JPH0829960B2 - Ultraviolet laser optical components - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION “Industrial field of application” The present invention relates to high-power ultraviolet light of approximately 360 nm or less, more specifically KrF or ArF excimer laser light, YAG 4th harmonic laser light (266 nm) and other The present invention relates to an optical member for an ultraviolet laser that can function as a lens, prism, filter, window, mirror, etalon plate, or a semi-finished product of these members before final finishing, which is incorporated in various devices using high-power ultraviolet laser light.

"Conventional technology" In recent years, wavelength-converted ultraviolet lasers such as the excimer laser and the YAG 4th harmonic have been used in lithography technology for LSI manufacturing, technology utilizing photochemical reactions, processing technology for cutting and grinding, laser fusion. It is attracting attention as one used for technology.

The materials for lenses, prisms, filters, windows, mirrors, etalon plates, and fibers that control ultraviolet lasers by transmitting, transmitting, refracting, reflecting, absorbing, and interfering are magnesium fluoride, calcium fluoride, barium fluoride. Fluoride such as or silica glass can be used, but silica glass is most suitable from the viewpoint of workability, size, striae and homogeneity of refractive index.

However, the silica glass constituting the various optics described above undergoes significantly stronger optical damage than other ionizing radiations such as X-rays and γ-rays when light in the ultraviolet wavelength range of about 360 nm to about 160 nm acts. Cheap.

For example, when an ultraviolet laser is irradiated for a long time, the network structure of silica glass is cut, and an absorption band of about 215 nm called a so-called E'center and another absorption band of about 260 nm are generated, and a transmission band of about 360 nm to about 160 nm is transmitted. Rate, and the optical characteristics are deteriorated. Therefore, it is structurally very difficult to improve the durability of silica glass with respect to the laser in the above wavelength range.

Furthermore, pulsed lasers, especially KrF or ArF excimer lasers in the short ultraviolet region of approximately 250 nm or less, have the strongest energy as compared to all other types of ultraviolet light, and the irradiation of the excimer laser provides a stronger optical energy. It has been confirmed that it is easily damaged.

[Problems to be Solved by the Invention] In order to eliminate such drawbacks, the applicant of the present invention is SUPRASIL-of high purity and high homogeneity, which is free of striae in all directions, has no birefringence, and does not generate bubbles or fluorescence. P10, SUPRASIL-1, SUP
We have developed synthetic silica glass bodies such as RASIL-W1 (trade name, manufactured by Shin-Etsu Quartz Co., Ltd.).
It is effective for near-ultraviolet rays of 360 nm or more, but K in the short ultraviolet region of approximately 360 nm or less, especially in the short ultraviolet region of approximately 250 nm or less.
Irradiation with rF or ArF excimer laser was extremely damaging.

The present invention has been made in view of the above-mentioned drawbacks of the prior art, and an object thereof is to provide an optical member for an ultraviolet laser capable of guaranteeing laser resistance and high transmittance.

"Means for Solving the Problem" The present inventor has found that the presence of oxygen gas in silica glass has a strong influence on the resistance to ultraviolet laser, and other dissolved gases, such as oxygen if the concentration of hydrogen gas is the same. It was found that the higher the gas content concentration, the lower the UV laser resistance, and the higher purity and more homogeneous synthetic silica glass body, especially OH group
A high-purity synthetic silica glass body containing 10 ppm by weight or more is used as a starting base material, and the glass body is maintained in an inert gas atmosphere and under a high temperature and high pressure for a predetermined time, and is contained in the glass body. ₂ Gas is substantially removed, and more specifically, the O ₂ content in the glass body is approximately 2 × 10 ¹⁸ (molecules / m ² ) in terms of the amount of released oxygen molecules when the temperature is raised to 1000 ° C. in vacuum.
An optical member for ultraviolet laser set as follows is proposed.

In the optical member for ultraviolet laser such as the lens for ultraviolet laser, there is no publication before the application of the present application, which describes the relationship between the deterioration of the optical characteristics due to laser irradiation and the O ₂ content in the glass body.

Therefore, the knowledge that the optical glass body having a large amount of O _{2 in the} glass body is susceptible to the deterioration of the optical characteristics due to the laser light irradiation is the technology that only the applicant of the present invention knew. There is a novel matter, and further O ₂ content
Naturally, the combination of OH group contents is a novel invention, which is extremely difficult in its creation process.

In addition, high purity synthetic silica glass means Li, Na and K
Alkali metal content consisting of 150 ppb or less, alkaline earth metal content consisting of Mg and Ca 100 ppb or less, Ti, Cr, F
A glass body in which the transition metal content of e, Ni, and Cu is set to 50 ppb or less.

The synthetic silica glass body preferably has striae in the triaxial direction removed and has a refractive index variation width (Δn) of 2 × 10 5.
^-6 is preferably a glass body set below, further, the present invention, KrF or ArF excimer laser light, YA
It is effective for G4 harmonic laser light (266 nm), but is not limited thereto.

"Operation" Now, the reason why the presence of oxygen gas adversely affects the resistance to ultraviolet laser is a technology that only the applicant of the present invention knew for the first time, and it is a novel matter. Although it is not clear, it is assumed as follows.

That is, when the silica glass body is irradiated with ultraviolet rays, oxygen gas contained in the glass body absorbs ultraviolet energy to generate various exciters by a photochemical reaction, for example, O, O ₂ , and O ₃ exciters are generated, These exciters transfer energy to the oxygen element in the silica glass structure, and as a result, the network structure in the silica glass structure is cut and the E'center (E-prime center), NBOH center (Non-Bridge Oxygen Hall center), etc. Is considered to form an absorption band of.

For this reason, the present applicant has previously proposed a technique for repairing the absorption band by doping the glass body with hydrogen gas and reacting it with the E'center or NBOH center [Japanese Patent Application No. 1-145226]. ], And such a technique is also effective in improving the laser resistance, but such a technique is
This is a coping therapy that assumes the generation of O, O ₂ , O ₃ exciters.

Further, the present applicant has further proposed a technique for removing oxygen vacancies present in the network structure of the silica glass body as much as possible [Japanese Patent Application No. 63-21
No. 361 (JP-A-1-197343)] is also disclosed, but such a technique also suppresses the formation of the absorption band at the beginning of laser irradiation due to the absence of oxygen defects. Contained oxygen gas [oxygen as a molecule]
As long as the existence of the above exists, there is no change in the scheme that the O, O ₂ and O ₃ exciters are formed, and as a result, the reduction in laser resistance cannot be completely prevented.

Therefore, the present invention is, as a completely new technique not existing in conventional publications, for removing the generation of the exciter from the root,
The oxygen gas (oxygen as a molecule) contained in the silica glass body is substantially removed, and more specifically, the O ₂ content in the glass body is released as oxygen molecules at a temperature of 1000 ° C. in vacuum. The amount is set to about 2 × 10 ¹⁸ (molecules / m ² ) or less.

Incidentally, the removal of O ₂ gas contained in the glass body is quite difficult to remove only by heat treatment in a reducing atmosphere or a vacuum atmosphere, and therefore, the present invention provides the high-purity homogeneous synthetic silica glass body. In a so-called inert gas atmosphere containing a rare gas such as helium or argon, or a nitrogen gas, and is maintained and removed for a predetermined time by a hot isostatic pressing (HIP) process.

Further, containing an OH group in the glass body, more specifically
If 10 wtppm or more of OH group is contained, the existence of OH group
Along with maintaining high homogeneity during processing, it has a favorable effect on ultraviolet laser resistance.

The reason is that the OH group serves as the terminal part of the glass network structure and is a so-called network terminator. When this network terminator is present in an appropriate amount, the deviation of the distance between elements in the glass network structure and the twist of the bond angle between elements are relaxed and stabilized, and further, the hydrogen of the OH group is absorbed in the absorption band such as the E ′ center. It is presumed to act on repair.

[Example] First, the present inventor has removed striae in the three-axis directions, and the refractive index fluctuation width (Δn) in the light use region is 2 ×.
SUPRASIL that is less than 10 ^-6 and has a birefringence of less than 5 nm / cm
Of P-10, OH group is selected about 700 wtppm silica glass body, the silica glass body is cut and ground to a diameter of 50 ×
After making a test piece of ^t 20 mm, the test piece was placed in an electric furnace in which a tungsten heater was placed in a clean stainless steel jacket, and 800 ~ 800 in a vacuum atmosphere of 1 × 10 ^-2 (Toor) or less. After degassing while heating to 1000 ° C., it was cooled to room temperature. The purpose of this process is
In order to clearly confirm the effect of the present invention, the dissolved gas in the silica glass that affects the ultraviolet laser resistance is degassed under the same conditions, so that the same conditions are set for the dissolved gas as the starting material for the experiment. Not an industrial requirement.

Next, the test piece that was not heat-treated (No.
6), HIP treatment (hot isostatic pressing) in 100% argon gas atmosphere, 1000 ° C, 2000atm for 20hrs (No.1), helium gas under the same treatment conditions
One that was HIP-treated in 100% atmosphere (No.2), one that was HIP-treated in the atmosphere under the same treatment conditions (No.2).
3) In a clean electric furnace that has been degassed as above, treated at 1000 ° C, 1atm, 20hrs in a 100% helium atmosphere (No. 4), the same as No. 4 in an atmospheric gas atmosphere. Prepare the one that was processed (No. 5), then cut and polish these processed test pieces, 20 × 20 × ^t 1 mm oxygen gas emission measurement sample, 30 × 30 × ^t 10 mm A sample for evaluation of KrF excimer laser resistance was prepared.

In addition, the oxygen gas measurement was performed at 4 ° C./min at 1000 ° C. after placing the sample chamber in a quartz chamber in a vacuum atmosphere.
After heating up to 1000 ° C., the temperature is maintained for 2 hours. This is a method in which oxygen gas and other various gases released at that time are introduced into a quadrupole mass spectrometer and the amount released is measured.

Moreover, in the KrF excimer laser resistance evaluation, the energy density per pulse was set to 400 mJ / cm ² , p and continuous irradiation was performed at 100 Hz, while a transmittance meter was used to detect the E'center absorption band. By measuring the change with time of the transmittance (approximately 215 nm), the number of irradiation pulses until the internal transmittance of each sample decreased by 1%, and the irradiation pulse 1 × in the 30φ area in order to evaluate the homogeneity of the refractive index The temporal changes in the homogeneity of the refractive index distribution before and after irradiation with 10 ⁶ pulses were measured.

 The measurement results are shown in Tables 1 and 2.

As a result, among the samples, those subjected to HIP treatment in an atmosphere of argon and helium, both of which have a decrease in internal transmittance and the homogeneity of the change in refractive index, are substantially free of changes with time, and are preferable KrF excimer laser resistant. And the amount of released oxygen gas is about 1 x 10
^{It was 18} (molecules / m ² ) or less. (No. 1 and 2) Also, although there is some change with time in the case of heat treatment under normal pressure in a 100% helium atmosphere, the laser resistance is improved by several steps compared to the case of not performing heat treatment. It was confirmed that (No.4) On the other hand, in the case of HIP treatment in the atmospheric gas atmosphere, both of the internal transmittance and the homogeneity of the refractive index were significantly deteriorated (No.3), and the heat treatment was performed under the atmospheric pressure under the atmospheric pressure. It is understood that the laser resistance of the products is worse than that of No. 3.

Therefore, it was confirmed from the above experimental results that the improvement of the laser resistance is inversely proportional to the amount of released oxygen gas, that is, the gas contained in the silica glass body.

Next, the inventor of the present invention, in order to investigate what effect the amount of OH groups in the silica glass has on the ultraviolet laser resistance,
After preparing a high-purity silicon tetrachloride raw material as in the above example, the OH group content was adjusted to 3 by the CVD soot remelting method (soot method).
Weight ppm or less, about 10 wt.ppm, about 220 wt.ppm containing high-purity silica glass lumps are synthesized, and the glass lumps are cut and ground to prepare test pieces with a diameter of 50 × ^t 20 mm, and then the same as above. Degassing-cooling at room temperature.

Then, the test piece was subjected to HIP treatment by argon gas 1
About the thing (No.7,8,9) which performed 1000 degreeC and 2000atm20hrs in 00% atmosphere, it irradiated with KrF excimer laser similarly to the said Example, and the fall of internal transmittance and refractive index change When the change in homogeneity with time was measured, the OH groups were also correlated, and favorable results could not be obtained for the laser resistance of test piece No. 7 of 3 ppm by weight or less.

Incidentally, for each of the samples, by atomic absorption spectrophotometry and neutron activation analysis, when the impurity analysis was performed, the total of Li, Na, K is 150 wt.ppb or less, the total of Mg, Ca is 100 w.
It was less than t.ppb and the total of Ti, Cr, Fe, Ni, Cu was less than 50 wt.ppb, and high purity was maintained even by various heat treatments.

As a comparative sample, SUPRASIL-P3 with stronger striae than the triaxial direction and refractive index fluctuation range (Δn) of 1 × 10 ^-5 was used.
As a result of HIP treatment using 0 as a starting base material under the conditions of Ar gas 100%, 1000 ° C., 2000 atm, a material that could be used as an optical member was not obtained in terms of homogeneity of refractive index.

[Advantages of the Invention] As described above, according to the present invention, laser resistance and high transmittance are guaranteed without deterioration of optical characteristics and homogeneity of refractive index even when irradiated with an ultraviolet laser for a long time. It is possible to obtain a possible optical member for ultraviolet laser.

 It has various remarkable effects.

Claims (3)

[Claims] 1. An alkali metal content of Li, Na and K of 150 ppb or less, an alkaline earth metal content of Mg and Ca of 100 ppb or less, and a transition metal content of Ti, Cr, Fe, Ni and Cu. Consisting of a high-purity synthetic silica glass body set to 50 ppb or less, and containing 10 wt ppm or more of OH groups in the glass body, the O ₂ content in the glass body, oxygen at a temperature of 1000 ℃ in vacuum Approximately 2 × 10 ¹⁸ (molecules /
m ² ) An optical member for an ultraviolet laser, which is set to the following. 2. The highly homogeneous synthetic silica glass body is free from striae in the triaxial direction and has a refractive index fluctuation width (Δn) of 2 ×.
The optical member for ultraviolet laser according to claim 1, which is a glass body set to 10 ^-6 or less. 3. A high-purity, highly-homogeneous synthetic silica glass body is maintained in an inert gas atmosphere and hot isostatic pressing (HIP) treatment for a predetermined time to obtain O ₂ in the glass body. The content is approximately 2 × 10 ¹⁸ (molecules / m ² ) in terms of the amount of released oxygen molecules at a temperature rise of 1000 ° C in vacuum.
An optical member for an ultraviolet laser characterized by being set as follows.