JPH09315815A - Fluorite and optical article using the same and exposing device for photolithography - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a fluorite hardly deteriorated in transmissivity characteristic even at the time of repeating the irradiation with a short wave high output light for a long period by purifying it to have the content of lanthanum and yttrium to that equal to or below a specific value. SOLUTION: This fluorite has >=70% transmissivity to the light of 135nm wave length, contains lanthanum (La) and yttrium (Y), the lanthanum content is <=1ppm and the yttrium content is <=10ppm. The fluorite further contains preferably at least one selected from strontium, aluminum, silicon, magnesium. The adequate content is 1-600ppm of Sr, 1-50ppm of Al, 1-50ppm of Si, 1-10ppm of Mg. In the fluorite, etch pit density may be controlled to <=10<4> .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to fluorite, an optical component using the same, and 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 be applied in the electronics, chemical, and energy industries.

[0003] Specifically, it is used for processing and chemical reaction of metals, resins, glass, ceramics, semiconductors and the like.

An apparatus for generating an excimer laser beam is known as an excimer laser oscillation apparatus. A laser gas such as Ar, Kr, Xe, KrF, or ArF filled in the manifold is excited by electron beam irradiation or discharge. Then, the excited atoms combine with atoms in the ground state to generate molecules that exist only in the excited state. This molecule is called excimer. Since the excimer is unstable, it immediately emits ultraviolet light and falls to the ground state. This is called a bond-free transition, and an excimer laser oscillation device is a device 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, KrF laser and ArF laser have wavelengths of 248 nm and 19 respectively.
It is light in a wavelength range called a vacuum ultraviolet region such as 3 nm, and an optical system having a high transmittance of light in such a wavelength range must be used. Fluorite (calcium fluoride single crystal) is a preferable glass material for such an optical system.

[0006]

However, although conventional fluorite shows satisfactory performance as an ordinary visible light optical system article, it repeats high-power light with a short wavelength for a long time like an excimer laser. Irradiation sometimes deteriorated its optical characteristics.

The present inventors, while searching for the cause, realized that it was influenced by the crystal structure and the impurities contained therein.

The present invention has been made in view of the above technical problems, and provides a fluorite whose transmittance characteristics are not easily deteriorated even when high-power light having a short wavelength is repeatedly irradiated for a long period of time. The main purpose is to do.

Another object of the present invention is to provide a fluorite suitable for an optical component for an excimer laser, particularly an optical component for an excimer laser of an exposure apparatus for photolithography.

Still another object of the present invention is to provide fluorite which can be a highly reliable optical article.

Still another object of the present invention is to provide a fluorspar which can be manufactured relatively inexpensively.

Still another object of the present invention is to provide an optical component for an excimer laser which does not deteriorate its optical characteristics even if it is repeatedly irradiated with light of short wavelength and high output for a long period of time.

Still another object of the present invention is to provide an exposure apparatus for photolithography, which can stably expose a fine pattern of 0.25 micron or less for a long period of time.

[0014]

The fluorspar of the present invention is 135
having a transmittance of 70% or more for light having a wavelength of nm, including lanthanum (La) and yttrium (Y), a lanthanum content of 1 ppm or less, and a yttrium content of 10 pp.
m or less.

The excimer laser optical component of the present invention has a transmittance of 70% or more for light having a wavelength of 135 nm, contains lanthanum (La) and yttrium (Y), and has a lanthanum content of 1 ppm or less and yttrium. It is characterized by being composed of fluorite whose amount is 10 ppm or less.

The exposure apparatus for photolithography of the present invention has a transmittance of 70% or more for light having a wavelength of 135 nm, contains lanthanum (La) and yttrium (Y), and has a lanthanum content of 1 ppm or less and yttrium content. An optical system having a lens made of fluorite having an amount of 10 ppm or less and a stage for holding a substrate to be exposed are provided.

The content ppm in the present invention is the weight (μg) of the target atom per 1 g of fluorite.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors produced a large number of fluorspars by changing the production conditions during the production of fluorspar. Then, considering the application, KrF and ArF excimer lasers were repeatedly irradiated for a long period of time, and their characteristics were measured. Then, an experiment was carried out in which an excimer laser was irradiated to an optical member formed into a disk shape having a thickness of 10 mm using only fluorite having good characteristics.

That is, the output is 30 mJ / cm.^TwoLaser
1 x 10^FourPulse irradiation and 1 × 10 ^FourR / H gamma rays
As a result of experiment of irradiating for 1 hour, some of the samples
Is colored. Both colored and uncolored samples are the first
The same absorption rate (transmittance) at 248 nm and 193 nm
Met. Therefore, the transmittance at the wavelength of the light used is
Even if a good product is sampled according to the standard, a test that is likely to deteriorate in the future
No distinction can be made between materials and non-degradable samples.

Therefore, as a result of analyzing the characteristics of the sample which was not deteriorated by the above experiment, the inventors of the present invention have determined that the product is a non-defective product based on the transmittance in the vicinity of 135 nm which is a wavelength much shorter than the wavelength of the excimer laser. I noticed that I could distinguish That is, when a fluorite optical member having a transmittance of 70% or more at a wavelength of 135 nm at the time of transmittance measurement is used for an excimer laser optical system before or after laser or gamma ray irradiation, the object to be processed is The irradiated laser light becomes stable.

FIG. 1 is a diagram showing an example of transmittance characteristics of fluorspar according to the present invention.

Further, in order to improve the crystallinity, the content of the specific element (atom) needs to be suppressed to a predetermined amount or less.

As such atoms, there are La and Y,
La is preferably 1 ppm or less and Y is preferably 10 ppm or less.

Thus, the content of each of La and Y is set to 1
According to the present invention of 0 ppm or less, the durability of the excimer laser is improved, the coloring of fluorite is prevented, and
The etch pit density (EPD) can be 10 ⁴ or less.

Further, it is possible to provide fluorite which has improved heat resistance and is resistant to distortion.

In order to obtain the fluorite having the above-mentioned characteristics, it is considered that one of the manufacturing conditions required at present is to repeat the refining process many times before growing the single crystal. However, repeating the refining process a number of times prolongs the manufacturing time required to obtain the final fluorspar, which is an obstacle to reducing the manufacturing cost of the fluorspar.

On the other hand, in some cases, it is better that a certain amount of impurities is present in the fluorite in a trace amount rather than not being present at all. For fluorite, strontium, aluminum,
Silicon and magnesium are one of them. If the content of these is too large, these oxides are formed in the fluorite, oxygen is easily taken in, and it becomes difficult to improve the characteristics of the fluorite.

If the content of these elements is too small, the optical characteristics do not change so much, and there is a tendency that other transition metals, which tend to deteriorate the characteristics, are contained in an amount more than the permissible amount. Strontium is 1-60 as a suitable content.
0 ppm, aluminum is 1 to 50 ppm, silicon is 1 to 50 ppm, and magnesium is 1 to 10 ppm.

Fluorite containing Sr, Al, Si, and Mg in the above range can have an oxygen content of 25 ppm or less, and a transition metal content other than Y and La can be reduced. You can

Thus, according to the present invention, it is possible to reduce the content of impurities such as oxygen, carbon and water, which tend to have a bad influence on the optical characteristics, without increasing the number of repetitions of the refining step excessively, and at 135 nm. 75% transmittance
It is possible to provide fluorite that is as expensive as the above.

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

FIG. 2 is a flow chart showing an example of manufacturing steps until the exposure apparatus is assembled.

First, raw fluorspar or 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 to mix. 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 which is easily vaporized by reacting with an oxide mixed in the synthetic fluoride raw material. Zinc fluoride is particularly desirable.

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

CaF + H ₂ O → CaO + 2HF CaO + ZnF ₂ → CaF ₂ + ZnO ↑ The scavenger addition rate is 0.05 mol% or more 5.0
0 mol% or less, more preferably 0.1 to 1.0
mol%. The generated ZnO evaporates under high temperature conditions in each process.

The mixture of the calcium fluoride powder thus obtained and the scavenger is placed in the crucible of the refining furnace shown in FIG. Thereafter, the heater is energized to melt the mixture. At this time, if necessary, AlF ₃ , SrF ₂ , Mg
F ₂ and Si may be added. Then, the crucible is lowered and the molten calcium fluoride is gradually cooled to grow crystals (purification step).

This step does not require temperature control as much as the single crystal growth step described later. Therefore, a grain boundary of the obtained crystal may exist.

The upper part of the crystals thus obtained, that is, the part which was finally crystallized over time, is removed. Since this portion is a portion where impurities are likely to be collected, this removing step removes impurities that adversely affect the characteristics.

The crystal is placed in the crucible again, and a series of steps of melting, crystallization and upper removal are repeated a plurality of times.

Then, after heating the crucible to about 1390 to 1450 ° C. to melt the assembly, the crucible is lowered at a rate of 0.1 to 5.0 mm per hour to partially radiate heat. Cooling. In this way, a single crystal is obtained.

Subsequently, the grown fluoride single crystal is heat-treated in the annealing furnace shown in FIG. 5 (annealing step). In this annealing step, the crucible 4 is heated to 900 to 1000 ° C. The heating time is 20 hours or more, more preferably 2 hours.
0-30 hours.

According to the above example, the change in the bulk density before and after the growth can be made extremely small as compared with the prior art, and the bulk density in the growth furnace can be increased.

Fluoride single crystal (fluorite) thus obtained
And the internal transmittance at a wavelength of 135 nm of 70% or more is extracted. An optical article is produced using the fluorite thus selected.

After that, the optical article is molded into a required shape (convex lens, concave lens, disk shape, plate shape, etc.) (molding step). Further, if necessary, an anti-reflection film may be provided on the surface of the fluoride crystal optical article. As the antireflection film, magnesium fluoride, silicon oxide, or tantalum oxide is preferable, and these can be formed by vapor deposition by resistance heating, electron beam vapor deposition, sputtering, etc. The optical article obtained by the present invention contains almost no water. Therefore, the adhesion with the antireflection film is improved, and the heat resistance is improved, so that the film formation by sputtering is easy.

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). Then, 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 (apparatus assembling step).

By using this exposure apparatus and irradiating the photosensitized resist on the substrate with the excimer laser light through the pattern of the reticle, a latent image corresponding to the pattern to be formed can be formed.

As the analysis method of the lanthanum and yttrium contents, there are used a fluorescent X-ray analysis method, an ICP emission analysis method, an ICP mass spectrometry method and the like.

The internal transmittance at 135 nm can be measured with a vacuum ultraviolet spectrophotometer.

[0051]

【Example】

(Example 1) A raw fluorspar ore having a large yttrium content of 12 ppm and a lanthanum content of 5 ppm and a large impurity content was ground to obtain powdered calcium fluoride. This and Zn
0.7% by weight of F ₂ was added to calcium fluoride to mix them.

Then, this mixture was put in a crucible of a refining furnace and heated to 1360 ° C., and then the crucible was lowered and gradually cooled to crystallize the raw material. The upper part of the crystallized calcium fluoride corresponding to the upper part of the crucible was removed by a thickness of several mm. The steps of heating, gradual cooling and removal were repeated, and a large number of calcium fluoride crystal block samples with different numbers of repeating steps were prepared.

Next, the above block was put into a crucible of a single crystal growth furnace together with silicon. As a scavenger, ZnF ₂ was put in a 0.1 wt% crucible. The furnace was evacuated to heat the crucible. Vacuum degree 6 × 10 ^-4
Torr and temperature were 1360 ° C.

The degree of vacuum is 2 × 10 ^-6 Torr and the temperature is 13
The temperature was kept at 60 ° C for 11 hours.

Next, the crucible was lowered at a speed of 2 mm / h. The rate of temperature decrease at this time corresponds to about 100 ° C./h.

Next, the calcium fluoride single crystal grown in the crucible of the annealing furnace and 0.1% by weight of ZnF ₂ were added. The temperature of the crucible evacuating the furnace is from room temperature to 900 ° C.
At a rate of 100 ° C / h for 20 hours at 900 ° C
Held. Then, the temperature was lowered at a rate of 6 ° C./h and cooled to room temperature.

The internal transmittance and the Y and La contents (ppm) of the fluorspar sample thus obtained are as follows.

[0058]

[Table 1] As a result, those having La of 1 ppm or less and Y of 10 ppm or less become fluorite with an internal transmittance of 70% or more, are not colored, and have a deterioration rate of 0%, and can be preferably applied as an optical system for KrF, ArF excimer laser. Is understood.

The deterioration rates are 193 nm and 248 nm.
It is the reduction ratio of the transmittance at and.

Example 2 In the refining process, Al
F _3, SrF _2, and the MgF _2, Si was added varying amounts with ZnF _2.

Other than that was the same as in Example 1.

The range of impurities contained in the thus-obtained fluorite analyzed was as follows.

[0063]

[Table 2] As shown in Table 2, in this embodiment, Sr, Al, Si,
Since the content of Mg is also within the predetermined range, the content of oxygen and carbon is 25 ppm without repeating the refining process.
The content of Ce, Fe, and Pb could be 1 ppm or less.

[0064]

According to the present invention, the following various effects can be achieved.

It is possible to provide fluorite in which the transmittance characteristics are not easily deteriorated even when light having a short wavelength and a high output is repeatedly irradiated for a long period of time.

It is possible to provide a fluorite suitable for an optical component for an excimer laser, particularly for an optical component for an excimer laser of an exposure apparatus for photolithography.

It is possible to provide fluorite that can be a highly reliable optical article.

It is possible to provide fluorspar that can be manufactured at a relatively low cost.

It is possible to provide an optical component for an excimer laser that does not deteriorate in optical characteristics even if it is repeatedly irradiated with light of short wavelength and high output for a long period of time.

It is possible to provide an exposure apparatus for photolithography that can stably expose a fine pattern of 0.25 micron or less for a long period of time.

[Brief description of drawings]

FIG. 1 is a graph showing an example of transmittance characteristics of fluorspar according to the present invention.

FIG. 2 is a flowchart for explaining an example of a manufacturing process up to exposure apparatus assembly.

FIG. 3 is a view showing a cross section of a refining device.

FIG. 4 is a view showing a cross section of a growth furnace used in a single crystal growth step.

FIG. 5 is a view showing a cross section of a growth furnace used in an annealing process.

[Explanation of symbols]

 1 chamber, 2 heat insulating material, 3 heater, 4 crucible, 5 fluorite, 6 crucible lowering mechanism.

Claims (5)

[Claims] 1. A light-transmitting material having a wavelength of 135 nm has a transmittance of 70% or more, contains lanthanum (La) and yttrium (Y), and has a lanthanum content of 1 ppm or less and a yttrium content of 10 ppm or less. Fluorite. 2. Further, strontium (Sr), aluminum (Al), silicon (Si), magnesium (M
The fluorspar according to claim 1, comprising at least one selected from g). 3. The fluorite according to claim 1, wherein the etch pit density is 10 ⁴ or less. 4. From fluorite having a transmittance of light having a wavelength of 135 nm of 70% or more, including lanthanum (La) and yttrium (Y), and having a lanthanum content of 1 ppm or less and a yttrium content of 10 ppm or less. An optical component for an excimer laser, which is characterized by: 5. From fluorite having a transmittance of light having a wavelength of 135 nm of 70% or more, including lanthanum (La) and yttrium (Y), having a lanthanum content of 5 ppm or less and a yttrium content of 10 ppm or less. An exposure apparatus for photolithography, comprising: an optical system having the following lens and a stage for holding a substrate to be exposed.