JP2011026142A - Method for growing fluoride crystal, fluoride crystal, and optical member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for growing a fluoride crystal, by which generation of cracks or cloudiness can be sufficiently prevented and a fluoride crystal showing good transmittance characteristics can be obtained in a high yield, and to provide a fluoride crystal and an optical member obtained by use of the above method. <P>SOLUTION: The method for growing the fluoride crystal is carried out by using a Bridgman furnace 10 and by housing a crystal raw material 3, in which a fluoride raw material is compounded, in a crucible 2, fusing the material and cooling to solidify the melt, wherein a fluoride having the total mass concentration of iron and zinc of less than 1.5 ppm is compounded as the fluoride raw material. From the viewpoint of suppressing aggregation of oxygen in a crystal, the mass concentrations of iron and zinc are preferably less than 0.5 ppm and less than 1 ppm, respectively. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a method for growing a fluoride crystal, a fluoride crystal, and an optical member, and more particularly, from ultraviolet to vacuum ultraviolet such as KrF laser (248 nm), ArF excimer laser (193 nm), and F ₂ laser (157 nm). The present invention relates to an optical member such as a lens, a prism, and a window material in an optical system such as a photolithography apparatus and a laser processing apparatus using a region as a light source, and a vacuum ultraviolet light source such as an Xe lamp (172 nm) and a Kr ₂ lamp (146 nm).

Along with the increase in the density of integrated circuits, a photolithography apparatus equipped with a light source having a very short wavelength in the vacuum ultraviolet region, such as a KrF laser, an ArF laser, or an F ₂ laser, is used to enable high-resolution exposure. Yes. Synthetic quartz has been used as a material for lenses of conventional photolithography apparatuses. However, since synthetic quartz has a low transmittance at the wavelength described above, an alternative lens material that exhibits a high transmittance even in the vacuum ultraviolet region is required.

  Fluoride crystals such as calcium fluoride and magnesium fluoride exhibit higher permeability in the vacuum ultraviolet region than synthetic quartz. Therefore, fluoride crystals are attracting attention as materials for lenses and window materials in optical systems such as a photolithography apparatus using a vacuum ultraviolet region as a light source and a vacuum ultraviolet light source.

  For the growth of fluoride crystals, the Bridgman method or the Czochralski method, which facilitates the enlargement of the crystal diameter, is used. In the Bridgman method, a fluoride raw material and a scavenger acting as a fluorinating agent are placed in a crucible made of carbon, etc., and after melting these crystal raw materials into a melt, the crucible is slowly lowered in a predetermined temperature gradient. As a result, the melt is cooled and solidified from one direction to grow a fluoride crystal.

  By the way, when a fluoride crystal is grown using the Bridgman method, cracks and turbidity may be included in the crystal. Cracks reduce the yield in optical material production, and turbidity is a factor that degrades optical characteristics such as crystal transmittance.

  Various investigations have been made so far on the investigation of the causes of cracks and turbidity and the countermeasures against them. For example, in the following Patent Document 1, it is considered that oxygen mixed at the time of growing a fluorite material is localized in the subgrain boundary or transition network of fluorite, and oxygen is aggregated by annealing treatment is considered to cause turbidity. As a countermeasure, when fluorite material is hermetically heat-treated, the fluorite dust removal method diffuses oxygen in the fluorite material to the outside of the fluorite material by reducing the pressure in the furnace to 1 Pa or less. Has been proposed. Further, in Patent Document 2 below, it is considered that the residual scavenger used during the growth is the cause of crystal cracking and turbidity. As a countermeasure, the melting point is 400 ° C. or more lower than the crystal growth melting point of the raw material. A method for producing a fluoride crystal using a specific fluoride having a scavenger as a scavenger has been proposed.

  On the other hand, there is a method for improving the crystal transmittance by adding a predetermined amount of strontium, magnesium or the like to fluorite by adding a fluoride such as strontium or magnesium to the crystal raw material (for example, the following patents) References 3, 4 and 5).

JP 2005-145727 A JP 09-227293 A JP 09-255329 A JP 09-315815 A JP 2007-126353 A

  The present invention can sufficiently prevent the occurrence of cracks and turbidity, and can provide a fluoride crystal exhibiting good transmittance characteristics with good yield, and a fluoride obtained using such a method. An object is to provide a crystal and an optical member.

  The present inventors focused on the purity of the fluoride raw material to be blended as the crystal raw material and grown the fluoride crystal by the Bridgman method. The total mass concentration of iron and lead in the fluoride raw material grew the crystal. The present invention has been found to be greatly correlated with the degree of occurrence of cracks and turbidity, and the present invention has been completed based on this finding.

  That is, the present invention is a method for growing a fluoride crystal by cooling and solidifying a melt of a crystal material mixed with a fluoride material, and the total mass concentration of iron and lead is 1.5 ppm as the fluoride material. Provided is a method for growing a fluoride crystal, characterized in that a fluoride that is less than the above is blended.

  According to the method for growing a fluoride crystal of the present invention, the use of the specific fluoride raw material can sufficiently prevent the occurrence of cracks and turbidity, and can produce a fluoride crystal exhibiting good transmittance characteristics with a high yield. Obtainable.

  The method for growing a fluoride crystal according to the present invention is characterized by blending, as a raw material, a fluoride having a total mass concentration of iron and lead of less than 1.5 ppm, including those described in the above-mentioned patent documents. Compared with this method, there is an advantage that the crystal production process is simple. That is, according to the present invention, cracking without greatly changing the growth conditions when growing the crystal, such as hermetic heat treatment and its condition setting, and selection of additives such as scavenger and strict adjustment of the addition amount thereof. It is possible to easily obtain a fluoride crystal that is free from turbidity or has a sufficiently small amount of good transmittance characteristics.

  In the method for growing fluoride crystals of the present invention, it is preferable to blend a fluoride having a mass concentration of iron of less than 0.5 ppm and a mass concentration of lead of less than 1 ppm as the fluoride raw material.

  The fluoride crystal growth method of the present invention provides calcium fluoride, magnesium fluoride, barium fluoride, strontium fluoride, lithium fluoride, rare earth fluoride, or a fluoride crystal that is a mixed crystal thereof. It is suitable for.

  The present invention also provides a fluoride crystal obtained by the method for growing a fluoride crystal of the present invention. The fluoride crystal of the present invention has few cracks and turbidity, and is excellent in transmittance characteristics, particularly in the vacuum ultraviolet region.

  The present invention also provides an optical member comprising the above-described fluoride crystal of the present invention. The optical member of the present invention can be applied to an optical system in the vacuum ultraviolet region.

  According to the present invention, it is possible to sufficiently prevent the occurrence of cracks and turbidity, and to obtain a fluoride crystal exhibiting good transmittance characteristics with good yield. Fluoride crystals and optical members can be provided.

It is a schematic diagram which shows the Bridgman furnace suitably used in order to perform the growth method of the fluoride crystal which concerns on this invention. 1 is an external view of a magnesium fluoride crystal obtained in Example 1. FIG. 2 is a view showing an appearance of a magnesium fluoride crystal obtained in Comparative Example 1. FIG. 6 is a view showing an appearance of a magnesium fluoride crystal obtained in Comparative Example 2. FIG.

  The method for growing a fluoride crystal according to the present invention is a method for growing a fluoride crystal by cooling and solidifying a melt of a crystal material mixed with a fluoride material, and the total mass of iron and lead as the fluoride material. A fluoride having a concentration of less than 1.5 ppm is blended.

  First, the crystal raw material used in the method for growing a fluoride crystal of the present invention will be described.

  Examples of the fluoride raw material blended with the crystal raw material include calcium fluoride, magnesium fluoride, barium fluoride, strontium fluoride, lithium fluoride, and rare earth fluoride.

  When the above-mentioned fluoride raw material is used in the method for growing a fluoride crystal of the present invention, calcium fluoride, magnesium fluoride, barium fluoride, strontium fluoride, lithium fluoride, rare earth fluoride, and mixed crystals thereof. Crystals can be grown.

  In the present invention, the fluoride blended as the fluoride raw material may be a total mass concentration of iron and lead of less than 1.5 ppm, but from the viewpoint of suppressing the aggregation of oxygen in the crystal, the mass concentration of iron is Those having a lead concentration of less than 0.5 ppm and less than 1 ppm are preferred.

  Furthermore, from the viewpoint of suppressing the aggregation of oxygen in the crystal, the fluoride compounded as the fluoride raw material has an iron mass concentration of 0.1 ppm or more and less than 0.5 ppm and a lead mass concentration of 0.5 ppm or more. Those less than 0 ppm are preferred.

  The fluoride satisfying the mass concentration of iron and lead can be prepared by, for example, a zone purification method, a vacuum distillation method, or the like.

  The mass concentration of iron and lead in the fluoride raw material can be confirmed by analytical methods such as fluorescent X-ray analysis, X-ray photoelectron spectroscopy, and ICP mass spectrometry. In the present invention, it is preferable to use ICP mass spectrometry from the viewpoint of performing highly accurate analysis on the order of ppm. Using these analytical methods, a fluoride whose mass concentration of iron and lead satisfies the above conditions may be selected and used as a fluoride raw material.

  As the fluoride raw material, it is preferable to use a powdery fluoride (commercially available product such as Grade-1) manufactured by Stella Chemifa Co.) in terms of impurity concentration. However, not only powdered fluorides but also fluorides such as sintered bodies, beads, chips, and bulks can be used. The fluoride raw material may be once melted and solidified.

  According to the method for growing a fluoride crystal of the present invention, by using the fluoride raw material described above, generation of cracks and turbidity can be suppressed, and good transmittance characteristics, particularly in the ultraviolet to vacuum ultraviolet region. A fluoride crystal having a high transmittance can be obtained with a high yield. The present inventors consider the reason why such an effect is obtained as follows. It is considered that iron and lead contained in the fluoride raw material have the function of forming oxygen and oxide in the crystal raw material and in the growth furnace, and making oxygen easily incorporated into the crystal. The present inventors consider that cracks and crystal defects occur when iron, lead, and oxides thereof serve as growth nuclei and inhibit crystal growth of fluoride crystals. In addition, the present inventors consider that turbidity in the fluoride crystal is observed with crystal defects and oxygen condensed in the subgrain boundary as a scattering center. By using a fluoride raw material with a total mass concentration of iron and lead of less than 1.5 ppm, the incorporation of oxygen into the crystal, the generation of cracks and crystal defects, and the agglomeration of oxygen into the subgrain boundary are effective. It is presumed that a fluoride crystal exhibiting good transmittance characteristics without cracks and turbidity was obtained.

  The fluoride compounded as the fluoride raw material is preferably one having a low content of metal impurities from the viewpoint of preventing inhibition of crystal growth by impurities. Such a fluoride can be obtained by a zone purification method, a vacuum distillation method, a unidirectional solidification method, or the like. In addition, the mass concentration of the element can be confirmed by the analysis method described above, similarly to the mass concentration of iron and lead.

  In the method for growing a fluoride crystal of the present invention, the crystal raw material is blended with a scavenger that can be vaporized below the melting point of the fluoride raw material for the purpose of fluorination of the oxide contained in the fluoride raw material. It is preferable to do.

  Examples of the scavenger include bismuth fluoride, zinc fluoride, sodium fluoride, lithium fluoride, silver fluoride, copper fluoride, and manganese fluoride. These fluorides can be blended singly or in combination of two or more. The blending amount of the scavenger is set to an amount sufficient to fluorinate all oxides contained in the fluoride raw material. Since excess scavenger remains in the crystal and is assumed to behave as an impurity, the compounding amount of fluoride as a scavenger is 0.001 to 0 with respect to 100 parts by mass of fluoride compounded as a fluoride raw material. It is preferably 1 part by mass.

  Next, a preferred embodiment of the method for growing a fluoride crystal of the present invention will be described with reference to the drawings. In addition, the growth method of the fluoride crystal by the Bridgeman method described below is only an example of the present invention, and the present invention is not limited to this.

FIG. 1 is a schematic view showing a Bridgman furnace preferably used for performing the method for growing fluoride crystals according to the present invention. A Bridgman furnace 10 shown in FIG. 1 includes a heater 4, a heat insulating material 5 surrounding the heater 4, and an airtight chamber 1 covering these. The inside of the chamber 1 has a structure that can be evacuated to about 10 ⁻⁵ Pa by a vacuum pump. The heater 4 forms a temperature gradient from the high temperature to the low temperature in the furnace from the upper part to the lower part. The crucible 2 for growing crystals is installed on the upper part of the shaft 6 inside the furnace. The material of the crucible 2 is carbon, quartz, platinum or the like, but is not particularly limited. A hole is provided in the upper part of the crucible 2 so that the inside of the crucible 2 is also evacuated. The crucible 2 can be moved up and down by the movement of the shaft 6, and the crystal is grown by lowering the crucible 2 along the temperature gradient.

  The crystal raw material 3 obtained by sufficiently stirring the above-described fluoride raw material and scavenger is accommodated in the crucible 2. The crucible 2 is installed in the upper part of the chamber 1 and the inside of the chamber 1 is evacuated. By heating the heater 4, the scavenger in the crucible 2 is vaporized and the oxide in the crystal raw material is fluorinated. In addition, it is necessary to hold | maintain temperature until reaction with the oxide in a raw material is completed. Thereafter, the temperature is raised to completely melt the crystal material 3, and the crucible 2 is slowly lowered to crystallize the melt from the lower part of the crucible 2. Since the crystal immediately after solidification has a high temperature and cracks are likely to occur due to thermal contraction, it is desirable to cool the heater 4 slowly to slowly cool the crystal after the melt is completely solidified.

  The fluoride crystal growth method described above includes a step of obtaining a crystal raw material by mixing a fluoride raw material and a scavenger, a step of obtaining a melt by melting the crystal raw material, and obtaining a fluoride crystal by cooling and solidifying the melt. Although it is a growth method by the Bridgman method including a step and a step of gradually cooling the fluoride crystal, various modifications can be made in the present invention. An example is the Czochralski method.

  According to the method for growing a fluoride crystal of the present invention, it is possible to sufficiently prevent the occurrence of cracks and turbidity, but further evaluate the cracks and turbidity contained in the grown fluoride crystal and further select the crystals according to the application. You may go. Evaluation of cracks and turbidity contained in the fluoride crystal can be performed visually. At this time, it is desirable to place the crystal on black paper or cloth and observe it under white light in order to clearly grasp the crystal state. Since the visual evaluation of turbidity depends on an individual's sense, it is more preferable to capture the influence of turbidity as a change in crystal transmittance. In this case, it is desirable to perform optical polishing on both surfaces in order to prevent scattering on the sample surface after cutting out the crystal serving as the measurement sample. When evaluating the transmittance, it is desirable to measure the transmittance in the wavelength range from the ultraviolet region to the vacuum ultraviolet region using a spectrophotometer.

  Next, the optical member of the present invention will be described. The optical member of the present invention comprises a fluoride crystal obtained by the above-described method for growing a fluoride crystal of the present invention.

Examples of the optical member include a semiconductor lithography lens, a laser window material, and a camera lens. Since the optical member of the present invention is made of the fluoride crystal according to the present invention having excellent light transmittance, it is possible to obtain sufficient optical characteristics when used as the above-mentioned application, and in particular, KrF Photolithography apparatus and laser processing apparatus using ultraviolet to vacuum ultraviolet light source such as laser (248 nm), ArF excimer laser (193 nm), F ₂ laser (157 nm), Xe lamp (172 nm) and Kr ₂ lamp ( 146 nm) and other optical members such as lenses, prisms and window materials in an optical system such as a vacuum ultraviolet light source.

  As the shape of the optical member of the present invention, the shape of the conventional optical member used in the above-described applications can be applied without particular limitation. For example, one surface is a flat surface and the other surface is a convex or concave curved surface, or both surfaces are convex or concave curved surfaces. The curved surface may be spherical or aspherical.

  The optical member of the present invention can be obtained by processing the fluoride crystal according to the present invention into a predetermined shape by a conventionally known method, and performing grinding, polishing or the like.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not restrict | limited to these.

<Growth of fluoride crystals>
Example 1
In order to grow fluoride crystals, a Bridgman furnace similar to that shown in FIG. 1 was prepared. As the crucible, a carbon crucible (inner diameter: 55 mm, straight body length: 200 mm) was used.

  Magnesium fluoride powder (Grade-1) (manufactured by Stella Chemifa) as a fluoride raw material, and bismuth fluoride powder (nominal purity 99.99%) (manufactured by Sigma-Aldrich) as a scavenger (magnesium fluoride powder ratio 0) .2% by weight). When the contents of iron and lead in the magnesium fluoride powder were measured using an ICP mass spectrometer, the mass concentration of iron was less than 0.2 ppm, and the mass concentration of lead was less than 0.5 ppm.

  The above-mentioned fluoride raw material and scavenger were sufficiently stirred using a mix rotor to prepare a crystal raw material, and this crystal raw material was accommodated in the above crucible.

After placing the crucible containing the crystal raw material in the chamber, the chamber was evacuated to about 10 ⁻⁴ to 10 ⁻⁵ Pa using an oil diffusion pump. After heating the heater from room temperature to 1400 ° C. at a rate of 60 ° C./h, the crucible was raised to the top of the furnace at a rate of 10 mm / h. Hold the crucible at the uppermost position for 20 hours and completely melt the crystal raw material in the crucible, then lower the crucible to a position where the magnesium fluoride is completely solidified at a rate of 2 mm / h to produce magnesium fluoride crystals Nurtured. Thereafter, the heater was gradually cooled at a rate of 20 ° C./h.

(Comparative Example 1)
Instead of 500 g of magnesium fluoride powder (Grade-1) (manufactured by Stella Chemifa), 380 g of magnesium fluoride powder (OP) (manufactured by Stella Chemifa) is blended, and the blending amount of bismuth fluoride powder is changed to 0.76 g. A magnesium fluoride crystal was grown in the same manner as in Example 1 except that. In addition, when the content of iron and lead in the magnesium fluoride powder was measured using an ICP mass spectrometer, the mass concentration of both iron and lead was 1 ppm.

(Comparative Example 2)
Magnesium fluoride powder (Grade-1) (manufactured by Stella Chemifa Corporation) was replaced with 500 g of magnesium fluoride powder (manufactured by Soekawa Rikagaku Co.), and the blending amount of bismuth fluoride powder was changed to 0.86 g. In the same manner as in Example 1, magnesium fluoride crystals were grown. When the contents of iron and lead in the magnesium fluoride powder were measured using an ICP mass spectrometer, the mass concentration of iron was 8 ppm, and the mass concentration of lead was 9 ppm.

<Evaluation of fluoride crystals>
The magnesium fluoride crystal obtained above was evaluated for cracks and turbidity and measured for transmittance according to the following method. The obtained results are shown in Table 1. Moreover, the figure which shows the external appearance of the magnesium fluoride crystal obtained in Example 1 and Comparative Examples 1 and 2 is shown in FIGS.

(Evaluation of cracks)
The crystal was visually observed under white light, and the ratio (%) of the length in the crystal growth direction of the region where no crack was found on the crystal growth surface to the length of the straight body of the crystal was calculated.

(Evaluation of turbidity)
The crystal was placed on a black cloth, and white light was irradiated from the side of the crystal to determine whether or not the color of the cloth could be observed.

(Measurement of transmittance)
A sample for measurement was cut out from the obtained crystal, and the incident surface and the transmission surface were optically polished. For the crystal obtained in Example 1, a disk-shaped sample having a radius of 38 mm was prepared. About the crystal | crystallization obtained in the comparative example 1, the plate-shaped sample of 31x12 mm was created. About the crystal | crystallization obtained by the comparative example 2, what was taken out along the crack of a crystal | crystallization was processed into the plate shape of 38x13 mm. The thicknesses of these samples were all 6 mm.

  About the sample created above, the transmittance | permeability in a vacuum ultraviolet region was measured using the spectrophotometer (Jasco, V-570). In addition, incident light was irradiated to the central part 4 mm x 2 mm range of the sample. The table shows the transmittance at 193 nm, which is the oscillation wavelength of the ArF excimer laser.

  In the magnesium fluoride crystal obtained in Example 1 (see FIG. 2), neither crack nor turbidity was found in the crystal. On the other hand, the crystal obtained in Comparative Example 1 (see FIG. 3) and the crystal obtained in Comparative Example 2 (see FIG. 4) have innumerable cracks throughout the crystal, and the whole crystal is cloudy white. It was.

  It was confirmed that the magnesium fluoride crystal obtained in Example 1 showed a high transmittance of 90% or more at a wavelength of 193 nm. On the other hand, the magnesium fluoride crystals obtained in Comparative Example 1 and Comparative Example 2 had low transmittance due to cracks and turbidity.

  From these results, the fluoride raw material is selected on the basis of the threshold value of the mass concentration of iron and lead (total mass concentration of less than 1.5 ppm, preferably less than 0.5 ppm of iron and less than 1 ppm of lead). However, it can be said that it is effective for realizing a magnesium fluoride crystal having a high transmittance suitable as a material for an optical member in the vacuum ultraviolet region.

DESCRIPTION OF SYMBOLS 1 ... Chamber, 2 ... Crucible, 3 ... Crystal raw material, 4 ... Heater, 5 ... Thermal insulation, 6 ... Shaft, 10 ... Bridgman furnace.

Claims (5)

A method for growing a fluoride crystal by cooling and solidifying a melt of a crystal material mixed with a fluoride material,
A method for growing fluoride crystals, wherein a fluoride having a total mass concentration of iron and lead of less than 1.5 ppm is blended as the fluoride raw material. The method for growing fluoride crystals according to claim 1, wherein a fluoride having a mass concentration of iron of less than 0.5 ppm and a mass concentration of lead of less than 1 ppm is blended as the fluoride material. The fluoride according to claim 1 or 2, wherein the fluoride crystal is calcium fluoride, magnesium fluoride, barium fluoride, strontium fluoride, lithium fluoride, rare earth fluoride, or a mixed crystal thereof. Crystal growth method. Fluoride crystals obtained by the method for growing fluoride crystals according to any one of claims 1 to 3. An optical member comprising the fluoride crystal according to claim 4.