a circumference. To the inner part having a width of r / 10. In addition, the birefringence index B ₁ in the central portion C and the birefringence index B ₂ in the outer peripheral portion O mean the average value of the birefringence obtained from the birefringence distribution measured in each region.

  The birefringence in the present invention can be measured by a birefringence measurement apparatus ((ABR-10A) high-precision measurement type). When the fluoride single crystal has a columnar shape as shown in FIG. 1A, the columnar fluoride single crystal is sliced into a disk shape having a thickness of 10 to 100 mm, and this is sampled. Can be measured as

When the value of B ₂ / B ₁ obtained from B ₁ and B ₂ thus measured exceeds the upper limit value, the birefringence of the outer peripheral portion O with respect to the birefringence of the central portion C is Since it is too large, precise imaging cannot be performed and sufficient resolution performance cannot be obtained. In general, the birefringence is more likely to occur in the outer peripheral portion than the central portion of the single crystal, and the value of B ₂ / B ₁ is almost ₁ in most cases, but contrary to this, the B ₂ / B ₁ When the value is less than the lower limit, as in the case where the value of B ₂ / B ₁ exceeds the upper limit, precise imaging cannot be performed, and sufficient resolution performance cannot be obtained.

In addition, the value of B ₂ / B ₁ in the fluoride single crystal of the present invention is obtained by the following formula because better resolution performance is obtained when the fluoride single crystal is used as a lens material for a stepper:
0.5 ≦ B ₂ / B ₁ ≦ 50
It is particularly preferable that the above condition is satisfied.

  Here, FIG. 1A shows a case where the shape of the fluoride single crystal is a columnar shape, but the shape of the fluoride single crystal of the present invention is such that the cross section perpendicular to the crystal growth direction axis has a diameter of 50 mm or more. The shape is not particularly limited as long as it has a substantially circular shape and can be processed into a stepper lens.

Furthermore, the fluoride single crystal of the present invention may be in an as-grown state or may be annealed. In the as-grown fluoride single crystal, when the value of B ₂ / B ₁ satisfies the above formula, excellent resolution performance can be obtained when used as a lens material for a stepper. Further, the fluoride single crystal of the present invention, the value of the B _{2 /} B ₁ in a state after annealing comprises also satisfies the condition of the above formula, in Azuguron state value of the B _{2 /} B ₁ is in the formula Also included are those in which a fluoride single crystal that satisfies the conditions is further annealed.

  Here, the annealing is performed, for example, by holding at a temperature lower than the melting point of the crystal (for example, 900 to 1300 ° C. for calcium fluoride) for 24 to 96 hours and cooling at a temperature lowering rate of 30 ° C./h or less. Done.

  Next, the method for growing a fluoride single crystal of the present invention will be described.

  The method for growing a fluoride single crystal according to the present invention includes a growth step for growing a single crystal in a crystal growth furnace having a temperature gradient in the crystal growth direction, and a uniform temperature below the melting point of the single crystal in the crystal growth furnace. A reheating step of heating the substrate to a single crystal and a cooling step of cooling the single crystal.

  Here, in the growing method of the present invention, a conventionally known growing method can be applied except that the above steps are performed, and the growing can be performed by the Bridgeman method, the Czochralski method, or the like. In addition, since it is relatively easy to grow a single crystal having a large diameter, it is preferable that the growing method of the present invention is performed by a vertical Bridgman (hereinafter abbreviated as VB) method.

Hereinafter, the method for growing a fluoride single crystal according to the present invention will be specifically described by taking a case where a calcium fluoride (CaF ₂ ) single crystal is grown by the VB method using a vacuum VB furnace equipped with a crucible shown in FIG. I will explain it.

As shown in FIG. 2, the crucible 1 is disposed inside a heater 2A in a vacuum VB furnace 2 as a single crystal growth apparatus by the vertical Bridgman method, and is moved up and down at a very low speed via a shaft 2B. The raw material M of calcium fluoride (CaF ₂ ) is melted and cooled, and this is crystallized in a seed crystal (seed) S made of a single crystal of calcium fluoride (CaF ₂ ), for example, in the (1,1,1) orientation It is for growing into a single crystal along the surface.

  The inside of the vacuum VB furnace 2 is depressurized by a vacuum pump 2C and heated by a heater 2A. In order to prevent the entire seed crystal S from being melted by the heating of the heater 2A, the shaft 2B of the vacuum VB furnace 2 is configured to form a cooling water circulation path.

  That is, the shaft 2B is formed of a double tube in which the upper end of the inner tube 2B1 is retracted from the upper end of the outer tube 2B2, and a cap-like heat transfer member 2D is fitted and fixed to the upper end portion. And this heat-transfer member 2D is comprised so that the lower part of the seed crystal S may be forcedly cooled by connecting to the center part of the bottom member 1C of the crucible 1 mentioned later.

  Here, as shown in FIG. 3, the crucible 1 of the present embodiment includes a crucible body 1A, a lid member 1B that covers the opening of the crucible body 1A, and a bottom member 1C that is fixed to the lower portion of the crucible body 1B. It is prepared for. The crucible body 1A is made of high-purity carbon material (C) as a material that has heat resistance and can improve the smoothness of the inner surface, and the inner surface is coated with glossy glassy carbon (GC). ing.

The crucible body 1A is formed with a raw material container 1D in which a raw material M of calcium fluoride (CaF ₂ ) (see FIG. 2) is stored. The raw material container 1D has a cylindrical wall surface 1H, a concave curved surface 1J formed continuously on the bottom member 1C side of the wall surface 1H, and a tapered shape formed continuously on the bottom member 1C side of the concave curved surface 1J ( A funnel-shaped cone surface 1F. Accordingly, the cone surface 1F constitutes the bottom of the raw material container 1D.

  In addition, a seed crystal accommodating portion 1E that accommodates a seed crystal S (see FIG. 2) is formed in the central portion from the crucible main body 1A to the bottom member 1C. The seed crystal housing portion 1E has a flat bottom surface and a cylindrical wall surface 1K that is continuous with the bottom surface and perpendicular to the bottom surface, and the diameter of the wall surface 1K substantially matches the diameter of the seed crystal S. In addition, the wall surface 1K of the seed crystal storage unit has a smaller diameter than the wall surface 1H of the raw material storage unit 1D.

  Further, a convex curved surface 1L is formed at the boundary portion between the cone surface 1F and the wall surface 1K of the seed crystal accommodating portion 1E, and the cone surface 1F and the wall surface 1K of the seed crystal accommodating portion 1E are smoothly passed through the convex curved surface 1L. It is continuous.

  On the other hand, the lid member 1B and the bottom member 1C are also made of a heat-resistant high-purity carbon material. A connecting cylinder portion 1C1 for fitting and fixing a heat transfer member 2D (see FIG. 2) fixed to the upper end portion of the shaft 2B of the vacuum VB furnace 2 protrudes from the bottom surface center portion of the bottom member 1C. ing.

In the crucible 1, if the cone angle θ of the cone surface 1F is too small, residual stress and strain are generated in the calcium fluoride (CaF ₂ ) crystal grown in the raw material container 1D. Polycrystal (heterophase) and birefringence are likely to occur. On the other hand, if the cone angle θ of the cone surface 1F is too large, the growth of a single crystal of calcium fluoride (CaF ₂ ) is likely to be hindered. Therefore, the cone angle θ of the cone surface 1F is preferably 95 ° to 150 °, and more preferably 120 ° to 130 ° in these ranges.

Further, when the concave curved surface 1J and the convex curved surface 1L are angular because the radius of curvature is too small, when the calcium fluoride (CaF ₂ ) melted in the raw material container 1D is crystallized by cooling, the concave concave curved surface is formed. Polycrystals (heterogeneous phases) are likely to occur with 1J and the convex curved surface 1L as nuclei. In addition, when calcium fluoride (CaF ₂ ) contracts by cooling, calcium fluoride (CaF ₂ ) adheres to these angular concave curved surface 1J and convex curved surface 1L, and residual stress and strain are generated in the crystal. Due to this, polycrystal (heterophase) and birefringence are likely to occur.

  Therefore, the curvature radii of the concave curved surface 1J and the convex curved surface 1L are set to a large curvature radius of 1/10 or more of the inner diameter (for example, 250 mm) between the wall surfaces 1H of the raw material container 1D. For example, the radius of curvature of the concave curved surface 1J is set to about 60 mm, and the radius of curvature of the convex curved surface 1L is set to about 50 mm.

Furthermore, when the surface roughness of the wall surface 1H and the cone surface 1F of the raw material container 1D is rough, when the calcium fluoride (CaF ₂ ) melted in the raw material container 1D is crystallized by cooling, the wall surface 1H and the cone Polycrystals (heterogeneous phases) are likely to be generated with minute irregularities such as the surface 1F serving as nuclei. In addition, when calcium fluoride (CaF ₂ ) contracts due to cooling, calcium fluoride (CaF ₂ ) adheres to the wall surface 1H and the cone surface 1F, resulting in residual stress and strain in the crystal. Polycrystal (heterophase) and birefringence are likely to occur.

  Therefore, in the crucible 1, the inner surface of the crucible extending from the wall surface 1H of the raw material container 1D of the crucible body 1A to the wall surface 1K of the seed crystal container 1E through the concave curved surface 1J, the cone surface 1F, and the convex curved surface 1L is The surface roughness according to the method is finished to Rmax 3.2 s or less. That is, the inner surface of the crucible body 1A made of the high-purity carbon material (C) is finished to about Rmax 6.4 s, for example, and the surface is coated with glassy carbon (GC) and finished to about Rmax 3.2 s. .

  And the crucible inner surface comprised of glassy carbon (GC) having a surface roughness of Rmax 3.2 s or less in this way has a contact angle with water droplets of at least 100 ° or less, for example 90 °.

  In the growing method of the present invention, the crucible 1 is used to first remove the lid member 1B of the crucible 1 and accommodate the seed crystal S made of calcium fluoride in the seed crystal accommodating portion 1E. Here, as the seed crystal S, a crystal having a cylindrical shape and a flat end surface and having a diameter substantially equal to the diameter of the wall surface 1K of the seed crystal accommodating portion 1E is used.

  After the seed crystal S is accommodated in the seed crystal accommodating part 1E, the raw material M of calcium fluoride is accommodated in the raw material accommodating part 1D, and then the raw material accommodating part 1D of the crucible body 1A is closed with the lid member 1B.

Next, the inside of the vacuum VB furnace 2 is depressurized to 10 ⁻⁴ Pa or less by the vacuum pump 2C, and the heater 2A is heated to around 1400 to 1500 ° C. At this time, as shown in FIG. 2, since no heater is provided in the lower part of the vacuum VB furnace 2 and the heater is not heated, the inside of the vacuum VB furnace extends from the upper part to the lower part (that is, in the crystal growth direction). ) Has a temperature gradient. Then, the crucible 1 is raised at a slow speed of about 10 mm / h by the shaft 2B and held at the raised position for about 10 hours. At this time, if the entire seed crystal S is melted, it becomes difficult to obtain a single crystal having a target crystal orientation. Therefore, the heat transfer member 2D is made of cooling water circulating in the shaft 2B from the inner tube 2B1 to the outer tube 2B2. Then, the lower part of the seed crystal S is forcibly cooled.

After melting the raw material M of calcium fluoride, the crucible 1 is lowered at a very low speed of, for example, about 1.0 mm / h, which is 1.5 mm / h or less by the shaft 2B, and lowered in the vacuum VB furnace 2 for about 5 hours. Hold in position. Thereby, the raw material M of molten calcium fluoride (CaF ₂ ) is cooled and grown into a single crystal along the crystal plane of, for example, the (1,1,1) orientation of the seed crystal S (growth step).

  Thereafter, with the crucible 1 held at the lowered position in the vacuum VB furnace, the heater 2A is heated to a temperature below the melting point of the grown single crystal. Here, the temperature of the heater 2A is appropriately adjusted depending on the type of single crystal, but when the single crystal is a calcium fluoride single crystal, the temperature is preferably 900 to 1300 ° C.

  Next, in a state where the heater 2A is heated as described above, the crucible 1 is raised by the shaft 2B at a speed of about 5 to 15 mm / h and held at the raised position for about 24 to 96 hours. Thereby, in the single crystal immediately after the growth, the temperature gradient generated from the upper part to the lower part and further from the central part to the outer peripheral part is eliminated, and the entire single crystal is reheated to a uniform temperature below its melting point (reheating step) ).

  Thereafter, the single crystal in the crucible 1 is controlled at 70 ° C. by controlling the heater 2A of the vacuum VB furnace 2 on and off while keeping the crucible 1 in the raised position in order to prevent quenching (cracking due to thermal shock). For example, at a cooling rate of about 30 ° C./h or less (cooling step).

In the calcium fluoride single crystal obtained by such a growth method, since the above-described reheating step is performed immediately after the crystal growth, the occurrence of residual stress and strain in the crystal is sufficiently suppressed. As a result, the calcium fluoride single crystal of the present invention in which the value of B ₂ / B ₁ satisfies the condition of the above formula can be obtained easily and reliably.

  The preferred embodiment of the method for growing a single crystal of the present invention has been described above, but the present invention is not limited to the above embodiment. For example, in the above embodiment, the wall surface 1K of the seed crystal housing portion 1E of the crucible 1 is cylindrical, but the shape of the wall surface 1K may be prismatic when accommodating a prismatic seed crystal. Good.

  Moreover, although the bottom surface of the seed crystal accommodating part 1E is a flat surface, the bottom surface of the seed crystal accommodating part of this invention is not restricted to a flat surface, A cone shape may be sufficient. However, in order to sufficiently cool the bottom of the seed crystal S in contact with the bottom surface of the seed crystal housing portion and prevent the entire seed crystal S from melting, the bottom surface of the seed crystal housing portion is the bottom portion of the seed crystal S. It is preferable that the gap between the bottom portion of the seed crystal S and the bottom surface of the seed crystal housing portion 1E be sufficiently small.

  Furthermore, although the case where the seed crystal is calcium fluoride has been described in the above embodiment, the method for growing a fluoride single crystal of the present invention is not limited to materials other than calcium fluoride (for example, fluoride). It is also applicable to the case of barium fluoride, magnesium fluoride, and the like.

  The fluoride single crystal of the present invention is obtained by such a growth method, that is, a growth step for growing a single crystal in a crystal growth furnace having a temperature gradient in the crystal growth direction, and the crystal growth furnace It is preferably obtained by a growth method including a reheating step for heating the single crystal to a uniform temperature below its melting point and a cooling step for cooling the single crystal. The fluoride single crystal thus obtained can provide sufficient resolution performance when used as a lens material for a stepper.

  Next, the lens of the present invention will be described.

  The lens of the present invention is used for a stepper for semiconductor lithography, for example, and is composed of the above-described fluoride single crystal of the present invention. The shape of such a lens can be applied without particular limitation to the shape of a lens conventionally used in a stepper, for example, one surface is a flat surface and the other surface is a convex or concave surface, or both surfaces are Examples thereof include a convex curved surface or a concave curved surface. The curved surface may be spherical or aspherical.

  These lenses can be obtained by processing the fluoride single crystal of the present invention into a predetermined shape by a conventionally known method, and performing grinding, polishing or the like. Such a lens can obtain sufficient resolution performance when used in a stepper.

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

Example 1
First, a crucible 1 shown in FIG. 2 was prepared. The crucible body 1A, the lid member 1B and the bottom member 1C were all made of high purity carbon (high purity carbon made by Nippon Carbon). The wall surface 1K of the seed crystal accommodating part 1E was cylindrical, and its inner diameter was 10 mm. The bottom surface of the seed crystal accommodating portion 1E was a flat surface perpendicular to the wall surface 1K. The wall surface of the raw material container was also cylindrical, and its inner diameter was 150 mm. The radius of curvature of the concave curved surface 1J was 60 mm, and the radius of curvature of the convex curved surface was 50 mm. Further, the inner surface of the raw material container 1D and the inner surface (wall surface) of the seed crystal container are coated with glassy carbon (Nisshinbo glassy carbon coat) having an impregnated layer thickness of 90, and the contact angle with water droplets is 90. It was made to become °.

  In this crucible 1, the lid member 1B was removed, and a cylindrical seed crystal S having a diameter of 10 mm and a length of 10 cm was accommodated in the seed crystal accommodating portion 1E of the crucible 1. Here, the material of the seed crystal S used was calcium fluoride, and the shape of the seed crystal S was flat at the end face.

  Subsequently, the raw material M of calcium fluoride was accommodated in the raw material accommodating part 1D. Subsequently, the raw material container 1D of the crucible body 1A was closed with the lid member 1B.

Next, the inside of the vacuum VB furnace 2 is depressurized to 10 ⁻⁴ Pa or less, the heater 2A is heated to about 1500 ° C., the crucible 1 is raised at a minute speed of about 10 mm / h by the shaft 2B, and the raised position is about 10 hours. Held on. At that time, the lower part of the seed crystal S was forcibly cooled through the heat transfer member 2D with cooling water circulating in the shaft 2B from the inner tube 2B1 to the outer tube 2B2.

After melting the raw material M of calcium fluoride, the crucible 1 was lowered at a very low speed of about 1.0 mm / h by the shaft 2B and held at the lowered position in the vacuum VB furnace 2 for about 5 hours. In this way, the molten raw material M of calcium fluoride (CaF ₂ ) was cooled and grown into a single crystal along the (1,1,1) -oriented crystal plane of the seed crystal S.

  Thereafter, the heater 2A is heated to around 1000 ° C., and the crucible 1 is raised again at a slow speed of about 10 mm / h by the shaft 2B and held at the raised position for about 50 hours, so that the grown single crystal is less than its melting point. Reheated at temperature.

  Thereafter, the single crystal in the crucible 1 is cooled at a cooling rate of about 30 ° C./h by turning on and off the heater 2A of the vacuum VB furnace 2 to form cylindrical calcium fluoride having a diameter of 150 mm and a length of 4 cm. A single crystal was obtained.

(Comparative Example 1)
A cylindrical calcium fluoride single crystal having a diameter of 150 mm and a length of 4 cm was obtained in the same manner as in Example 1 except that reheating was not performed.

(Example 2)
After growing a calcium fluoride single crystal in the same manner as in Example 1, each was heated to around 1000 ° C. and annealed under the condition of holding for 50 hours to obtain a calcium fluoride single crystal of Example 2.

(Measurement of birefringence)
The calcium fluoride single crystals obtained in Examples 1 and 2 and Comparative Example 1 were sliced into discs each having a thickness of 30 mm. For this disk-shaped single crystal, a birefringence distribution is measured by a birefringence measuring apparatus ((ABR-10A) high-precision measurement type), and a cross section perpendicular to the crystal growth direction axis of the single crystal is obtained from the measurement data. The values of the birefringence B _{1 at the} center and the birefringences B ₂ and B ₂ / B _{1 at} the outer periphery were determined. The results are shown in Table 1.

(Example 3, Comparative Example 2)
The calcium fluoride single crystals obtained in Example 2 and Comparative Example 1 were processed into a biconvex lens shape, and the lenses of Example 3 and Comparative Example 2 were obtained.

[Evaluation of resolution performance]
When the resolution performance when the lenses obtained in Example 3 and Comparative Example 2 were used in a stepper for semiconductor lithography was evaluated, the lens of Example 3 can form a precise image and has good resolution performance. On the other hand, with the lens of Comparative Example 2, precise imaging could not be formed and the resolution performance was insufficient.

(A) is a perspective view which shows typically one Embodiment of the fluoride single crystal of this invention, (b) is a model of a cross section perpendicular | vertical to the crystal growth direction axis | shaft of the fluoride single crystal of (a). FIG. It is a schematic diagram which shows schematic structure of the vacuum VB furnace provided with the crucible used in the growth method of the fluoride single crystal of this invention. It is sectional drawing which shows the structure of the crucible shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Crucible, 1A ... Crucible body, 1B ... Lid member, 1C ... Bottom member, 1D ... Raw material accommodating part, 1E ... Seed crystal accommodating part, 1F ... Conical surface, 1H ... Wall surface of raw material accommodating part, 1J ... Concave curved surface, DESCRIPTION OF SYMBOLS 1K ... Wall surface of seed crystal accommodating part, 1L ... Convex curved surface, 1N ... Bottom surface, 2 ... Vacuum VB furnace, 2A ... Heater, 2B ... Shaft, 2C ... Vacuum pump, 2D ... Heat transfer member, 10 ... Fluoride single crystal, a ... crystal growth axis, C ... central raw material of M ... calcium fluoride (CaF _2), O ... outer peripheral portion, P ... center, S ... seed crystals of calcium fluoride (CaF _2), S1 ... seed crystal End face.

Claims (6)

A fluoride single crystal whose cross-sectional shape perpendicular to the crystal growth direction axis is a substantially circular shape having a diameter of 50 mm or more, wherein the birefringence at the center in the cross-section is B ₁ and the birefringence at the outer periphery is B _2. Where B ₂ / B ₁ has the following formula:
0.1 ≦ B ₂ / B ₁ ≦ 100
A fluoride single crystal characterized by satisfying the above conditions. A growth step of growing a single crystal in a crystal growth furnace having a temperature gradient in the crystal growth direction;
A reheating step of heating the single crystal to a uniform temperature below its melting point in the crystal growth furnace;
A cooling step for cooling the single crystal;
The fluoride single crystal according to claim 1, wherein the fluoride single crystal is obtained by a growth method comprising:   The fluoride single crystal according to claim 2 in an as-grown state obtained after the cooling step.   The fluoride single crystal according to claim 2, wherein the fluoride single crystal is obtained by performing an annealing step of annealing the single crystal after the cooling step. A growth step of growing a single crystal in a crystal growth furnace having a temperature gradient in the crystal growth direction;
A reheating step of heating the single crystal to a uniform temperature below its melting point in the crystal growth furnace;
A cooling step for cooling the single crystal;
A method for growing a fluoride single crystal, comprising: The lens which consists of a fluoride single crystal as described in any one of Claims 1-4.

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