JP2003221297A - Method for producing calcium fluoride crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a calcium fluoride crystal in which the transmittance through the crystal at a wavelength for use can be improved. <P>SOLUTION: In production processes for purifying, growing and annealing the calcium fluoride crystal, the scavengers to be used as antioxidants are added such that a solid-powdered scavenger and a gas scavenger are added both at the same time or each at different times, wherein the solid-powdered scavenger comprises one of carbon, Teflon (R), and zinc fluoride, or mixtures thereof; and the gas scavenger comprises hydrogen fluoride, carbon fluoride and fluorine. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a calcium fluoride crystal used in an exposure apparatus for photolithography.

[0002]

2. Description of the Related Art Excimer lasers are attracting attention as the only high-power lasers that oscillate in the ultraviolet region, and are expected to find applications in the electronic, chemical, and energy industries. A device that generates excimer laser light is known as an excimer laser oscillator. A laser gas such as Ar, Kr, Xe, KrF, ArF, or F2 filled in the manifold is excited by electron beam irradiation or discharge. Then, the excited atoms combine with the atoms in the ground state to generate molecules that exist only in the excited state. This molecule is called an excimer.

Since excimers are unstable, they immediately emit ultraviolet light and fall to the ground state. This is called a bond-free transition. An excimer laser oscillator is one that multiplies the ultraviolet light obtained by this transition in an optical resonator composed of a pair of mirrors and extracts it as laser light.

Among excimer laser beams, ArF laser and F2 laser have wavelengths of 193 nm and 15 respectively.
It is light in a wavelength range called a vacuum ultraviolet region such as 7 nm, and an optical system having a high transmittance of light in such a wavelength range must be used.

CaF 2 crystal is an optimum optical material for such purpose.

On the other hand, as an optical lens material used in an exposure apparatus for photolithography, it is necessary that the refractive index has good homogeneity, the birefringence is small, and the polishing accuracy can be improved.

Japanese Unexamined Patent Publication (Kokai) No. 9-227293 proposes a method for producing a crystal using a solid fluoride such as zinc fluoride, bismuth fluoride, lithium fluoride or sodium fluoride having a melting point of 400 ° C. lower than that of the crystal as a scavenger. is there.

[0008]

However, although the conventional CaF2 crystal exhibits satisfactory performance as an article for an ordinary visible light optical system, CaF2 crystal as a lens material used in an exposure device in the vacuum ultraviolet wavelength region. Crystals are required to have a high internal transmittance, and further, when irradiated with light of short wavelength and high output like an excimer laser for a long period of time, their optical characteristics may deteriorate, and the refraction that influences the imaging performance. There are problems such as homogeneity of the index, high birefringence, and difficulty in obtaining surface accuracy during polishing.

In particular, in the case of an optical component used in a photolithography exposure machine using an F2 excimer laser having a short wavelength among excimer lasers, the optical performance required for a CaF2 crystal as a lens material is extremely severe. .

The optical performances required of the CaF 2 crystal as the lens material are the internal transmittance, laser durability, refractive index homogeneity and birefringence as described above.

Further, it should be a crystalline material that can obtain surface accuracy in the polishing process for forming a lens.

The present inventors, while investigating the cause, found that the decrease in the internal transmittance of the crystal was greatly influenced by the oxidation of the crystal and the residual of the scavenger used for preventing the oxidation. I noticed that it is very important to make the scavenger work so as not to reduce the transmittance.

The present invention has been made in view of the above technical problems, and an object of the present invention is to provide a method for producing a calcium fluoride crystal capable of improving the internal transmittance of the crystal at the wavelength used.

[0014]

In order to achieve the above object, the present invention provides both a solid powder and a gas when a scavenger used as an antioxidant is added in the steps of refining and growing calcium fluoride crystals and in the annealing manufacturing process. Are added simultaneously or at different addition times.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings.

The present inventors manufactured a large number of CaF2 crystals by changing the scavenger addition conditions at the time of manufacturing the CaF2 crystals, and measured the optical characteristics (spectral transmittance, γ-ray durability) of the crystals.

As for the spectral transmittance, a spectroscope capable of measuring a vacuum ultraviolet wavelength region of 120 nm to 300 nm and an ultraviolet-visible spectroscope for evaluating generation of a color center after γ-ray irradiation were used.

Gamma ray durability is simpler than laser durability, and it is possible to know whether or not a large color center is generated in the entire crystal.

Further, it is possible to judge the tendency of residual scavenger constituent elements in the crystal and a slight amount of oxidation. As the irradiation condition, γ rays having an irradiation intensity of 1 × 10 6 R were irradiated for 1 hour to give a total amount of 1 × 10 6 R.

From these evaluations, it was found that crystals having good internal transmittance have less impurities, especially scavenger constituent elements, and less oxidative deterioration of the crystals.

Needless to say, it is important as a raw material that rare earths and other elements that are particularly poor in transmittance and laser durability are contained in the raw material in an extremely small amount, and as a result of the raw material selection, the above evaluation results are obtained. It was

A preferred manufacturing process of the present invention will be described below with reference to the drawings.

First, a high-purity CaF 2 synthetic raw material is prepared as a raw material.

Then, the calcium fluoride raw material and the scavenger are mixed. At this time, calcium fluoride and the scavenger may be put in a container and the container may be rotated and mixed. As a scavenger, zinc fluoride,
It is preferable to use bismuth fluoride, sodium fluoride, lithium fluoride or the like, which is more likely to bond with oxygen than the grown fluoride.

A substance is selected which becomes an oxide that is easily vaporized by reacting with the oxide mixed in the synthetic fluoride raw material. Zinc fluoride is especially preferred.

For example, a zinc fluoride scavenger converts calcium oxide generated by the presence of water into calcium fluoride.

CaF 2 + H 2 O → CaO + 2HF CaO + ZnF 2 → CaF 2 + ZnO ↑ In Japanese Patent Laid-Open No. 9-315818, a large amount of a scavenger is added, and the required performance of a lens used in a photolithography exposure apparatus using an F 2 excimer laser is satisfied. I can't.

A method 1000 for manufacturing a CaF 2 crystal and an optical element according to the present invention using the crucible lowering method will be described below with reference to the accompanying drawings. However, the present invention is not limited to the crucible descent method.

FIG. 1 shows a flowchart of a method of manufacturing an optical element using CaF 2 of the present invention.

First, a synthetic raw material of high-purity CaF2 is prepared as a raw material, and the CaF2 raw material and the scavenger are mixed (step 1100). The raw material for synthesizing high-purity CaF2 is produced by treating calcium carbonate with hydrofluoric acid. Although the present invention does not exclude a method of treating CaF2 rough with hydrofluoric acid to remove impurities (eg, SiO2), high-purity CaF2 is a powder unlike rough and has a bulk density (about 10 to about 10). 20 μ), which is very small and preferable. When mixing a CaF2 raw material and a scavenger into a container (or crucible), the Ca
It is preferable that the F2 raw material and the scavenger are put in and rotated to ensure uniform mixing.

As the scavenger, zinc fluoride, cadmium fluoride, manganese fluoride, bismuth fluoride, sodium fluoride, lithium fluoride, etc. are more easily bonded to oxygen than the grown fluoride and decomposed and evaporated. Easy one is desirable. A substance is selected which becomes an oxide which is easily vaporized by reacting with the oxide mixed in the fluoride raw material. Zinc fluoride is especially preferred.

Here, it is important to control the addition amount of the scavenger. For example, Japanese Patent Laid-Open No. 9-315815
Since the amount of scavenger added disclosed in the publication is too large, the lens and the diffraction grating manufactured from the CaF2 crystal do not satisfy the required performance of the exposure apparatus for photolithography using the F2 excimer laser.

The amount of scavenger added in the present embodiment is 0.001% by weight or more and 0.05% by weight or less, and more preferably 0.02% by weight. If the added amount is large, the scavenger residue causes a decrease in the homogeneity of the refractive index and a decrease in the internal transmittance and the laser durability.
In other words, since the amount of such scavenger remaining is small, the amount of impurities and crystal defects (particularly, dislocation density) contained in the CaF2 crystal are reduced to provide a high-quality CaF2 crystal.

The addition amount of the scavenger can be lowered to such a value by the combination of the solid scavenger and the gas scavenger described below and the vacuum baking process (steps 1200 and 1400).

One of the features of the present invention is that a vacuum baking process (steps 1200 and 1400) is provided before the crystal growth process (step 1500) described later. Note that the same effect as the vacuum baking process can be obtained by the structure of the heater 206 of the crystal growth furnace 200 described later (for example,
Temperature control and temperature control (for example, to slow down the crystal growth rate, to gently optimize the descending temperature gradient near the solid-liquid interface when pulling down the crucible that lowers the temperature so that segregation of impurities concentrates at the top), It can also be obtained by controlling process conditions, etc. (longer evacuation time).

The mixture of CaF 2 powder and scavenger thus obtained is subjected to vacuum baking treatment (step 1200). The vacuum baking process is performed in order to remove the water content of the mixture by heating the mixture, and the vacuum baking process of step 1200 is performed as a process different from the purification process before the purification process (step 1300) described later. . The vacuum baking process is also performed in step 1400, but the process of step 1200 absorbs a larger amount of water than the process of step 1400, so that the effect of removing water is great.

In the vacuum baking process, first, the mixture 320 is put in a carbon crucible 304 and loaded into a mesh table 310 in the vacuum chamber 302 of the vacuum baking furnace 300 shown in FIG. Here, FIG. 5 shows a vacuum baking furnace 30.
It is a schematic sectional drawing of 0.

Therefore, in this embodiment, the mixture 320 of the CaF 2 raw material and the scavenger is heated together with the crucible 304. Instead of using the vacuum baking furnace 300, the mixture 320 may use the refining furnace 100 used in the refining process (step 1300) described below. That is, in the latter case, the refining furnace 100 of FIG. 2 serves both as the vacuum baking process and the refining process. However, in general, the refining furnace 100 is more expensive than the general-purpose vacuum bake furnace 300, and in order to secure the time for the refining process by the refining furnace 100, it is better to configure it as separate furnaces as in the present embodiment. preferable.

The vacuum bake furnace 300 includes a vacuum chamber 3
The internal space of 02 is defined by the mesh base 310 and the heat insulating material 308, is thermally insulated by the heat insulating material 308, and is maintained in a vacuum or reduced pressure environment via the dry pump 330.
The mesh table 320 is heated by the heater 306. In the internal space of the vacuum chamber 302, nitrogen, helium, argon, neon,
A gas flow environment may be formed in the internal environment by providing a nozzle that supplies an inert gas that promotes the removal of water such as krypton or xenon. This is because the number of gas molecules is increased in a depressurized environment to generate water molecules and a scavenger product (for example, in the case of the above Chemical Formula 2, Zn
This is to accelerate the removal of O, O2, Zn) and other adsorbed contaminants.

In the vacuum bake furnace 300, the heater 306 is set to about 30 after the inert gas is introduced from the nozzle.
Heat to 0 ° C to about 900 ° C to remove water, then
The dry pump 330 evacuates the inside of the chamber 302 to a vacuum degree of about 10 Pa, and the heater 306 draws about 1
Water and inert gas are removed by heating at 50 ° C to 200 ° C.

After the vacuum baking process is performed, the mixture is subjected to a purification process (step 1300). Purification is performed by removing impurities (eg, carbonic acid) and CaF2.
Is a step of purifying the water, and includes the actions of dehydration, scavenging reaction, removal of scavenger products, removal of scavenger residues, melting and solidification. In the refining process, the mixture 120 is the crucible 10 of the refining furnace 100 shown in FIG.
It can be put in 4.

In FIG. 2, reference numeral 102 denotes a chamber of the refining furnace 100, which is connected to a vacuum exhaust system including a rotary pump 150, a mechanical booster pump 160, a turbo molecular pump 170, and an abatement system 180. 1
The exhaust pump of 50 to 170 is not limited to these types. As the detoxification system (trap), both a cooling trap for cooling exhaust gas to deposit and remove harmful components in the exhaust gas and a high temperature trap for thermally decomposing exhaust gas can be applied.

The chamber 102 is thermally insulated by a heat insulating material 108 and houses a crucible 104 and a heater 106. Reference numeral 101 is a gas scavenger supply port connected to a gas supply system (not shown). The crucible 104 is made of, for example, carbon, has a substantially cylindrical shape, and is rotatably supported by the crucible support mechanism 140. If necessary, crucible support mechanism 14
0 is configured so that the crucible 104 can be lowered. The rotation by the crucible support mechanism 140 is performed in order to make the temperature of the crucible 104 uniform. In this embodiment, the crucible 304 and the crucible 104 in the vacuum baking furnace of FIG. 5 are the same, but they may be separate members (that is, the mixture in the crucible 304 may be transferred to the crucible 104).

The chamber 102 is a cooling pipe 1 for temperature control.
It is also connected to 30. Heater 106 and cooling pipe 1
The temperature in the chamber 102 can be controlled by 30. The thermocouple 110 is made of platinum, for example, and measures the temperature of the crucible 104 from the vicinity of the outer wall of the crucible 104.

After that, the heater 106 is energized and the crucible 1
The mixture 120 in 04 is heated to dehydrate and remove other adsorbed substances, and promote the impurity removal reaction by the scavenger shown in Chemical Formulas 1 and 2. As described above, dehydration and the like are performed during the refining process of step 1300, but the vacuum baking process in steps 1200 and 1400 is a separate process.

In order to accelerate the reaction sufficiently, the temperature for heating the raw material must be slowed in this temperature range. At the temperature at which the reaction is completed, the refining furnace 100 is evacuated by a vacuum evacuation system to a vacuum degree of 1 × 10 −3 Pa or more.

Next, the CaF 2 raw material is completely melted through 350 ° C. to 1100 ° C., which is the temperature range of the scavenging reaction.

After the melting, the molten state is maintained for several hours to several tens of hours in order to remove the residual harmful elements. During this period, the solid scavenger added to the calcium fluoride is harmful to the optical properties if it remains in the crystal, so the amount was made very small. Therefore, for the scavenger that is insufficient during melting, a gas scavenger is added to the furnace at appropriate times to prevent crystal oxidation. Alternatively, the refining furnace 100 is evacuated to vacuum after the CaF2 feedstock is completely melted. Since it is not necessary to arrange the lattice arrangement of the CaF2 crystal, the power of the heater 106 is turned off when the CaF2 raw material is melted.

Then, the crucible 104 is lowered and the raw material of CaF 2 melted is gradually cooled to grow crystals. CaF2
Since it is not necessary to arrange the crystal lattice arrangement, it is not necessary for the crucible to descend slowly. Although the present invention includes the case where the crucible 104 is not lowered, the effect of removing impurities is improved by lowering the crucible 104.

Since this step does not require temperature control as much as the single crystal growth step (step 1500) described later, the obtained crystal may be a polycrystal or one having grain boundaries.
The upper portion of the crystals thus obtained, that is, the portion that has finally crystallized over time is removed. Since this portion is a portion where impurities are easily collected (that is, segregation), by removing this portion, impurities that adversely affect the characteristics are removed. Again, this crystal is put into the crucible 104, and a series of steps of melting, crystallization, and upper part removal are repeated a plurality of times. still,
If necessary, an inert gas may be introduced into the chamber 102. A suitable temperature range for the purification process is about 1390
~ About 1450 ° C.

The single crystal growth step is a step of growing a single crystal of CaF 2 to improve the crystal quality (that is, to arrange the lattice arrangement). A suitable growth method is selected according to the size of the crystal and the purpose of use. The purified crystal 220 is put into the crucible 204 housed in the chamber 202 of the crystal growth furnace 200 shown in FIG.

In FIG. 3, 202 is a crystal growth furnace 200.
Of the rotary pump 250, mechanical booster pump 260, turbo molecular pump 270,
It is connected to an evacuation system consisting of the abatement system 280. The chamber 202 is insulated by a heat insulating material 208, and houses a crucible 204 and a heater 206. The crucible 204 is made of, for example, carbon, has a substantially cylindrical shape, has a high hermeticity, and is supported by a crucible supporting / pulling down mechanism 240 so as to be rotatable and vertically movable. The rotation by the crucible supporting / pulling-down mechanism 240 is performed in order to make the temperature of the crucible 204 uniform.

In the present embodiment, the crucible 310 and the crucible 204 in the vacuum baking furnace shown in FIG. 5 are the same, but they are separate members (that is, the mixture in the crucible 304 is transferred to the crucible 204). Is also good. The chamber 202 is also connected to a cooling pipe 230 for temperature control. Heater 2
A desired temperature gradient can be formed in the vertical direction shown in FIG. 3 by the 06 and the cooling pipe 230. Thermocouple 210
Measures the temperature of the crucible 204 from the vicinity of the outer wall of the crucible 204.

Thereafter, the heater 206 is energized and the crucible 2 is heated.
The secondary raw material (crystal) 220 of CaF 2 in 04 is about 14
The CaF2 crystal is completely melted by heating to about 20 ° C. Thereafter, the crucible 204 was gradually lowered at a speed of 2 mm / h (passing a predetermined temperature gradient) to melt the Ca.
The F2 crystal is gradually cooled to grow a single crystal.

Then, the grown fluoride single crystal is shown in FIG.
Heat treatment is performed in the annealing furnace 400 shown in (annealing step)
(Step 1600). The annealing process is performed by growing Ca
In this step, the F2 single crystal is heat-treated to remove the strain that causes crystal cracking. The grown single crystal 420 is put into the crucible 404 housed in the chamber 402 of the annealing furnace 400 shown in FIG.

In FIG. 4, 402 is an annealing furnace 400.
Of the rotary pump 450, mechanical booster pump 460, turbo molecular pump 470,
It is connected to an evacuation system consisting of the abatement system 480. The chamber 402 is insulated by a heat insulating material 408, and houses a crucible 404 and a heater 406. The crucible 404 is, for example, made of carbon, has a substantially cylindrical shape, and is configured in a multi-stage manner, and is supported by a crucible support member 440.

The chamber 402 is a cooling pipe 4 for temperature control.
It is also connected to 30. Heater 406 and cooling pipe 4
The temperature of the chamber 402 can be controlled by 30. The thermocouple 410 is made of platinum, for example, and measures the temperature of the crucible 404 from the vicinity of the outer wall of the crucible 404.

In the annealing step, the crucible 404 was set to about 900
℃ ~ about 1000 ℃ uniformly heated to a solid Ca
The strain of the F2 crystal is removed. A heating temperature of about 1140 ° C. or higher is not preferable because it causes structural changes and the like. The heating time is about 20 hours or more, more preferably about 20 to about 30 hours. In the annealing process, crystal dislocations are reduced by passing through the annealing. afterwards,
The temperature of the CaF2 crystal is returned to room temperature while maintaining the strain-free state.

After that, the optical article is molded into a required shape (convex lens, concave lens, disk shape, plate shape, etc.) (molding step).

Since the dislocation density in the CaF 2 crystal is small at the time of polishing, the partial reduction in surface accuracy is very small, and high-precision processing is possible below the allowable value.

By combining various lenses obtained in this way, an illumination optical system suitable for an excimer laser, particularly an ArF excimer laser, can be constructed (an optical system assembly step). An exposure apparatus for photolithography can be configured by combining an excimer laser light source, an optical system having a lens made of calcium fluoride, and a stage that can move the substrate.

[Example] A necessary amount of a commercially available high-purity calcium fluoride powder raw material and 0.02% by weight of ZnF 2 which is a scavenger with respect to calcium fluoride were added and both were mixed and uniformly dispersed. Filled.

Then, in a vacuum drying oven, 150 ° C., 2
Degas for 4 hours, transfer to a refining furnace, and heat and melt at 1420 ° C. The melting time is set to an appropriate time, for example, 30 hours, in order to remove harmful elements among the solid scavenger constituent elements, but during this period, the added solid scavenger was in an extremely small amount, so that it was insufficient in the antioxidant function. Scavenger supplies gas scavengers in a timely manner.

After removing the harmful unnecessary elements, the material was slowly cooled to solidify the raw material. The harmful unnecessary portion of the surface layer of the calcium fluoride block is removed to obtain a secondary raw material before crystal growth.

Next, the above block was placed in a crucible for single crystal growth which had been previously vacuum dried. As a scavenger, ZnF2 is put in a 0.005 wt% crucible.

The furnace was evacuated to heat the crucible, the degree of vacuum was 6 × 10 −4 Torr, and the temperature was raised to 1420 ° C.
In order to dissolve and degas calcium fluoride in the crucible, the degree of vacuum is set to 2 × 10 −6 Torr and the temperature is set to 1420 ° C.
I kept it for 0 hours.

Next, the crucible was lowered at a speed of 2 mm / h to grow a good quality CaF 2 single crystal. It is needless to say that the pulling down rate at this time should be taken into consideration depending on the size and shape of the crystal to be produced, since it is desirable to correspond to the crystal growth rate. Generally, the larger the size of the crystal, the slower the pulling rate needs to be.

Next, the calcium fluoride single crystal grown in the crucible of the annealing furnace and 0.002% by weight of ZnF 2
I put it in. After evacuating the furnace and raising the temperature of the crucible from room temperature to 900 ° C at a rate of 100 ° C / h, 50 hours 90
Hold at 0 ° C. Then, it is lowered at a rate of 5 ° C./h,
Cooled to room temperature. At this time, the cooling rate needs to be slowed down as the size of the crystal becomes larger. That is, unless it is delayed, it becomes difficult to make the birefringence extremely small.

Throughout the whole process, the solid scavenger used is LiF, NaF, CdF2, KF, ZnF2, etc.
Similar effects can be obtained with other fluorides.

As the gas scavenger, fluorine gas, fluorocarbon gas and hydrogen fluoride gas are effective.

[0071]

According to the present invention, the crucible used for degassing in the refining furnace and the growth furnace and the degassing of the raw material in vacuum are pretreated in a separate step (including the load lock prechamber). The amount of solid scavenger mixed in the raw material is minimized, and the use of a gas scavenger prevents the scavenger constituent elements from remaining in the crystal and oxidation of the crystal. 10
The transmittance (excluding the surface reflection loss of mm thickness) can be improved. This makes it possible to improve the transmittance of the objective lens system of the exposure machine and obtain a good light amount of the optical system.

[Brief description of drawings]

FIG. 1 is a flowchart showing a method for producing a CaF 2 crystal according to the present invention.

FIG. 2 is a schematic cross-sectional view of a refining device used in the refining step of the manufacturing method shown in FIG.

FIG. 3 is a schematic cross-sectional view of a crystal growth furnace used in the single crystal growth step of the manufacturing method shown in FIG.

4 is a schematic cross-sectional view of an annealing furnace used in the annealing step of the manufacturing method shown in FIG.

5 is a schematic cross-sectional view of a vacuum baking furnace used in the vacuum baking step of the manufacturing method shown in FIG.

FIG. 6 is a diagram showing an example of a spectral transmittance of a crystal having a low short wavelength transmittance due to a shortage of a scavenger.

FIG. 7 is a diagram showing an example of a spectral transmittance of a crystal depending on a scavenger addition amount.

FIG. 8 is a diagram showing an example of spectral transmittance of the crystal of the present invention.

[Explanation of symbols]

100 refining furnace 200 Crystal growth furnace 300 vacuum bake oven 400 annealing furnace

Claims (2)

[Claims] 1. A scavenger used as an antioxidant in the steps of refining and growing a calcium fluoride crystal and in an annealing production step, at the time of addition, both a solid powder and a gas are added simultaneously or at different addition times. A method for producing a calcium fluoride crystal, which is characterized. 2. The scavenger is characterized in that the solid powder is one of carbon, Teflon, zinc fluoride, or a mixture thereof, and the gas is hydrogen fluoride, carbon fluoride, or fluorine. A method for producing the calcium fluoride crystal described in the above.