JPH0761823A - Production of silica glass having ultraviolet resistance and silica glass optical member - Google Patents

Abstract

PURPOSE:To improve ultraviolet resistance, reduce generation of structural defects and optimize the chlorine content, the OH content and the metal impurity content by specifying the growth rate in formation of a silica glass ingot. CONSTITUTION:Oxygen gas and hydrogen gas of a silica glass burner are mixed and burnt and a silicon chloride compound as the raw material, e.g. silicon tetrachloride diluted with a carrier gas (oxygen gas in general) is supplied and injected through its central part at a rate of about 5 to 20g/min. The injected silicon chloride compound is allowed to react with water generated by combustion of the hydrogen gas and the oxygen gas and the produced silica glass soot is deposited and melted on a target made of an opaque silica glass plate and designed so as to perform a rotational motion, a shaking motion respectively in a prescribed period and a descending motion at <=2mm/hr at the same time, keep the position of the ingot at a constant distance from the burner and control the growth rate of the ingot to <=2mm/hr. Thereby, the objective silica glass of a silica glass ingot preform having >=800ppm OH group content, <=50ppm chlorine content and <=20ppb metal impurity content can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silica glass optical member used in an optical system of an ultraviolet laser of 300 nm or less such as excimer laser lithography and a method for manufacturing the same.

[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 device called a stepper is used. The light source of this stepper is g-line (436 nm) to i-line (365 nm), and further Kr with the recent high integration of LSI.
Shortening wavelengths are being promoted to F (248 nm) and ArF (193 nm) excimer lasers. Generally, an optical glass used as an illumination system of a stepper or a projection lens is i
Since the light transmittance is low in the wavelength region shorter than the line, synthetic quartz glass is used instead of conventional optical glass.

However, even synthetic quartz glass, when used for a long time under high-power ultraviolet light or excimer laser light, has a reduced light transmittance or other optical characteristics. . These deteriorations are caused by the structural defect called E'center due to the irradiation of the ultraviolet laser at the absorption band of 215 nm and NBOHC (Non-Bridging Oxygen Hole C).
It is thought that one of the causes is the appearance of a 260 nm absorption band due to a structural defect called "enter".

30 such as excimer laser lithography
Since quartz glass optical members used for optical systems of ultraviolet lasers of 0 nm or less are required to have high resistance to ultraviolet rays, various studies have been made to reduce the above structural defects. These will be briefly described below. It is known that the dissolved chlorine contained in the synthetic quartz glass exists in the state of Si-Cl, Cl ₂ , HCl, etc. inside the quartz glass. It is expected that chlorine in any existing form is easily converted into harmful structural defects when irradiated with ultraviolet rays. In other words, if the concentration of chlorine contained in the synthetic quartz glass is high, the UV resistance will be poor.
This means that, as described in JP-A-5-32432, high UV resistance can be obtained by suppressing the chlorine concentration in the synthetic quartz glass to 200 ppm or less. Also European patent
As described in EP0525984A1, it is known that high UV resistance can be obtained by synthesizing quartz glass using a chlorine-free raw material.

On the other hand, as described in JP-A-4-295018, when the content of OH groups is increased, the light transmittance in the ultraviolet region is increased and the ultraviolet resistance is improved. Quartz glass optical members having a high OH group concentration of 800 ppm to 1200 ppm are used. Further, the metal impurities contained in the synthetic quartz glass are also a cause of structural defects and deteriorate the ultraviolet resistance, so that the synthetic quartz glass in which the metal impurities contained are reduced is used.

Further, in order to further improve the ultraviolet resistance of the synthetic quartz glass, a technique for improving the ultraviolet resistance by heat-treating the obtained quartz glass ingot in a hydrogen atmosphere (JP-A-1-201664) and A technique (JP-A-3-109233) for improving the excimer resistance by doping hydrogen molecules has been proposed. It is considered that the hydrogen molecules thus doped into the quartz glass ingot have an effect of changing (terminating) the structural defects generated by the ultraviolet irradiation into a stable structure.

[0007]

The problems of the prior art will be described. First, regarding the content of chlorine, JP-A-5-3243
In No. 2, it has been proposed to suppress the chlorine concentration in the synthetic quartz glass to 200 ppm or less, but as described in the means for solving the problem, 200 ppm or less is insufficient. With such a concentration of chlorine, deterioration due to ultraviolet irradiation is large. On the other hand, as described in European Patent EP0525984A1, even if silica glass is synthesized using a chlorine-free raw material, it may increase other structural defects on the contrary and avoid deterioration of ultraviolet resistance. Absent. Further, even if the contained OH group concentration is kept at 800 ppm or more, if the chlorine concentration is not reduced, the UV resistance may deteriorate.
That is, it is meaningless to optimize the contained chlorine concentration, the contained OH group concentration, and the contained metal impurity concentration independently.

Furthermore, even if they are optimized, the silica glass basic structure Si--O--Si itself has an incomplete structure, for example, KA.
wazu et.al.:Journal of Applied Physics, 73 (1993) 164
There should be no stress structure as described in 4. Next, the problems in improving the ultraviolet resistance by introducing hydrogen molecules will be described. In order to introduce hydrogen molecules into the quartz glass ingot, there is a problem that the heat treatment (hydrogen treatment) must be performed again after forming the glass ingot once. In other words, with this method, heat is applied at least twice until the introduction of hydrogen molecules,
It is disadvantageous both in terms of cost and time. Further, in order to introduce hydrogen molecules in the secondary treatment, the treatment must be carried out in a hydrogen atmosphere, and there is a risk of ignition and explosion. Further secondary treatment requires considerable labor for the ingot after formation, and therefore the possibility of inclusion of impurities cannot be denied. In addition, in recent years, as the lens diameter used for photolithography technology has become larger, it takes a considerably long time to evenly introduce hydrogen molecules into a large-diameter quartz glass ingot by secondary treatment, considering the diffusion coefficient. Have.

Further, the biggest problem of hydrogen treatment is that it has only an effect of terminating structural defects generated by hydrogen molecules, and cannot suppress generation of structural defects themselves. Therefore, even if it contains a dissolved hydrogen molecule, it cannot suppress the generation of defects itself, and therefore cannot be a fundamental solution to further improve the ultraviolet resistance.

Therefore, in the present invention, it is possible to obtain sufficient ultraviolet resistance as a quartz glass optical member for an ultraviolet laser without performing a secondary treatment such as hydrogen treatment.
It is an object of the present invention to provide a quartz glass that suppresses the generation of structural defects itself and further optimizes the content chlorine concentration, the content OH group concentration, and the content metal impurity concentration.

[0011]

The inventors of the present invention have investigated the cause of structural defects when irradiated with ultraviolet rays in order to obtain manufacturing conditions in which the structure itself of quartz glass is stable during ingot formation. did. as a result,
It was discovered that the growth rate during the formation of a quartz glass ingot has a great influence on the UV resistance. That is, if the growth rate of the ingot is too fast, after the quartz glass soot is deposited on the target and melted, it vitrifies without removing the incomplete structure, and these incomplete structures are exposed to ultraviolet rays. It is a precursor to defect generation and therefore
It was concluded that the excimer laser resistance was worse than when the growth rate was low. From this result, it was found that the growth rate at the time of forming the quartz glass ingot needs to be set to 2 mm / hour or less.

Further, by simultaneously optimizing the growth rate of the ingot and the conventional concentration of chlorine content, metal impurity concentration, and content OH group concentration, the ultraviolet resistance of the quartz glass is further improved. It is possible. As a result of repeating the experiment, it was found that it is necessary to set the content chlorine concentration to 50 ppm or less, the content OH group concentration to 800 ppm or more, and the content metal impurity concentration to 20 ppb or less for each element.

[0013]

In the method for producing synthetic quartz glass, silicon chloride such as silicon tetrachloride is used as a raw material, and the raw material is decomposed by an oxyhydrogen flame to form silica soot, which is rotated, shaken, and lowered. Since the silica glass ingot is obtained by depositing and melting on the target, it is expected that the silica glass structure will be diversified depending on the manufacturing conditions.

The growth rate of the ingot is mainly determined by the supply amount of the raw material and the descent rate of the target. Until now, no research focusing on the growth rate of ingots has been done, but the higher the growth rate, the higher the productivity and the better the yield. Therefore, in the past, for example, the production was performed at a growth rate of 2 mm / hour or more. Was there. The present invention is characterized in that the growth rate is set to 2 mm / hour or less in order to remove the incomplete structure in the ingot.

As an example of the incomplete structure, Si-Si bond, Si-OO-Si bond and the like have been proposed in the literature and the like, but the stoichiometry of silica glass obtained in the present invention is such. The absence of incomplete structures due to deviations from the ratio has been confirmed by absorption measurements with a vacuum ultraviolet / ultraviolet / visible / infrared spectrophotometer. That is, g-line (436 nm) to i-line (365 nm)
nm), and KrF (248 nm) wavelength has an internal transmittance of 99.
Achieved 9% or more, and approximately 99.9% or more at the wavelength of ArF (193 nm).

In the quartz glass network, the tetrahedra made up of oxygen atoms and silicon atoms are cross-linked, and the Si-O-Si bond angle has a certain distribution because it is glass. This Si-O-Si bond angle distribution contains structurally unstable ones. It is considered that this unstable Si-O-Si bond angle component causes deterioration of ultraviolet resistance. That is, it is not an incomplete structure due to deviation from the stoichiometric ratio. As such an unstable structure, for example, when the Si-O bond distance is deviated from the most stable distance,
A bond having an unstable bond angle such as a three-membered ring or a four-membered ring structure is considered.

At this time, when the OH group is contained, it is not necessary to crosslink, so that the Si—OH bond angle can approach the most stable structure. Therefore, it is presumed that the higher the OH group content is, the higher the UV resistance is. In the present invention,
Set the growth rate of the ingot to 2 mm / hr or less, and
By setting the content of the group to 800 ppm or more, it becomes possible to provide a quartz glass optical member that is more excellent in ultraviolet resistance than ever before and sufficiently satisfies the requirements as an optical member for ultraviolet rays.

Similarly, regarding the content of Cl (50 ppm or less) and the content of metal impurities (20 ppb or less), by combining with the growth rate of the ingot (2 mm / hour or less), a quartz glass optical member excellent in ultraviolet resistance can be obtained. It becomes possible to provide.

[0019]

Example 1 A high-purity quartz glass ingot uses high-purity silicon tetrachloride as a raw material, and an oxygen gas and a hydrogen gas are mixed and burned by a quartz glass burner, and the raw material gas is supplied from a center portion to a carrier gas (usually It was diluted with oxygen gas) and ejected, deposited on a target, melted, and synthesized. At the time of synthesis, the raw material gas is reacted with water generated by combustion of surrounding oxygen gas and hydrogen gas, and vitrified and deposited on the target made of an opaque quartz glass plate below the burner. Rotates and oscillates at a constant cycle, and further descends at the same time to keep the position of the ingot at the same distance from the burner at all times (see Japanese Patent Application Nos. 5-22293 and 5-22294).

At this time, in Example 1, the raw material supply amount was 5 g.
/ min, set the target descent speed to 1 mm / hour,
By keeping the ingot head constant during synthesis,
A quartz glass ingot having a diameter of 180 mm and a length of 550 mm was synthesized at a growth rate of 1 mm / hour. Further, as Comparative Example 1, a quartz glass ingot having a diameter of 180 mm and a length of 480 mm synthesized at a growth rate of 4 mm / hour with a raw material supply rate of 20 g / min was prepared.

Regarding these quartz glass ingots,
The OH group content was measured by infrared absorption spectroscopy (the amount of absorption by the OH group at 1.38 μm is measured).
It was 1200 ppm and Comparative Example 1 was 1070 ppm. In addition, when the chlorine content was examined by activation analysis, it was found that
m and Comparative Example 1 were 70 ppm.

Further, metal impurities contained (Mg, Ca, Ti, Cr, F
Quantitative analysis of e, Ni, Cu, Zn, Co, Mn) was carried out by inductively coupled plasma emission spectroscopy, and it was found that the concentration was 20 ppb or less and the purity was high. Test pieces for ArF excimer laser irradiation having a diameter of 60 mm and a thickness of 10 mm were cut out from each of the quartz glass ingots of Example 1 and Comparative Example 1, and two surfaces facing each other in the thickness direction were optically polished. Next, these two test pieces were heated to 700 ° C in the same heat treatment furnace while exhausting to 10 ^-5 Torr with a diffusion pump.
After holding it for 60 hours (vacuum annealing), it was cooled to room temperature and subjected to dehydrogenation gas treatment to eliminate the influence of dissolved hydrogen molecules on laser resistance. The measurement of hydrogen molecule concentration was performed by a laser Raman spectrophotometer. Quantification is performed by setting the sample on the sample table and then measuring the intensity at 800 cm ^-1 and 4135 cm ^-1 of the Raman scattered light in the direction perpendicular to the sample generated when the sample is irradiated with Ar + laser (output 800 mW). By taking the ratio (VSKhotimchenko et al., J. Ap.
pl. Spectrosc., 46, 632-635 (1987)). As a result, the dissolved hydrogen molecule was detected at the detection limit (1x10 ¹⁶ molecule
s / cm ³ ) or less.

The test pieces of Example 1 and Comparative Example 1 thus prepared were subjected to irradiation test with ArF excimer laser light at one pulse energy density: 100 mJ / cm ² / pulse and repetition: 100 Hz, and absorption at 193 nm was measured. Coefficient, absorption coefficient = ln (transmittance after irradiation / transmittance before irradiation) /
The change in the thickness of the test piece was investigated. The results are shown in Fig. 1. As shown in FIG. 1, it can be seen that the excimer laser resistance of Example 1 in which the silica glass ingot was synthesized at a growth rate of 1 mm / hour was significantly improved as compared with Comparative Example 1.

[0024]

Example 2 In Example 2, the feed rate was set to 20 g / min, the target descending rate was 1.2 mm / hour, and the growth rate of 1.2 mm / hour was maintained by keeping the ingot head constant during synthesis. A quartz glass ingot having a diameter of 280 mm and a length of 600 mm synthesized at the time was obtained. In addition, as Comparative Example 2, a diameter of 12 mm synthesized at a growth rate of 20 mm / hour with a raw material supply rate of 40 g / min.
A quartz glass ingot having a length of 5 mm and a length of 500 mm was prepared.

Regarding these quartz glass ingots,
When the OH group content was measured, it was 1200 ppm in Example 2 and 660 ppm in Comparative Example 2. Further, when the chlorine content was examined, Example 2 was 23 ppm, and Comparative Example 2 was 130 ppm.
In addition, contained metal impurities (Mg, Ca, Ti, Cr, Fe, Ni, Cu, Zn, C
It was found that the concentration of o, Mn) was less than 20 ppb, which was high purity.

ArF having a diameter of 60 mm and a thickness of 10 mm from each of the silica glass ingots of Example 2 and Comparative Example 2
A test piece for excimer laser irradiation was cut out, and two surfaces facing each other in the thickness direction were subjected to optical polishing. These two test pieces were then placed in the same heat treatment furnace at 10 ^{-5 with a} diffusion pump.
While evacuating to Torr, the temperature was kept at 700 ° C for 60 hours (vacuum annealing), cooled to room temperature, and subjected to dehydrogenation gas treatment to eliminate the influence of dissolved hydrogen molecules on laser resistance. As a result, dissolved hydrogen molecules were detected at the detection limit (1x10 ¹⁶
molecules / cm ³ ) or less.

The test pieces of Example 2 and Comparative Example 2 thus prepared were subjected to irradiation test with ArF excimer laser light at one pulse energy density: 100 mJ / cm ² / pulse, repetition: 100 Hz, and absorption at 193 nm The change in coefficient was investigated. The results are shown in Fig. 2. As shown in FIG. 2, it can be seen that the excimer laser resistance of Example 2 in which the silica glass ingot was synthesized at a growth rate of 1.2 mm / hour was significantly improved as compared with Comparative Example 2.

[0028]

As described above, according to the present invention,
It is possible to obtain a synthetic quartz glass optical member which is essentially excellent in ultraviolet resistance with little reduction in transmittance with respect to high-power irradiation of excimer laser light or ultraviolet light for a long period of time.

[Brief description of drawings]

FIG. 1 shows a change in absorption coefficient of 193 nm when the test pieces of Example 1 and Comparative Example 1 were irradiated with ArF excimer laser.

FIG. 2 shows changes in absorption coefficient of 193 nm when the test pieces of Example 2 and Comparative Example 2 were irradiated with ArF excimer laser.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hiroyuki Hiraiwa 3 2-3 Marunouchi, Chiyoda-ku, Tokyo Inside Nikon Corporation

Claims (5)

[Claims] 1. A method for producing quartz glass, wherein a silica glass compound is hydrolyzed by an oxyhydrogen flame, and the produced quartz glass soot is deposited on a target and melted to form a quartz glass ingot, which comprises growing the quartz glass ingot. A method for producing quartz glass, characterized in that the speed is 2 mm / hour or less. 2. A silica glass optical member using a silica glass ingot obtained by hydrolyzing a silicon chloride compound with an oxyhydrogen flame, depositing the resulting silica glass soot on a target, and melting the silica glass ingot as a base material. A quartz glass optical member for ultraviolet rays, wherein the growth rate of the quartz glass ingot is 2 mm / hour or less. 3. The silica glass optical member according to claim 2, wherein the silica glass ingot has an OH group concentration of 800 ppm or more. 4. The silica glass optical member according to claim 2, wherein the silica glass ingot has a chlorine concentration of 50 ppm or less. 5. The quartz glass optical member according to claim 2, wherein the concentration of metal impurities contained in the quartz glass ingot is 20 ppb or less, respectively.