JPH10203899A - Fluorite little in alkaline earth metal impurities and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain fluorite little in alkaline earth metal impurities and not producing color centers, even when irradiated with the photons or particles of ArF excimer laser light or γ-rays, thereby not lowering transmittance by controlling the concentrations of the alkaline earth metal impurities to specific values or lower. SOLUTION: This fluorite has preferably a total alkaline earth metal impurity concentration of IE 18 atom/cm<3> , more preferably a strontium concentration of IE 18 atom/cm<3> among the alkaline earth metal impurities. The fluorite is obtained by a Bridgman method comprising a process for thermally melting polycrystals and subsequently growing a single crystal from the melted polycrystals, a process for plurally repeating the first process and subsequently cutting out the growing direction lower portions of the obtained plural single crystal ingots, and a process for collecting the cut plural single crystal ingot lower portions, thermally melting the mixture and subsequently growing a single crystal from the melted mixture. Since Mg, Sr and Ba contained in the powder of raw materials and belonging to the same group as that of Ca are mostly left, a conventional method utilizes segregation coefficients to give the fluorite little in the impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to fluorite used in an ultraviolet optical system and a method for producing the same. In particular, various devices using an ArF excimer laser, such as a stepper,
The present invention relates to an optical member such as a lens and a window material used for a CVD device, a nuclear fusion device, and the like.

[0002]

2. Description of the Related Art In recent years, VLSIs have become highly integrated and highly functional, and a fine processing technique on a wafer is required. As a method of the fine processing, a method using an optical lithography technique is generally performed. This VLSI
Of these, taking a DRAM as an example, a capacity of 256 M or more has recently become a reality. Processing line width is 0.35μ
Since the size is as small as m or less, a projection lens of a stepper, which is a mainstream of the optical lithography technology, is required to have high imaging performance (resolution and depth of focus). In order to satisfy this requirement, the exposure wavelength is gradually shortened, and K
Steppers using an rF excimer laser (wavelength: 248 nm) as a light source have come to the market. 248
There are very few optical materials that can be used for optical lithography at wavelengths of nm or less, and two types of fluorite and quartz glass are used.

[0003] In the case of producing fluorite used in the ultraviolet or vacuum ultraviolet region, a crystal is grown by a method called the Bridgman method to produce calcium fluoride crystals, that is, fluorite. As the raw material, artificially chemically synthesized calcium fluoride is used. Examples of the method of synthesizing calcium fluoride include "a method using a reaction between calcium carbonate and ammonium fluoride or ammonium hydrogen fluoride", "addition of excess hydrofluoric acid to calcium carbonate, digestion, or calcium chloride. And a method of melting calcium chloride and a mixture of a fluoride and a chloride of an alkali metal, and the like. A growth crucible filled with calcium fluoride synthesized and purified in this manner and filled with a fluorinating agent such as PbF ₂ is placed in a vacuum electric furnace (growing apparatus) to 10 ⁻⁵ to 10 ⁻⁵ .
Keep in a vacuum atmosphere of 10 ^over 6 Torr. Next, the temperature in the growing apparatus is gradually increased to react the raw material with the fluorinating agent, and then gradually raised to the melting point of fluorite or more (1370 ° C. to 1450 ° C.), and react with the excess fluorinating agent. While volatilizing the product, the raw material is melted. In the crystal growth stage, 0.1
By lowering the growing crucible at a speed of about 5 mm / H, fluorite is obtained by gradually crystallizing from the lower part of the crucible.

[0004]

However, even with fluorite manufactured by such a method, a color center is generated when irradiated with high-energy photons or particles such as ArF excimer laser light and γ-rays. And the transmittance decreases,
There was a very large problem in practice.

[0005]

Means for Solving the Problems Accordingly, the present inventors have:
In the manufacture of fluorite, as a result of intensive research on fluorite, which hardly generates a color center, it was found that, for fluorite having a small amount of impurities as described below, the decrease in transmittance due to the light irradiation did not pose a problem. The present invention provides a fluorite having a total alkaline earth metal impurity concentration of 1E18 atom / cm ³ or less, more preferably an alkaline earth metal impurity with a strontium concentration of 1E18 atom / cm ³ or less. It is.

In the present invention, 1E18 atom / cm
³ indicates 1 × 10 ¹⁸ atom / cm ³ . In the present invention, such fluorite is obtained by the Bridgman method. Further, in the method for producing such fluorite, the present invention provides a production method including at least the following steps. A first step: a step of heating and melting the polycrystal to grow a crystal to obtain a single crystal ingot; a second step: performing the first step a plurality of times to obtain a lower part in a growth direction of the obtained plurality of single crystal ingots. Cutting step, third step: a step of combining the lower portions of the plurality of single crystal ingots cut out in the second step, heating and melting the single crystal ingots to grow crystals, thereby obtaining a single crystal ingot.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the conventional fluorite manufacturing method, impurities such as magnesium, strontium and barium, which are alkaline earth metals of the same family as calcium, contained in the raw material powder remain almost unpurified. There was a problem. The impurity concentrations in the synthesized calcium fluoride raw material and calcium fluoride grown by the conventional Bridgman method were measured. The results were as follows.

[0008]

[Table 1]

[0009]

[Table 2]

As described above, since magnesium, strontium, and barium are similar to calcium and have similar properties, calcium fluoride in the synthesized powder contains several ppm or more, particularly about 200 ppm of strontium. Further, even if a crystal is grown using this powder raw material, impurities remain in the crystal as it is in the conventional method.

For these three types of impurities, the segregation coefficient is determined by W. BOLLMAN, Crystal Research and Technology, 16,
In 521 (1981), magnesium is 0.15, strontium is 0.705, and barium is 0.18. The definition of the segregation coefficient is

[0012]

(Equation 1)

Where k ₀ : segregation coefficient, c (s):
Impurity concentration in solid phase, c (l): impurity concentration in liquid phase. If the solid phase and the liquid phase are in an equilibrium state, the crystal will always grow according to this equation. In the Bridgman method, if the segregation coefficient k ₀ > 1, the impurity concentration decreases with the crystal growth, and conversely. If k ₀ <1, the impurity concentration increases. If k _{0 to} 0, impurities are most preferably eliminated because they are rejected as the crystal grows.

Both magnesium and barium have a segregation coefficient k
₀ is 0.2 or less, which is smaller than that of strontium. However, this coefficient has a meaning when the crystal growth is performed while maintaining the equilibrium state between the solid phase and the liquid phase, and is not satisfied when the growth rate is high. Furthermore, as the diameter of the crystal becomes larger, the influence of the convection of the liquid phase (natural convection, surface tension convection, etc.) increases, and a problem arises in that the equilibrium state cannot be maintained.

Therefore, in order to realize the equilibrium state, it is necessary to reduce the crystal growth rate as much as possible and to devise the convection of the liquid phase as much as possible. However, the lower the crystal growth rate, the longer the production time and the higher the cost.
The limit is about 3 mm / H. In order to prevent convection of the liquid phase, the configuration of the vacuum electric furnace is very important. A growth apparatus having a structure having a heater at an upper portion to suppress the convection of the liquid phase as shown in FIG. 2 is used. Furthermore, as the degree of cooling from the crystallized solid phase increases as the crystal grows, the output of the upper heater and the ceiling heater is gradually increased together with the crystal growth.

In this way, when the crystal is grown so that the segregation coefficient k ₀ has a meaning, magnesium and barium other than strontium are eliminated during the growth process, and hardly remain in the main part of the crystal. be able to. Also, for strontium, it is possible to obtain fluorite with less impurities by removing the latter half of the crystal growth and growing the crystal again, or by cutting out the product from the initial part of the growth.

Although it is conceivable to sufficiently purify the impurities before the crystal growth, the purification of the powder raw material is very expensive. Therefore, although the powder raw material has little alkaline earth impurities, it is not indispensable, but is not indispensable and has no problem in general purity. In addition, an ArF excimer laser (wavelength 193 nm) is set to 100 mJ / cm.
² pulse was examined a relationship between the absorption coefficient and the strontium concentration in 193nm when the 100 Hz, 10 ⁴ shots, was as shown in Figure 1. The strontium concentration was analyzed by ICP-AES, and the absorption coefficient was calculated from the transmittance including the reflection loss using a test piece having a thickness of 10 mm. It was found that as the strontium concentration decreased, the absorption coefficient also decreased, and hardly changed when the concentration was less than about 1E18 atom / cm ³ .

The decrease in transmittance of the ArF excimer laser has a linear relationship with the energy density at a pulse of 100 mJ / cm ² · pulse or less. Therefore, the lower the energy density applied to the fluorite, the higher the limit value of impurities. However, it is unavoidable that the light irradiated on the optical component close to the laser light source has a pulse of several tens mJ / cm ² · pulse. The fluorite of the present invention is very effective, especially when used in such places.

[0019]

EXAMPLE 5 Commercially available calcium fluoride powder raw material 5
Add a small amount of a fluorinating agent to 0 kg, put it in a carbon container,
Set in a vacuum electric furnace. The furnace KoNoboru heated which is evacuated to a vacuum degree of 10 ^@ 5 Torr, 1370 higher than the melting point
After holding at １４1450 ° C. for one day, the heater is turned off. Next, the once melted polycrystal is grown in a vacuum electric furnace as shown in FIG. 1370 above melting point
After raising the temperature to １４1420 ° C. to melt the polycrystal, the growth crucible was lowered at a very low speed.

The obtained crystal ingot φ250 × t30
0 alkaline earth metals (Mg, Sr, B
a) Concentration distribution is 10 or 20 or 100, 200 or 270 ± 5 mm from the bottom crystallized with the growing crucible.
× 10 × 10mm bulk is cut out and crushed,
It was made into a solution with an acid and analyzed by ICP-AES (manufactured by Seiko Denshi). Table 2 shows the results. Although the concentrations of Mg and Ba were not so high, the concentration of Sr was higher as the position became higher in the growth direction of the crystal ingot.

Therefore, the crystal ingot is grown three times by the above method, cut at a distance of 100 mm from the bottom of the obtained three crystal ingots, only the lower part is filled in a growth crucible, and the crystal is grown again. Further, the same operation is performed once again to eliminate impurities in the crystal ingot. As a result of performing the crystal growth three times in total, the Sr concentration below 100 mm from the bottom of the obtained crystal ingot was 1E18 atom / cm ³ .

In order to compare the conventional fluorite (fluorite having a single crystal growth) with the fluorite of the present invention (fluorite grown by crystal growth using the cut lower part), the respective fluorites were compared. A sample of φ30 × t10 was cut out from the single crystal, the surface was polished, and the transmittance including the reflection loss at 193 nm (sample 1)
0 mm thick) with a Varian Cary 5 under a nitrogen atmosphere, and an ArF laser at 100 mJ / cm.
² pulse, transmittance of 100 Hz, 10 ⁴ shots after the irradiation was measured in the same manner. The internal transmittance was determined from the reflectance in consideration of multiple reflection at 193 nm, and the change before and after irradiation was shown in Table 3.
Shown in

[0023]

[Table 3]

As described above, the fluorite of the present invention has excellent durability against ArF excimer laser.

[0025]

The light source of the stepper is considered to be an ArF excimer laser after the KrF excimer laser. In both light sources, the optical materials used are mainly fluorite and quartz glass. Fluorite has recently become available with a large diameter and high quality, but the problem of a decrease in transmittance due to excimer laser irradiation is a major problem when used as a stepper optical system. The present invention is very effective for this problem,
By using the fluorite of the present invention for photolithography using an ArF excimer laser as a light source, sufficient imaging performance can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between an absorption coefficient at 193 nm and a strontium concentration when fluorite of the present invention is irradiated with an ArF excimer laser (wavelength 193 nm).

FIG. 2 is a schematic view of an apparatus for growing a fluorite single crystal.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Crystal ingot 2 Crystal growth surface 3 Crucible for growth 4 Heater 5 Insulation material

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masaki Shiozawa 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nikon Corporation (72) Inventor Shuichi Takano 1642-26 Kumakawa, Fussa-shi, Tokyo 26 Applied Optical Laboratory Co., Ltd. (72) Inventor Hidemi Nishikawa 1642 Kumakawa, Fussa-shi, Tokyo 26-applied light laboratory

Claims (7)

[Claims] (1) The total concentration of alkaline earth metal impurities is 1
Fluorite having an E18 atom / cm ³ or less. 2. A fluorite having a strontium concentration of 1E18 atom / cm ³ or less among alkaline earth metal impurities. 3. The fluorite according to claim 1, wherein the fluorite is manufactured by a Bridgman method. 4. The total concentration of alkaline earth metals is 1E18.
An optical member for ultraviolet optics, wherein fluorite having an atom / cm ³ or less is used. 5. An optical member for ultraviolet optics, wherein fluorite having a strontium concentration of 1E18 atom / cm ³ or less among alkaline earth metal impurities is used. 6. The optical member for ultraviolet optics according to claim 4, wherein said ultraviolet light is an ArF excimer laser. 7. A method for producing fluorite by the Bridgman method of heating and melting a raw material to grow a crystal by producing a single crystal ingot, comprising at least the following steps: a first step of producing fluorite. : A step of heating and melting the polycrystal to grow a crystal to obtain a single crystal ingot; a second step: a step of performing the first step a plurality of times to cut out a lower portion of the obtained single crystal ingot in a growth direction; Third step: a step in which the lower portions of the plurality of single crystal ingots cut out in the second step are heated and melted together to grow crystals to obtain a single crystal ingot.