JPH1187808A - Manufacture of optical element for arf excimer laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To set a quantity of light loss due to absorption and scattering to 2% or less, by polishing the surface of an optical element to a specific roughness, by performing ultraviolet ray washing, and by forming an optical thin film for transmitting an ArF excimer laser beam to the optical element being subjected ultraviolet ray washing. SOLUTION: A crucible for upbringing, where fluorine calcium being synthesized and purified is used as a raw material and is filled with a fluorination agent, is placed in a vacuum electric oven, temperature in the vacuum electric oven is gradually increased, the raw material is allowed to react with the fluorination agent. Furthermore, the temperature is slowly increased up to 1,370-1,450 deg.C, and the raw material is dissolved and is gradually crystallized from the lower part of a crucible for upbringing for obtaining fluorite. The fluorite is used as an optical material and the surface is polished by an abrasive using diamond powder so that the surface roughness becomes 5 Å or less, and the polished surface of the optical material is subjected to ultraviolet ray washing. An optical thin film for transmitting ArF excimer laser beam is formed at the optical element being subjected to ultraviolet ray washing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing an optical element used in an optical system using a high-pulse laser as a light source.

[0002]

2. Description of the Related Art Conventionally, in an optical lithography technique for exposing and transferring a fine pattern of an integrated circuit on a wafer such as silicon, an exposure apparatus called a stepper is used. The light source of this stepper has been shortened in wavelength from g-line to i-line with the recent high integration of LSI. As LSIs become more highly integrated, stepper light sources include KrF excimer laser (λ = 248 nm) and Ar
It has shifted to an F excimer laser (λ = 193 nm). General optical glass can no longer be used for the optical system of such an excimer laser stepper, and is limited to materials such as quartz glass and fluorite having high light transmittance at short wavelengths.

[0003] In particular, in the case of a stepper, for example, φ20
Since the optical system is configured by using a large number of lenses having a large diameter such as 0 mm × t20 mm and having a large thickness, the optical path length is very long. Therefore, to increase the light transmittance of the entire optical system of the stepper,
It is required to increase the transmittance of each lens. The internal transmittance of silica glass and fluorite used in the optical system of an excimer laser stepper is 99.8% / c.
m preferably 99.9% / cm or more (internal absorption 0.2%
/ Cm, or 0.1% / cm or less).
Therefore, developments aimed at increasing the transmittance of the optical material in the ultraviolet light region are being pursued.

Optical elements such as prisms, lenses, and reflecting mirrors form optical thin films of various structures in order to maintain desired optical characteristics on the optical material. The anti-reflection film is formed in order to reduce loss of light amount, flare, ghost, and the like due to surface reflection of the optical material. On the other hand, the reflection film is formed to efficiently reflect the incident light on the surface of the optical material. In addition, the film material has a low absorption and high laser resistance which is less likely to cause a loss of light amount due to absorption, a change in a substrate surface due to absorption heat generation, and a film destruction with respect to a KrF excimer laser or an ArF excimer laser as a light source. desirable.
That is, the types of film materials that can be used at the wavelength of the excimer laser are mainly limited to fluorine compounds such as magnesium fluoride (MgF ₂ ) and some oxides. Conventionally, a combination of these film materials is used to design a film in which the light amount loss due to light absorption of the film itself at the light source wavelength is 0%, and the thin film is formed on an optical material.

[0005]

However, even if an antireflection film having a good internal transmittance and a loss of light amount of the film itself expected to be 0% is formed, the transmittance of an optical element manufactured by an actual manufacturing process is increased. However, there was a problem that the value was lower than the design value. Therefore, an object of the present invention is to solve the above problems and to provide a method for manufacturing an optical element for an ArF excimer laser in which the light loss due to absorption and scattering is 2.0% or less.

[0006]

In an optical element for an ArF excimer laser, it is generally considered that the light quantity loss is caused by internal absorption of the optical material itself, surface loss (absorption, reflection, scattering) after film formation. However, as described above, even if a thin film designed to have zero light loss is actually formed on a material having a high internal transmittance, a desired high transmittance is not obtained.

The inventors of the present invention have further examined the process of manufacturing an optical element. As a result, the surface of the optical element before forming a thin film is polished to a surface roughness of 5 ° or less.
It has been found that it is essential to achieve high transmittance of the optical element. If the surface roughness of the optical material is larger than 5 mm,
Not only does the transmittance decrease due to the loss of the material surface, but also the optical performance of the film formed thereon cannot be exhibited as designed.

[0008] Further, in the step after polishing to a surface roughness of 5 mm or less, washing is carried out to remove the abrasive. In particular, the organic solvent used in the drying step for removing the washing water is a raw material. The film is formed while remaining on the surface,
It has been found that this affects the transmittance. Therefore, in the present invention, high transmittance of the optical element is achieved by performing ultraviolet cleaning after the polishing step, particularly after the wet cleaning step after polishing to remove organic substances.

As described above, the present invention provides a method of manufacturing an optical element used in an optical system using an ArF excimer laser as a light source, wherein the step of polishing the surface of the optical material to a surface roughness of 5 ° or less; An optical system for an ArF excimer laser, comprising: a step of cleaning the polished optical material surface with ultraviolet light; and a step of forming an optical thin film that transmits ArF excimer laser light on the optical element cleaned with ultraviolet light. It is intended to provide a method for manufacturing an element.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION An optical material used in the present invention has no internal absorption.
It is preferably 8% / cm or more. The factor of the internal absorption of the optical material is an impurity or a structural defect contained other than the constituent composition of the optical material. Therefore, as a method for producing quartz glass with few impurities and structural defects, a Si compound gas as a raw material gas and a carrier gas for sending the Si compound gas, and a gas for combustion and heating (for example, H ₂ , O ₂₎
Gas or the like is ejected from a burner, and a quartz hydrolysis method of depositing quartz glass on a target in a flame is generally used.

As a method for producing fluorite, bridging is used.
A method called the Ziman method is used. This method
Using purified and purified calcium fluoride as raw material,
bF _TwoGrowth crucible filled with a fluorinating agent such as
Place in a vacuum electric furnace (growing device) and adjust the temperature inside the growing device.
After gradually raising the raw material and reacting with the fluorinating agent,
The temperature gradually rises to above the melting point (1370 ° C to 1450 ° C)
And melt the raw material. 0.1-5 m at the crystal growth stage
By lowering the growing crucible at a speed of about m / H
The fluorite is gradually crystallized from the lower part of the crucible to obtain fluorite.

According to such a manufacturing method, contained metal impurities which cause internal absorption are reduced to T in quartz glass.
As a result of quantitative analysis of i, Cr, Fe, Ni, Cu, Zn, Co, and Mn by inductively coupled plasma emission spectroscopy, the respective concentrations were 20 ppb or less.
Can be 1 ppm or less.
The synthetic quartz glass and fluorite thus obtained are
It is cut and processed to obtain an optical material having a desired shape.

Next, a step of polishing the surface of the optical material is performed. The polishing step is usually performed in the order of grinding (spherical grinding) → smoothing (sanding) → polishing. The removal amount (removal depth) of the material surface during smoothing is 20 to 200 μm
And about several μm for polishing. Generally, CeO ₂ is used for quartz glass and diamond powder is used for fluorite as an abrasive.

By such a polishing step, the surface roughness is 5Å.
Obtain the following optical material. The reason for setting the surface roughness to 5 ° or less is as follows. It is considered that the surface loss is caused by the surface roughness of the optical material. In general, the equation for calculating the numerical value of the surface roughness from the amount of surface scattering is represented by the following equation. R _rms is a notation of surface roughness (Roughness), and represents root-mean-square roughness.

[0015]

(Equation 1)

Therefore, at λ = 248 nm, the amount of light loss due to surface roughness (Total Integrated Scattering: T
IS) 0.1% or less of surface roughness R _rms
= 6.2 or less, and it can be seen that at λ = 193 nm, the surface roughness R _rms needs to be about 5 ° or less. By polishing the optical material according to the above-described manufacturing method to a surface roughness corresponding to the wavelength of the light source, a spectral transmittance close to the theoretical transmittance of the optical material at each wavelength can be obtained.

Generally, the spectral transmittance in consideration of multiple reflection is as follows.

[0018]

(Equation 2)

(A: absorption coefficient, t: thickness of sample) Here, R is the reflectance when light is incident perpendicularly to the glass surface.

[0020]

(Equation 3)

[0021] (n _g: the refractive index of the sample, n _0: refractive index of air, k _g: extinction coefficient of the sample) The theoretical transmittance, (2), the spectral transmittance as defined in (3) Where only the reflection loss exists, that is, the calculated value of the spectral transmittance when the internal absorption coefficient is 0, or the calculated value of the spectral transmittance when the sample thickness is infinitely small, and is calculated as a = 0. it can. The refractive index at each wavelength of quartz glass and fluorite was measured by the minimum declination method. The refractive index of air was calculated by the formula adopted at the International Conference on Spectroscopy. Each is shown in Table 1.

[0022]

[Table 1]

Therefore, from the above, the theoretical transmittance of quartz glass is 92.12% at 248 nm, and 90.87% at 193 nm.
% For fluorite, 93.06% at 248 nm, 193
It is determined to be 92.27% in nm. An abrasive remains on the surface of the optical material polished in the polishing step. Since the absorption edge of CeO ₂ which is an abrasive is about 400 nm and the absorption edge of diamond is about 250 nm, it is necessary to sufficiently remove the abrasive by hand wiping or wet cleaning such as ultrasonic cleaning. Hand wiping can be performed with an organic solvent that does not erode the optical surface at all, and is particularly effective for optical materials that are liable to cause latent damage, but has the disadvantage of requiring skill and poor workability. Therefore, ultrasonic automatic cleaning has become mainstream. FIG. 2 shows a typical ultrasonic automatic cleaning process. (A) The degreasing step is a step of dissolving and removing the oils and fats such as an adhesive and a grinding fluid used in the polishing process using perchlorethylene as a solvent, and (b) the emulsifying step is performed using perchlorethylene and an emulsifier in the previous step. (C) a washing step for dissolving and removing water-soluble contaminants with a detergent; (d) a washing step for removing any contaminants; (e) pure water washing. The step comprises removing contaminants of underwater impurities, and (f) the dehydration and drying step comprises steps for maintaining the cleaned optical surface and removing moisture from the optical surface. As a drying agent, an organic solvent such as alcohol is used. FIG. 3 shows a spectral transmittance chart of acetone, ethanol, isopropyl alcohol (IPA) and water, which are typical drying agents. The measuring device used was a commercially available high-precision spectral transmittance meter. This measuring device is a double beam method, inserts an empty glass cell into the reference,
The measurement was performed after removing the reflection loss due to the glass cell. Also,
The measurement accuracy obtained by continuously transmitting light in the wavelength range of 190 to 400 nm is ± 0.1%.

Except for acetone, in an optical member used at a wavelength longer than about 250 nm, even if the desiccant used in the dehydration and drying steps remains on the surface of the optical material, the contribution to the light quantity loss is small but shorter. It is conceivable that the contribution increases as the wavelength increases. Therefore, in the present invention, ultraviolet cleaning is performed after each step of ultrasonic cleaning.

As a general ultraviolet cleaning method, a method using a low-pressure mercury lamp made of synthetic quartz glass as a light source is known. This light source emits 185 nm and 254 nm ultraviolet light. Since its energy is larger than the binding energy of many organic compounds, when absorbed by an organic substance, it breaks a chemical bond and can generate radicals and molecules in an excited state. On the other hand, ultraviolet light of 185 nm is absorbed by oxygen molecules to generate O ₃ . O ₃ absorbs ultraviolet light having a wavelength of 254 nm to generate active oxygen, which reacts with radicals of the organic substance and molecules in an excited state, thereby decomposing the organic substance.

The present inventors have examined the change in transmittance due to the effect of removing the desiccant by using ultraviolet (UV-O ₃ ) cleaning useful for removing the desiccant. The horizontal axis indicates the UV cleaning time and the vertical axis indicates the change in transmittance, and the measurement results are shown in FIGS. FIG. 4 is a graph showing the change with time of the ultraviolet ray cleaning time and the transmittance of quartz glass when the measurement wavelength is set to 248 nm and the ArF laser is set to 193 nm. FIG. 6 is a diagram showing a time-dependent change in ultraviolet light cleaning time and fluorite transmittance when the measurement wavelength is 193 nm for an ArF laser.

Dry cleaning methods for removing the desiccant include ion, plasma, and electron beam irradiation. Among them, ultraviolet cleaning is industrially excellent and is a useful method. It was adopted. For the measurement of the transmittance of quartz glass and fluorite, the same measuring device as in the case of the solution measurement was used. The measurement sample needs to remove the influence of internal absorption in order to evaluate the light loss on the surface. Therefore, a plurality of samples having different thicknesses are prepared in advance, and the respective spectral transmittances at 248 and 193 nm are measured.
At each wavelength, the internal absorption (slope) at which the transmittance changes depending on the sample thickness and the sample thickness are 0
In the case of (section), the transmittance is determined. Since the difference between the transmittance of the section and the theoretical transmittance is the amount of light loss on the surface, a measurement sample was prepared using an optical material having a thickness and quality that can sufficiently evaluate the amount of light loss on the surface. The measurement accuracy of the transmittance is ± 0.02% at 248 nm and ± 0.02% at 193 nm.
0.1%.

The sample for measurement was mixed at a mixing ratio of 1: 1 before washing with ultraviolet light.
The value at the left end of the horizontal axis is the transmittance when the degreasing is performed by hand wiping with an ethanol and acetone solution, and the value at the horizontal axis is 0 when the IPA vapor drying is performed. In addition, Table 2 shows the amount of increase (%) in transmittance due to ultraviolet cleaning.

[0029]

[Table 2]

From the above results, it was found that ultraviolet cleaning was effective in cleaning optical members used at shorter wavelengths in addition to the conventional IPA vapor drying cleaning. Further, it was found that it is preferable to carry out the ultraviolet ray cleaning time of the optical material for at least 5 minutes or more in order to obtain a sufficient transmittance, and that the irradiation time of 10 minutes or more is still sufficient. In Examples of the present invention, quartz glass and fluorite were used as optical materials, and after surface polishing, automatic ultrasonic cleaning was performed, and then ultraviolet cleaning was performed for about 10 minutes. The UV cleaner is a commercially available UV dry processor (Oak Co., Ltd., V
UM-3073-B) was used.

After the ultraviolet cleaning step, a film forming step is performed. Hereinafter, an example in which a simple anti-reflection film is formed is shown. In addition to the following anti-reflection film, the thin film formed in the present invention is provided for improving the characteristics of an optical element such as a wavelength selection film. It can be any thin film that can be used. There is no particular limitation on a film formation method, and an arbitrary film formation method such as a vacuum evaporation method, a sputtering method, or an ion plating method is used.

On the optical element thus obtained, which has been washed with ultraviolet light, a substance (LaF ₃ ) having a high refractive index with respect to the optical material is deposited on the material as an antireflection film, and then a low refractive index is applied to the material. (MgF ₂ ) was deposited on both surfaces of the substrate. FIG. 1 shows the relationship between the standing time from the end of material cleaning to film formation and the transmittance at 193 nm. As a reference, the transmittance when the material was formed without UV cleaning was also measured (not cleaned). Even optical materials that have been cleaned with ultraviolet light are contaminated by leaving them in the air. In particular, since the surface activated by ultraviolet cleaning is easily contaminated, it is preferable to form a film immediately after cleaning.

As described above, in the method of manufacturing an optical member for an ArF excimer laser, it is important to perform ultraviolet cleaning as a pre-process of film formation in order to obtain desired film characteristics.
After cleaning an optical material having a surface roughness of approximately 5 mm or less with ultraviolet light, approximately 19
By forming a thin film having a film design at 3 nm, an optical member for an ArF excimer laser having an optical loss of 2.0% or less due to absorption can be manufactured. This manufacturing method is particularly effective for transmission type optical elements (lenses, prisms, etc.), and is also effective for half mirrors.

[0034]

As described above, according to the present invention, after the polishing step for reducing the surface roughness of the optical material to 5 ° or less, ultraviolet cleaning is performed, and then, a desired optical thin film is formed. It has become possible to manufacture an optical member for an ArF excimer laser having a loss of 2.0% or less.

[Brief description of the drawings]

FIG. 1 is a diagram showing a time-dependent change of a substrate standing time and a film characteristic (spectral transmittance) after ultraviolet cleaning.

FIG. 2 is a view showing a typical ultrasonic automatic cleaning step.

FIG. 3 is a spectral transmittance chart of a representative drying agent and water.

FIG. 4 is a diagram showing a time-dependent change in ultraviolet light cleaning time and transmittance of quartz glass at 248 nm.

FIG. 5 is a diagram showing a time-dependent change in ultraviolet light cleaning time and transmittance of quartz glass at 193 nm.

FIG. 6 is a graph showing a time-dependent change in ultraviolet light cleaning time and transmittance of fluorite at 193 nm.

Claims (5)

[Claims] 1. A method of manufacturing an optical element used in an optical system using an ArF excimer laser as a light source, wherein the step of polishing the surface of the optical material so as to have a surface roughness of 5 ° or less; And a step of forming an optical thin film, which transmits ArF excimer laser light, on the optical element cleaned with ultraviolet light, thereby producing an optical element for ArF excimer laser. 2. The method for manufacturing an optical element for an ArF excimer laser according to claim 1, wherein the wavelength of the optical material is 1
A method for producing an optical element for an ArF excimer laser, wherein the internal transmittance at 93 nm is 99.8% / cm or more. 3. The method for manufacturing an optical element for an ArF excimer laser according to claim 1, wherein the ultraviolet cleaning is performed by using a low-pressure mercury lamp made of synthetic quartz glass. A method for manufacturing an optical element for an excimer laser. 4. The method of manufacturing an optical element for an ArF excimer laser according to claim 1, wherein a wet cleaning step is provided between said surface polishing and said ultraviolet cleaning. Of manufacturing an optical element for an ArF excimer laser. 5. An optical element for an ArF excimer laser, wherein the optical loss obtained by the method according to claim 1 is 2.0% or less.