JPH0323236A - Optical member for laser light - Google Patents

Abstract

PURPOSE:To impart excellent laser light resistance to the optical member for laser light over a long period by using a high-purity synthetic quartz glass material freed of oxygen defects and in which appropriate amts. of the OH group and occluded hydrogen are incorporated in the glass structure to form the member. CONSTITUTION:A high-purity synthetic quartz glass material contg. >=100ppm OH group, having no oxygen defects in its glass structure and in which gaseous hydrogen is occluded is prepared. The content of the gaseous hydrogen is appropriately set so that the number of the hydrogen molecules to be liberated when the glass material is heated to 1000 deg.C under vacuum is controlled to >=1X10<20> (molecules/m<2>). An optical member for laser light to be used for the laser light over the wavelength region at <= about 400 nm is made from the quartz glass material. A decrease in transmissivity, variations in refractive index, etc., are not caused in the optical member even if the member is irradiated with excimer laser light for a long time.

Description

[Detailed description of the invention] The invention relates to lenses, window members, mirrors, prisms, filters, etalon plates, and other optical systems for laser beams, and in particular specifies wavelengths of approximately 400 nm or less. Related to optical system components for laser beams used in wavelength castles. ``Prior technology'' In recent years, with the miniaturization and high integration of LSIs, for example, phosphorography technology for drawing circuit patterns on wafers has also been drawing submicron units. Although the development of technology is urgent,
Considering the recent steady progress in optical systems, light sources, photoresists, etc., it is presumed that optical lithography will become the mainstream, but one drawback of optical lithography is that the exposure wavelength is large, so the resolution is limited by diffraction. As a solution to this problem, shortening the wavelength of light is being considered. However, when ultraviolet rays of 400 ns or less are used to shorten the wavelength of light, the light transmittance of lenses using conventional optical glass decreases rapidly when the wavelength used is around 385 nm (iiJi). In other words, heat generation occurs due to light absorption, which disturbs the focal position and other characteristics of the lens. For this reason, it is possible to change the lens material from conventional optical glass to quartz glass and to use a laser beam with a narrow spectral width in order to prevent the occurrence of chromatic aberration when using quartz glass. The most complete type of laser is the excimer laser. However, the excimer laser light has extremely high power compared to conventionally used light sources, and κrF(
248ns), XeC (308nm). ArF(
When using a short wavelength laser beam with an oscillation wavelength of about 350 nm or less, such as 193 ns), even if the optical system member for the laser beam is made of quartz glass, if the laser beam is irradiated for a long time, Optical systems such as lenses are damaged, causing a decrease in transmittance, an increase in the absolute refractive index, fluctuations in the refractive index distribution, and fluorescence, which eventually leads to cracks, especially when the energy density is high. ．． One of the causes of the decrease in transmittance is said to be due to metal impurities present in the quartz glass, and for this reason, highly purified SiCl4 etc. are used in the optical system instead of using natural quartz as a starting material. using a silicon compound. Forming high-purity synthetic quartz glass while minimizing the contamination of metal elements,
We tried to eliminate the above-mentioned drawbacks by manufacturing lenses for laser beams using the laminated quartz glass as a base material, but we were unable to obtain satisfactory results as an optical system for high-output, short-wavelength laser beams. ．． Therefore, the applicant first formed the base material for manufacturing the lenses, etc., in other words, the laser optical system element, from high-purity synthetic quartz glass, and reduced the OH group content in the glass structure to 300PP. The applicant has proposed a laser optical system element set as described above (#Application No. 82-323882, hereinafter referred to as the first technology), and furthermore, the present applicant has proposed a synthetic quartz glass structure for forming the laser optical system element. We are proposing a laser optical system system that can achieve the desired effect even when the OH group content in the glass structure is reduced to 100pp by further reducing the impurity concentration contained therein. ．．
(Application filed on May 30, 1997, hereinafter referred to as the second prior art) ``Problem to be solved by the invention'' However, the laser optical system element subject to the above-mentioned OH group content and purity regulations cannot be solved by itself. It is true that laser resistance is improved, but it has been found that the desired effect cannot always be achieved when using this element to form lenses or other optical systems. Therefore, the present invention proposes that although the laser optical system elements themselves according to the prior art can achieve the desired laser resistance, they are used to form lenses and other optical system members. We discovered the reason why laser resistance deteriorates in some cases, and focused on this cause to create the present invention. In other words, the invention of wood. The purpose of the present invention is to provide an optical system member for laser light that further improves the laser performance without causing a decrease in transmittance or fluctuation in refractive index distribution even when irradiated with excimer laser light for a long time. ．． ``Means for Solving the Problems'' The process that led to the present invention will be explained step by step.■ First, the invention is based on a high-purity wood containing at least 100 ppm or more, preferably approximately 300 ppm or more. As mentioned above, the laser optical system member is formed using silica glass as the starting material. Improving ultraviolet resistance properties such as fluorescence properties, refractive index, and transmittance by increasing the OHTi concentration in the glass structure is a technique that the present inventors first discovered in the technique of the first prior application. Furthermore, in the second prior art, the impurity concentration contained in the silica glass structure is highly purified, more specifically, the metal element is set to 50 ρρb or less, and the OH group concentration is set to 100 ppm or more.
It is stated that it is possible to improve the lasing properties of chuji when irradiated with ultraviolet rays in the wavelength range up to 400 nm. ■However, optical system members for laser beams, including general lens members, are not made by cutting, polishing, or coating the base material as is to produce the desired lens, but by first heat-treating the base material to eliminate internal distortion. Since the desired lens is manufactured by the above-mentioned processing means after removing the above-mentioned materials, even if laser resistance is guaranteed on the starting base material side, defects that occur during the processing process may cause the finished part to fail. There are cases where the optical system components for laser light cannot achieve the desired laser properties. In particular, in the case of fused silica glass materials, the fusion methods and raw materials used for the synthesis vary widely, so even if heat treatment is performed under the same conditions, the formed glass structure will differ, and the surface will also differ. Unlike optical glass, the annealing point of quartz glass material is as high as 1120°C, so the heat treatment temperature must be set at least to a high temperature after the annealing point #. For this reason, it is expected that the structural changes caused by the heat treatment described above will cause various problems that will reduce the laser properties of the chu. ■The first is the FW1 problem of oxygen defects. When heat treatment is performed at a high temperature as described above, oxygen defects are generated or remain due to differences in the amalgamating method and heating atmosphere conditions, and the present inventors believe that the glass structure (Si
If a typical oxygen-deficient type defect shown by the following formula (■) or a typical oxygen-excess type defect shown by the following formula (2) is present in (02), the optical properties are easily deteriorated by laser beam irradiation! ! I lost it. (See Japanese Patent Application No. 83-21381.) The second feature of the present invention is that the optical system member is made of quartz glass material with substantially no oxygen defects in its glass structure. More specifically, if the starting base material contains oxygen-excess defects, heat treatment is performed in a reducing atmosphere, while if oxygen-deficient defects are present in the starting base material, heat treatment is performed in an oxidizing atmosphere. In both cases, the concentration of oxygen defects can be reduced, and oxygen defects can be substantially eliminated. The reason why the presence of oxygen defects has a negative effect on optical properties is not clear, but it is presumed to be due to the following reasons. That is, if oxygen defects exist in addition to impurities in the glass structure, the bonds between the elements that make up the glass structure become weaker than the bonds between the elements of ideal silica glass, and the laser beam Bonds are easily broken by energy, and it is presumed that the breaking of bonds between elements in silica glass causes structural changes and changes in the refractive index. Similarly, the presence of impurities or oxygen defects acts as a precursor and forms various color centers after laser light irradiation, resulting in a decrease in transmittance. It is thought that the fluorescent coffin length and intensity of the quartz glass during laser irradiation are determined, and this makes it easier for fluorescence to occur. "Substantially no oxygen defects" in Naoki's invention
means that when the concentration of defective oxygen atoms and the concentration of excess oxygen atoms in the glass structure are measured with reference to the Shelby (1138G) method, the measured values are below the detection limit, specifically for the ideal glass structure (Si02). In contrast, there is a shortage or an excess! *It is estimated that the number of elementary atoms in the glass Ig is approximately 10r1 or less. By the way, in the case of oxygen-excess type defects, the excessive oxygen atom concentration is 10r7 (glass 1
(per g) corresponds to about 3 ppm, and if it is 1019 pieces, it corresponds to about 300 ppm. Shelb here! (1980) method, the excess oxygen concentration of oxygen-excess defects is determined by measuring the infrared absorption of OH groups generated when reacting with hydrogen at high temperatures. Does the measurement of the missing oxygen concentration of S element-defective defects decrease when reacted with oxygen gas at high temperatures? ．． 8eV (1113n
m) is quantified by measuring the absorption peak. Oxygen deficiency defects can also be detected using the method described by Hosono et al.
This can also be done by detecting fluorescence (approximately 290 ns). Furthermore, the detection of oxygen-excess defects was performed by Nagasawa et al. (19El8).
This is also possible by measuring the ultraviolet transmittance of a quartz glass sample and detecting the presence of an absorption band with a resistance of 3.8 eV (approximately 325 n). Next, the second problem in heat treatment is The problem is the degassing of the absorbed hydrogen atoms. That is, as mentioned above, since the heat treatment temperature is high, the absorbed hydrogen atoms in the quartz glass structure are degassed during the heat treatment, and the hydrogen concentration decreases. It was confirmed that the performance of the chu laser, which had been maintained at a predetermined level or higher before the heat treatment, deteriorated due to a decrease in By performing heat treatment in a hydrogen gas atmosphere, hydrogen gas is occluded at a high concentration in the optical system member. Molecular release amount is at least IX 1020
(molecules/rn') or more. The amount of released hydrogen molecules can be measured by preparing a sample of the optical system member cut into 40 x 2 Q After raising the temperature to 1000℃, fi
Hold at 1000℃ for 2 hours. The various gases released at that time can be introduced into a quadrupole mass spectrometer and the amount released can be measured. ■The reason why the present invention is limited to optical system members for laser beams used for laser beams with specific wavelengths up to 400n■ is to take into consideration the stability of optical characteristics since the photon energy is small in the wavelength range of 400n■ or more. This is because there is no need. ``Experimental Example'' The light accident that led to the invention of wood will be explained based on a specific experimental example. After distilling the raw material silicon tetrachloride to remove impurities, we prepare high-purity silicon tetrachloride stored in a stainless steel container with a Teflon run, and use the high-purity silicon tetrachloride raw material to perform the direct method and CvD soot. By remelting fusion method,
A plurality of high-purity quartz glass ingots were synthesized, each of which was free from RMR in three directions and had a refractive index variation range (Δn) of 2XI04 in the optical use region. Then, from the ingot R, the content of OH groups is 2><5 PPI or less,
I5 (l ppm (soot method) 400 ppm (
An ingot with an OH group concentration of 100% (direct method) was selected. Next, the above-mentioned ingots each having a concentration of 0}1 group were placed in a quartz glass chamber in an atmosphere heating furnace, and the first group of ingots (realistic 1~2, ratio -1~3) was heated with argon gas. In an oxygen gas atmosphere diluted with
Heat treatment is performed at a temperature of around 000℃. Next, for the first ingot group (actual -z ~ 2, ratio -1), in a hydrogen gas H2 atmosphere with HC text added,
After holding each for a certain period of time at approximately 800 to 700℃,
Slow cooling was performed according to a certain program until the temperature reached 00°C or less, and then air cooling was performed. Then, for each ingot subjected to the heat treatment, alkali metal elements Li, Ha, K, alkaline earth metal elements M
ggCa and transition metal element Ti. Cr, Fe, Ni
When we analyzed the content of each element in Cu, we found that the alkali metal elements were 0.05 ppm or less, the alkali metal elements were around 0.01 ppm, and the transition metal elements were 0.01 ppm.
High purity was maintained at less than ppm. Then, a part of such an ingot was cut into a desired size, and the excess oxygen concentration and deficient oxygen concentration were measured based on the Shelby (+980) method, and the temperature was raised to 1000°C under vacuum based on the measurement method described above. The table below shows the results of measuring the amount of hydrogen molecules released at different times. Next, as a pseudo optical member, 40X
A test piece for excimer laser irradiation experiments was prepared by cutting it into a desired size of 30X tlo mra and mirror-finishing both sides.
8n■), and the energy density per pulse is 100.2
00,400 (mJ/crn"@pulse) and number of irradiation pulses IX105, IX 10'.IX
Irradiation was performed under certain irradiation conditions from a combination of 107 pulses. After the irradiation, each test piece was examined for changes in refractive index distribution using an interferometer, and for normalization using a transmittance meter.
Measure the fluorescence intensity using a fluorometer, and the results are shown in the table below. As can be understood from the ratio -1 in the table below, when the OH group content is spp■ or less, no oxygen defects are detected and the amount of hydrogen gas released is less than IXI020 (rinolecules/
m″) or more, the chu laser property is below the average level, and as shown in ratio-2 and ratio-3, when the OH group content is <10
Even if the concentration is 0 ppm or more and no oxygen defects are detected, the amount of hydrogen gas released is I x 102 (+ (mole
Cu! as/rrf), the laser resistance is still at the average level, and as shown in ratio-4, the OH group content is 100 pp or more and the amount of hydrogen gas released is lX
It was demonstrated that when the OH group content was more than 1020 (molecules/rn") and oxygen defects were detected, the laser resistance was poor. On the other hand, when the OH group content was IQQpp- or more, oxygen defects were detected. and the amount of hydrogen gas released is IX1020 (iol
The laser properties of the samples with a diameter of 1.2 m/m' or more were extremely high. These experimental results show that the effects of the wood invention are smoothly achieved. "Effects of the Invention" As described above, according to the present invention, by removing oxygen defects and effectively mixing OR groups and occluded hydrogen in a high-purity glass structure, excimer laser light can be irradiated for a long period of time. It is possible to obtain an optical system member for a laser beam that can further improve the laser performance without causing a decrease in transmittance or a change in the refractive index distribution even in the case of lithography. It is fully applicable not only to equipment and other highly integrated circuit manufacturing equipment, but also to laser optical system base materials used in laser fusion equipment and other high-power excimer lasers. It has various effects such as.

Claims (1)

[Scope of Claims] 1) In a laser beam optical system member used for laser light in the ultraviolet wavelength range of about 400 nm or less, the optical system member is made of high-purity synthetic quartz glass containing an OH group concentration of at least 100 ppm or more. An optical system member 2 for laser light, characterized in that the optical system member is made of a quartz glass material substantially free of oxygen defects in its glass structure and containing hydrogen gas. ) The hydrogen gas content in the quartz glass material is set such that the amount of hydrogen molecules released when the glass material is heated to 1000°C under vacuum is at least 1×10^2^0 (molecular
The optical system member for laser light according to claim 1), characterized in that the optical system member is set to be equal to or more than les/m^2).