JPH0558667A - Optical member - Google Patents

Abstract

PURPOSE:To provide an optical member which does not cause the irregularity of its refractive index even on a large optical member especially having a thickness of >=30mm and which is thereby free from the lowering of the image- forming performance and laser resistance caused by the irregularity of the refractive index. CONSTITUTION:This invention is characterized in that the virtual temperature and the homogeneity are set to 800-1000 deg.C and a DELTAn of 1X10-5, respectively, even on a synthetic silica glass optical member having a thickness of >=30mm, thereby permitting to reduce both the deterioration and the spherical aberration of the optical member by the irradiation of laser. In order that the virtual temperature is set to 800-1000 deg.C, it is premised that the content of OH groups is >=100 (wt.ppm).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical member mainly incorporated in various devices using an ultraviolet laser, and can function as a lens, a prism, an etalon plate, or a semi-finished product of these members before final finishing. It relates to an optical member.

[0002]

2. Description of the Related Art In recent years, ultraviolet lasers, particularly KrF excimer lasers and other short ultraviolet lasers, have been used in lithography technology for manufacturing LSI, technology utilizing photochemical reaction, processing technology for cutting and grinding, laser fusion technology. It is attracting attention as something.

Lenses, prisms, filters, windows, mirrors, etalon plates, which control ultraviolet lasers by transmitting, transmitting, refracting, reflecting, absorbing, and interfering with them,
Fluoride such as magnesium fluoride, calcium fluoride, barium fluoride or silica glass can be used as the material of the fiber, but silica glass is most suitable from the viewpoint of workability, size, striae and homogeneity of refractive index, and Among such silica glasses, synthetic silica glass has been widely used in recent years as an optical member for high-power ultraviolet light because a glass body of high purity and high homogeneity can be obtained.

In order to obtain a desired laser resistance when a high-power ultraviolet light having a wavelength of about 360 nm or less is applied, the applicant of the present invention forms the optical member with a high-purity synthetic silica glass material. Silica glass with at least 5 OH groups
An optical member containing 0, preferably 100 (wt.ppm) or more has been provided. (JP-A-3-5338, etc.)

The reason why the OH group content affects the above-mentioned optical characteristics is not critical, but it is considered as follows. When silica glass is irradiated with strong laser light for a long time, the bonds between the elements that make up the glass network structure are gradually broken, and as a result the transmittance decreases, absorption bands appear, and in the worst case cracks occur. Resulting in. However, the cleavage between these elements is also largely repaired by the presence or movement of the OH groups present in the silica glass or the hydrogen atoms in the OH groups, and even when cracks occur, a large amount of OH groups is generated. It is considered that if included, the generation of absorption bands is reduced for the above reason, resulting in less light absorption and fewer cracks.

Therefore, it is necessary to increase the OH group content in proportion to the increase in energy density due to the shortened wavelength of the used light as in the excimer laser. For example, in the laser beam of 250 nm or less, the OH group concentration is 100. (Wt / p
pm) or more. Now, in evaluating the optical characteristics, particularly homogeneity, of such silica glass, He-N is measured from the disc surface of the disc-shaped optical member.
The in-plane refractive index is caused by the difference in the transmission speed of the in-plane light that interferes with the light that is reflected by the mirror surface on the opposite side and returns to the surface after being irradiated with the e-laser. Was calculated.

[0007]

However, in the above-mentioned measuring method, since the thickness direction (length) is averaged, only an averaged value can be obtained.
In the synthetic silica glass formed to have a thickness of 0 mm or more, even if a portion having a non-uniform refractive index is generated, it cannot be detected.

For this reason, when a lens and other optical parts are processed / manufactured by using such synthetic silica glass, a phenomenon in which spherical aberration is partially increased and the imaging performance thereof is deteriorated has been observed. It was also found that when the above-mentioned optical components were irradiated with a laser beam having a very high energy such as an excimer laser, the deterioration was partially caused from the non-uniform portion. We also found that it is practically difficult to apply it to a laser device.

In view of the above-mentioned drawbacks of the prior art, the present invention does not cause a variation in the refractive index even in a large-sized optical member having a thickness of 30 mm or more. An object of the present invention is to provide an optical member whose laser resistance does not deteriorate.

[0010]

According to the present invention, the optical components having a large spherical aberration and the components deteriorated by laser irradiation are examined by a laser Raman spectrophotometer. Knowing that
The invention was achieved based on the findings. That is, the present invention has the fictive temperature of 800 to 800 even from an optical member made of synthetic silica glass having a thickness of 30 mm or more from the above knowledge.
The feature is that the homogeneity at 1000 ° C. is set to Δn of 1 × 10 ⁻⁵ or less, which can reduce both deterioration due to laser irradiation and spherical aberration. The setting of the fictive temperature to 800 to 1000 ° C. can be easily realized by selecting the annealing condition as shown in the examples below. However, setting the fictive temperature at 800 ° C. or lower actually requires heating for a long time (about two months), which is industrially disadvantageous.

Although it is not clear why the deterioration due to laser irradiation does not easily occur when the fictive temperature is set to 800 to 1000 ° C., the following is considered. First, when the fictive temperature reaches 800 to 1000 ° C, the amount of precursor in the silica glass decreases. Second, the fictive temperature is 800
It is considered that when the temperature rises to about 1000 ° C., the density decreases, the structure relaxes, and it becomes difficult for defects to be generated by laser irradiation.

Next, in order to set the fictive temperature to 800 to 1000 ° C., it is premised that the OH group content is 100 (wt.ppm) or more. Bare OH group content 10
Setting it to 0 (wt.ppm) or more is not only preferable in terms of laser resistance as described above, but also when the OH group content is 100 (wt.ppm) or less, the fictive temperature is 100.
It becomes difficult to set the temperature below 0 ° C. In addition, the inclusion of an OH group content of 1000 (wt.ppm) or more tends to cause problems in manufacturing.

Therefore, the second feature of the present invention is that O
The point is that H group is contained in an amount of 100 to 1000 (wt.ppm). Further, in order to obtain laser resistance even in the above glass body, in order to obtain laser resistance upon irradiation with a high power laser in the short ultraviolet region of about 250 nm or less, particularly KrF excimer laser, as disclosed in JP-A-3-88742. H
_It is necessary to set the content of ₂ gas molecules to 1 × 10 ¹⁶ molecules / cm ³ or more.

[0014]

EXAMPLE First, in order to produce a high-purity synthetic quartz glass, a high-purity silicon tetrachloride stored in a Teflon lannig stainless steel container was prepared by distilling a raw material silicon tetrachloride to remove impurities. Using the high-purity silicon tetrachloride raw material, 500 (wt · ppm) of OH groups was contained by a flame hydrolysis method (direct method). High purity synthetic silica glass ingot and OH group of 150 by CVD soot method
A plurality of high-purity synthetic silica glass ingots containing (wt.ppm) and a plurality of high-purity synthetic silica glass ingots each having an OH group content of 5 (wt.ppm) or less were synthesized by the CVD soot method + Cl ₂ treatment.

Then, each of the ingots above the softening point is added.
Repeated heating / cooling operation, and due to the weight of each heating
Change the direction of softening to remove internal striae. I.e.
By repeating the operation of 3
Folding rate fluctuation range (△ n) is 2 × 10^-6 Ingots suppressed below
To manufacture.

After placing each of the ingots in an electric furnace in which a tungsten heater is placed in a stainless jacket, the ingots are heated / slowly cooled in a temperature curve shown in FIG. I went. The outer periphery of each of the annealed cylindrical ingots was ground by 10 mm, and the upper and lower surfaces were further cut to form the outer periphery 1.
A cylindrical test piece having a diameter of 00φ and a thickness of 30 mm was prepared. The test piece of the direct method was used in Example 1 and the test piece manufactured by the soot method was used in Example 2 [OH group content 150 (wt.pp.
m)] and Comparative Example 1 [(OH group content 5 (wt · ppm
Below)].

Next, the annealing conditions of the ingot manufactured by the direct method are changed, and the annealing / annealing is performed by heating / slow cooling according to the temperature curve 2 shown in FIG. 1 (B). 10m
m shaving and further cutting the upper and lower surfaces to give a circumference of 100φ and a wall thickness of 3
A 0 mm cylindrical test piece was prepared. (Comparative example 2)

Next, a KrF excimer laser was applied to each of the above test pieces at an energy density of 100 mJ / cm ² and 100 H.
The content of H ₂ after irradiation with 3 × 10 ⁷ p at the frequency of z was measured, and the consumption of H ₂ was calculated.

As a result, in Example 1, the H ₂ content was 1 × 10 ¹⁷ to 5 × 10 ¹⁶ (molecules / cm ³ ) before and after laser irradiation.
Although it has been reduced to 1 ×, it is the limit of the defect occurrence of laser resistance.
It satisfies 10 ¹⁶ (molecules / cm ³ ).

Also in Example 2, H before and after laser irradiation
₂Content is 1 x 10¹⁷From 2 × 10¹ 6 (molecules / cm³)What
Although it was reduced to 1 × 1 which is the limit of the defect occurrence of laser resistance.
0¹⁶ (molecules / cm³) Is satisfied. On the other hand, in Comparative Example 1
Is H before laser irradiation₂Content is 1 x 10¹⁶(molecules /
cm³) Or less, and below the laser-resistant defect generation limit.
It was In Comparative Example 2, H before laser irradiation₂Content is implemented
Same as Example 1, but 1 × 10 after use¹⁶(molec
ules / cm³) Significantly reduced to less than the following, and laser resistant defect occurrence
It was confirmed that it fell below.

Next, when a convex lens was manufactured / processed from each of the test pieces and the spherical aberration of this lens was measured, the spherical aberrations of Comparative Examples 1 and 2 were about twice as bad as those of Example 1. Things were confirmed. Next, the setting of the virtual temperature of each of the test pieces is performed by The American Physical Society, Vol.28, No.
Laser Raman spectroscopy is used as described in 6, pp 3266-3721, September, 1983. First, to briefly explain the measurement method, first, as a comparative sample, an OH group of 5
A small piece (5 cm square, length 20 mm) of synthetic silica glass of about 00 (wt · ppm) was prepared, and this small piece was heated at 1200 ° C. for 2 hours and then rapidly cooled in water. Sample 1, and heated at 1000 ° C. for 20 hours. Post-quenched sample 2 produced at 900 ° C. for 120 hours and then sub-quenched sample 3 produced at 800 ° C. for 1200 hours then sub-quenched sample 4 produced by Raman spectrophotometer A range of 150 to 650 cm ^-1 is measured, and the following three peak areas are measured.

150-650 cm ^-1 (W1, peak area AW1), 470-520 cm ^-1 (D1, peak area AD1) 580-640 cm ^-1 (D2, peak area AD2) Next, from these three peak areas, D2 The ratio (I) of the peak areas of is calculated. I = {AD2 / (AW1-AD1-AD2)}

The relationship between this (I) and the fictive temperature is shown in a graph, and the fictive temperature is estimated from the (I) of the sample for which the fictive temperature is unknown as a standard line (calibration curve). When the fictive temperature of each of the test specimens was measured by this method, the fictive temperature was 1000 ° C. or higher in almost the entire area in Comparative Example 1, and the fictive temperature was 10 in the peripheral portion in Comparative Example 2.
There is a location of 00 ° C. or higher, and for Example 1, both the central region and the peripheral region have fictive temperatures within 850 to 950 ° C., and for Example 2, the central region and the peripheral region. Virtual temperature of all green areas is 900 ℃ -1000 ℃
It was confirmed that it was inside.

The content of each of the alkali metal elements Li, Na, K, the alkaline earth metal elements Mg, Ca and the transition metal elements Ti, Cr, Fe, Ni, Cu is analyzed for each of the above-mentioned test bodies. When,
All have a total content of Li, Na and K of 150 ppb or less, M
Total content of g and Ca is 100ppb or less, Ti, Cr, Fe, Ni
High purity was maintained with a total Cu content of less than 50 ppb.

[0025]

As described above, according to the present invention, even in a large-sized optical member whose thickness is set to 30 mm or more, the refractive index does not fluctuate, so that the imaging performance resulting from the fluctuation or the like occurs. It is possible to obtain an optical member in which the laser resistance does not deteriorate. And so on.

[Brief description of drawings]

FIG. 1 is a heat treatment temperature distribution diagram of an example (A) of the present invention and a comparative example (B).

Claims (2)

[Claims] 1. An optical member made of synthetic silica glass having a wall thickness of 30 mm or more containing 100 to 1000 (wt.ppm) of OH groups, a fictive temperature of 800 to 1000 ° C., and a homogeneity Δ.
Optical member characterized by setting n to 1 × 10 ⁻⁵ or less _2. The content of H ₂ gas molecules is 1 × 10 ¹⁶ molec.
The optical member according to claim 1, wherein the optical member is set to ules / cm ³ or more.