JPH01212247A - Production of base material for laser optical system - Google Patents

Abstract

PURPOSE:To obtain a base material for a laser optical system having durability to high-output short-wavelength laser and high quality by subjecting high-purity quartz glass to a heat treatment while passing gaseous hydrogen halide between the inside tube and outside tube of a double tube chamber made of quartz glass. CONSTITUTION:The quartz double tube chamber 2 is installed in a heating furnace 1 and high-purity quartz glass ingots 3 are installed in the chamber 2. While the gaseous hydrogen halide, gaseous halogen or the gas obtd. by diluting the gases with Ar, etc., is passed between the inside tube 21 and outside tube 22 of the chamber 2, the ingots are subjected to a heating treatment at 500-1,300 deg.C to obtain the base material for the optical system used for laser light of about <=400nm wavelength region. The excimer laser resistance is greatly improved if the high-purity quartz glass in which oxygen deficiency type defects exist is subjected to the heat treatment in the same manner as mentioned above while an oxidizing gas is kept passed in the inside tube 22 and if said quartz glass in which oxygen excess type defects exist is subjected to the heat treatment while a reducing gas is kept passed in said tube.

Description

Detailed Description of the Invention "Industrial Application Field" The present invention is applicable to lasers such as lenses, window members, mirrors, prisms, filters, etc. used in laser stepper devices, laser oscillation devices, laser CVD devices, laser fusion devices, etc. The present invention relates to a method of manufacturing an optical system base material, and particularly to a method of manufacturing a laser optical system base material that can guarantee durability and high quality against high-power laser light in a short wavelength range.

"Prior art" In recent years, the development of phosphorography technology for drawing integrated circuit patterns on wafers has progressed rapidly.
(Pitsu) Pattern line width corresponding to DRAM: 1#L■, furthermore, pattern line width corresponding to 4M bit DRAM: 0.8
JL* and other technologies that can draw finer line widths are being developed, and these fine line width writing technologies require relatively high brightness light sources, highly sensitive resists, stable optical materials, etc. Optical lithography is generally used because it has the various conditions necessary to draw ultra-fine line widths, but the problem with optical lithography is that its resolution is limited by diffraction because the exposure wavelength is large. There is.

To solve this problem, it is possible to increase the NA (numerical aperture) of lenses and other optical systems used in the optical lithography and to shorten the wavelength of light.

We have entered an era in which high HA optical systems have exceeded HA (numerical aperture) of 0.4, and a prototype lens with an HA of 0.8 has been developed, but as the HA increases, the depth of focus becomes shallower. Therefore, increasing the HA to improve the resolution has reached its limit.

Therefore, the next consideration was to shorten the wavelength of light. However, in order to shorten the wavelength of light, for example, approximately 400
When ultraviolet rays of nm or less are used, in lenses using conventional optical glass, the light transmittance decreases rapidly when the wavelength used is around 3B5n+s (i-line), and in other words, light absorption and heat generation due to the light absorption occur. occurs, which disturbs the focal position and other characteristics of the lens.

In order to eliminate this drawback, it has been proposed to replace the lens material with quartz glass or fluorite, but when using quartz glass or the like, it is very difficult to correct chromatic aberration due to the wide optical spectrum width of normal ultraviolet light. It is.

For this reason, it may be possible to use an excimer laser, which is a high-power pulsed laser that oscillates mainly in the ultraviolet region, in combination with a laser beam with a narrow spectrum width, in combination with the quartz glass, etc.; , KrF (248n layer), Xe (41(308n*)
, ArF (193 nm) is advantageous.

However, all of the excimer lasers have a wavelength of 35
Because the wavelength is short, less than 0 nm, the uniformity of the refractive index of these optical materials is equivalent to the wavelength used by conventional mercury lamps.
This is more than an order of magnitude higher than in the case of a line (43Gam) or 1 line (385!III) (lXl0-7 in Δn
IX 10-6, Δn: refractive index variation range), but among the above lens materials, fluorite has problems with refractive index uniformity, maximum dimension, hygroscopicity during processing, and mechanical strength. Therefore, quartz glass is the only material that can be used as a base material for laser optical systems that can guarantee durability and high quality for laser light in the short wavelength range.

``Problem to be solved by the invention'' However, when using a laser light source in the short wavelength range as described above, even if the laser optical system is manufactured using quartz glass, high-power pulsed light such as excimer laser If the laser beam is irradiated for a long period of time, the quartz glass lens will be damaged and distorted over time, causing birefringence. A problem arises in that the refractive index distribution fluctuates and eventually cracks occur.

In particular, for a quartz glass lens for lithography using an excimer laser, the refractive index variation width Δn is lX10
It is said that a value of 4 or less is preferable, and when the above-mentioned changes in the optical properties of quartz glass occur, the optical axis and focal position of the lens change, resulting in fine and! ! #It becomes extremely difficult to form a bright pattern.

In addition, when irradiated with short wavelength laser light of 300 nm or less, even silica glass, which has higher optical stability than conventional optical glass, generates fluorescence, especially in projection exposure type equipment such as excimer laser steppers. Fluorescence from lenses and other optical systems is sensitive to the photoresist on the wafer together with laser light, making it difficult to form a 1g bright pattern.

In view of the shortcomings of the prior art, the present invention has been developed to reduce transmittance and increase absolute refractive index even when a light source is irradiated with short wavelength laser light of 400 nm or less, more specifically 350 nm or less, for a long time. Manufacture of a base material for a laser optical system that can guarantee durability and high quality even with high output and short wavelength laser beams, without changing the refractive index distribution or cracking. The purpose is to provide a method.

"Means for solving the problem" In order to achieve the technical problem, the present invention has finally achieved the desired effect as a result of repeated thought and error while conducting the experiments described below. This has led to the realization of a method for manufacturing a laser optical system base material.

The outline will be explained step by step.

First, changes in optical properties such as a decrease in transmittance of a quartz glass material and generation of fluorescence caused by irradiation with short wavelength laser light are generally caused by Li, Ha,
) Lg, AI, Ca, Ti, Cr, Mn, Fe
, Go, Ni, Cu, Zn, Ge, and other impurity metal elements.

Therefore, the present inventors used highly purified silicon compounds such as 5iCI4 to form extremely high-purity synthetic quartz glass by eliminating the contamination of metal elements as much as possible by oxyhydrogen flame decomposition method, etc. A window etc. (test piece) for the short wavelength laser beam was manufactured using quartz glass material as a base material in an attempt to eliminate the above drawbacks, but the result was still satisfactory as an optical system for high output short wavelength laser beam. was not obtained.

In order to investigate the reason, we checked the state of the glass structure of the synthetic silica glass material and found that internal strain existed in the glass structure, even though the metal elements were below the detection limit. was confirmed.

Next, we fabricated the test piece using a quartz glass material obtained by subjecting the high-purity synthetic quartz glass to a 7-neal treatment to remove distortion as a base material, and conducted an experiment to confirm the effectiveness.The results were still satisfactory. was not obtained.

When we investigated the reason for this, we found that the distortion was removed by annealing, but conversely, the metal elements in the high-purity synthetic silica glass structure increased during the annealing.
It was found that the base material was contaminated.

Therefore, the present inventors arranged a chamber with a double tube structure consisting of an inner tube and an outer tube made of quartz glass in the heat treatment furnace in which the annealing is performed, and with the base material housed in the chamber. Although 7-neal treatment was performed, the above-mentioned drawbacks could not be solved.

Next, the inventors conducted repeated experiments while flowing various gases between the inner tube and the outer tube, and found that halogen compound gas such as hydrogen chloride gas, halogen gas such as chlorine gas, Only when heat treatment is performed while flowing an inert gas such as nitrogen gas.

Satisfactory results were obtained as a base material for laser optical systems.

That is, in the present invention as described in claim 1), when the base material is housed in a chamber with a double tube structure consisting of an inner tube and an outer tube made of quartz glass, a halogen gas is inserted between the inner tube and the outer tube. Laser optical system base material that can guarantee durability and high quality by performing heat treatment while flowing compound gas, halogen gas, diluent gas containing these, inert gas, or even a mixture of these gases. It is possible to provide the following.

In this case, it is desirable that the temperature of the heat treatment is arbitrarily selected between 500 and 1300°C; however, at temperatures lower than 500°C, it becomes difficult to remove oxygen defects.
This is because at temperatures above 00°C, the quartz double tube chamber is disadvantageously deformed.

Furthermore, when supplementary experiments were conducted on the invention, it was confirmed that the excimer laser characteristics change depending on the type of gas flowing through the inner tube of the quartz glass chamber.

Therefore, we analyzed the materials with improved excimer laser characteristics and those with deteriorated excimer laser characteristics, and found that the base materials with improved excimer laser characteristics all had a silica glass structure (S+
It was confirmed that the oxygen defects present in 02), specifically the oxygen-deficient type defects represented by the following formula 0, or the oxygen-excess type defects represented by the following formula 0, were substantially removed.

Therefore, the second invention as set forth in claim 2) provides that, in addition to the requirements set forth in claim 1), in the inner tube, the atmosphere is selected from one or more of oxidizing and reducing atmospheres. The main point of the laser optical system base material is that by heat-treating the base material, oxygen defects existing in the base material structure are substantially removed, thereby further guaranteeing durability and high quality. It is possible to provide the following.

The reason why the presence of oxygen defects adversely affects the optical properties is not clear, but it is presumed to be due to the following reasons.

That is, when oxygen defects exist in the glass structure, the bonds between the elements constituting the glass structure become weaker than the bonds between the elements of ideal silica glass, and the energy of the laser beam breaks the bonds. It is presumed that the bond between the elements of the silica glass is broken, causing a change in density and a change in the refractive index. Similarly, it is considered that the presence of oxygen defects acts as a precursor and forms various color centers during laser beam irradiation, resulting in a decrease in transmittance, and that fluorescence is more likely to occur as the color centers are formed. be done.

Incidentally, among oxygen defects, ugliness-excess type defects can be detected by measuring the increase in the infrared absorption peak of OH groups generated when reacting with hydrogen at high temperatures, and oxygen-deficient defects can be detected by measuring the increase in the infrared absorption peak of OH groups generated when reacting with hydrogen at high temperatures.
Vacuum ultraviolet range? , the presence of an absorption peak at 8 eV (IHnm), and decreases when reacted with oxygen at high temperatures? , 8 eV absorption peak.

``Example'' A preferred embodiment of the present invention will be described in detail below by way of example with reference to one drawing. However, there are no specific descriptions of the dimensions, materials, shapes, relative arrangements, etc. of the component parts described in this example, and the scope of this invention is not intended to be limited to only the percentages. , is merely an illustrative example.

The progress that led to the present invention will be explained in detail in a step-by-step manner through specific experimental progress.

A. Claimed invention First, silicon tetrachloride as a raw material is distilled to remove impurities and then high purity silicon tetrachloride stored in a Teflon-lined stainless steel container is prepared, and using this raw material,
Multiple high-purity transparent quartz glass lumps are directly synthesized by oxyhydrogen flame hydrolysis method.

In this case, in any of the quartz glass lumps, Li, Ha, Mg, Ca, Ti
The content of gold metal impurity elements such as Cr, Fe, Cu, and AI was below the detection limit (Table 2).The OH group content of each of the silica glass ingots was approximately 700 ppm.

The high-purity synthetic silica glass block thus formed was then heat-treated in a normal annealing furnace to remove strain, cut into dimensions of 30 x 20 x 1 Gmm, and mirror-finished on both sides to produce test pieces for excimer laser irradiation experiments. 9
A side mold was made, and 248n (KrF) was applied to the nine test pieces.
), the energy density per pulse is 200, 400. floo (mJ/crr
f a pulse) and irradiation pulse IX 104 ,
IX 105, IX 1G' (pulse)
Irradiation was performed under irradiation conditions consisting of a combination of.

After the irradiation, each specimen was subjected to changes in refractive index distribution using an interferometer, solarization using a transmittance meter, and fluorescence intensity measurement using a fluorescence measuring device. ) As shown in experiment number 1), no satisfactory results were obtained as an optical system for high-output, short-wavelength laser light.

Therefore, as a result of analyzing the impurity metal elements of the glass lump after the heat treatment, it was found that the content of impurity metal elements was significantly increased compared to that before the heat treatment, as shown in Table 2. . The reason for this is assumed to be that impurity elements contained in the heat insulating material of the heating furnace contaminate the quartz glass lump during the heat treatment.

Therefore, the wood inventors set up a quartz double tube chamber 2 in a horizontal cylindrical heating furnace 1 as shown in FIG. 3
The heat treatment was carried out at a temperature of about 1000°C, and the glass lumps thus formed after the heat treatment were tested in the same manner as above, and the experimental results -
As shown in experiment number 2) in the list, no satisfactory results were obtained as an optical system for high output short wavelength laser light.

Next, while flowing hydrogen chloride gas diluted with nitrogen gas, argon gas, or helium gas between the outer tube 21 and the inner tube 22 of the chamber 2, and selectively changing the concentration, about 1000 The heat treatment was performed at a temperature of around ℃. The heat-treated glass lump thus formed was also tested in the same manner as above, and as shown in experiment number 4-5) in the list of experimental results, 1.
Samples heat-treated while flowing hydrogen chloride diluted with inert gas between the quartz double tubes have high excimer laser resistance.
Satisfactory results were obtained as an optical system for high-power, short-wavelength laser light.

Next, although the quartz glass lump heat-treated while simply flowing nitrogen gas between the outer tube 21 and the inner tube 22 of the chamber 2 has some excimer laser resistance, it is suitable for use with high-power short wavelength laser beams. As an optical system, the results were somewhat unsatisfactory. (In the experiment number (experiment number 05, as a result of the impurity metal element analysis of each sample), contamination with impurity metal elements was detected, but experiment number 1) 2)
In 3), an increase in the content of impurity metal elements was observed;
The degree of increase is 2! Experiment No. 3), in which heat treatment was performed while flowing only nitrogen gas between the inner tube 22 and the inner tube 22, had the smallest increase.

In addition, when a similar experiment was conducted using salt ice male CI2 instead of the hydrogen chloride gas MCI used in experiment number 05),
Almost the same excellent results as in Experiment No. 4) and 5) were obtained. (
Experiment number 8)? )) B. The outer tube 2 is made of high-purity quartz glass synthesized by the soot remelting method in which #elementary defects exist in claim 2)! and the inner tube 22, while flowing hydrogen chloride gas diluted with argon gas, and flowing oxygen gas diluted with argon gas or hydrogen gas diluted with argon gas into the inner tube 22.
The heat treatment was performed at a temperature of around ℃. The heat-treated glass lump thus formed was also tested in the same manner as above, and as shown in the following experimental results table (Table 3), the oxidizing atmosphere shown in Experiment No. 8) was obtained. The oxygen defect concentration was substantially removed in the heat-treated sample, and the excimer laser resistance was significantly improved. However, it was revealed that when heat treated in the reducing atmosphere shown in Experiment No. 9), the oxygen defect concentration increased and the excimer laser resistance deteriorated.

Furthermore, in order to confirm the above-mentioned effect, we attempted a similar experiment on a high-purity quartz glass lump synthesized by the plasma method in which oxygen-excess type defects exist. , we were able to remove the oxygen defect concentration to below the detection limit and significantly improve the excimer laser resistance, but samples heat-treated in an oxidizing atmosphere had an increased oxygen defect concentration and the excimer laser resistance decreased. It became clear that the condition worsened significantly (Experiment Example 10)
11)12)).

"Effects of the Invention" As described above, according to the present invention, by using a high-purity quartz glass material and subjecting the quartz glass material to special treatment, it is possible to resist high-power laser light in a short wavelength range. A method for manufacturing a laser optical system base material that can guarantee durability and high quality can be provided, and the laser optical system base material manufactured based on the present invention can only be used in lithography equipment and other highly integrated circuit manufacturing equipment. In addition, it has various outstanding effects, such as the fact that it is fully applicable to laser optical system base materials used in laser fusion devices, laser fusion devices, and other high-power excimer lasers.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a heat treatment furnace used in the present invention.

Claims (1)

[Claims] 1) In a method for manufacturing a laser optical system base material used for laser light in a specific wavelength range of about 400 nm or less, after manufacturing the laser optical system base material using a high-purity silica glass ingot, The base material is housed in a chamber with a double tube structure consisting of an inner tube and an outer tube made of quartz glass, and a halogen compound gas, a halogen gas, or any of these is contained between the inner tube and the outer tube. A method for manufacturing a laser optical system base material, which is characterized by performing heat treatment while flowing one or more gases selected from diluent gas or inert gas 2) Laser light in a specific wavelength range of about 400 nm or less In the manufacturing method of the laser optical system base material used in With the base material housed inside, one or more gases selected from among a halogen compound gas, a halogen gas, a diluent gas containing these, or an inert gas are provided between the inner tube and the outer tube. A method for producing a laser optical system base material, characterized in that the heat treatment is performed while flowing a selected one or more of oxidizing or reducing atmospheric gases into the inner tube.