JPH0624997B2 - Optical components for laser light - Google Patents

Description

The present invention relates to a lens, a window member, a mirror, a prism, a filter, an etalon plate, and other optical systems for laser light, and particularly in a specific wavelength range of about 400 nm or less. The present invention relates to a laser beam optical system member used.

"Conventional technology" With the recent miniaturization and high integration of LSIs, development of submicron drawing technology is urgently required for lithography technology for drawing circuit patterns on wafers, for example. It is estimated that optical lithography will be the mainstream from the steady progress of light sources, photoresists, etc., but the drawback of optical lithography is that it has a large exposure wavelength, which limits the imagination due to diffraction. As a solution, shortening the wavelength of light is being studied.

However, when ultraviolet rays of 400 nm or less are used to shorten the wavelength of light, the light transmittance of the lens using conventional optical glass drops sharply from around the working wavelength of 354 nm (i-line). In other words, heat is generated due to light absorption, and the focal position and other characteristics of the lens are disturbed.

For this reason, it is conceivable to replace the conventional optical glass with quartz glass as the lens material and to use a laser beam having a narrow spectrum width in order to prevent the occurrence of chromatic aberration when quartz glass is used. The excimer laser has the highest degree of perfection.

However, the excimer laser light has extremely large power as compared with the conventionally used light source, and the KrF (248n
m), XeCl (308 nm), ArF (193 nm) or the like using a short-wavelength laser light with an oscillation wavelength of about 350 nm or less, even if quartz glass is used for the laser light optical system member. When the laser light is irradiated for a long time, the optical system such as a lens is damaged, and the transmittance is decreased, the absolute refractive index is increased, the refractive index distribution is changed, and fluorescence is generated. There is a problem that a crack is generated.

It is said that one of the causes of the decrease in the transmittance and the like is due to the metal impurities present in the quartz glass. Therefore, the optical system was highly purified without using natural quartz as a starting base material. A silicon compound such as SiCl ₄ is used to form a high-purity synthetic quartz glass while eliminating the mixture of metal elements as much as possible, and a lens for laser light is manufactured using the synthetic quartz glass as a base material. Although it was attempted to solve the problem, a satisfactory output as an optical system for high power and short wavelength laser light could not be obtained.

Therefore, the present applicant has previously formed a base material for manufacturing the lens and the like, in other words, forming the laser optical system element body with high-purity synthetic quartz glass, and the OH group content in the glass structure is 300 ppm or more. (Japanese Patent Application No. 62-323882, hereinafter referred to as the first prior art), and the applicant of the present invention further proposes that the laser optical system element body in the synthetic quartz glass structure forming the laser optical element body is It proposes a laser optical system element body capable of achieving a desired effect even when the OH group content in the glass structure is relaxed to 100 ppm by further reducing the impurity concentration contained in the glass structure. (Applied on May 30, 1989, hereinafter referred to as second prior art) "Problems to be solved by the invention" However, the laser optical system element with the OH group content and purity regulated is certainly in itself. It was found that although the laser resistance is improved, the desired effect cannot always be achieved when a lens or other optical system is formed using the element body.

Therefore, in the present invention, although the laser optical system element body according to each of the above prior arts can obtain desired laser resistance, lenses and other optical system members are formed using the element body. In this case, the reason why the laser resistance deteriorates was found, and the present invention was created by focusing on the cause.

That is, the present invention provides an optical system member for laser light, which is further improved in laser resistance without causing a decrease in transmittance and a change in refractive index distribution even when irradiated with excimer laser light for a long time. To aim for things.

[Means for Solving the Problems] The process leading to the present invention will be described step by step.

First, as described above, the present invention is characterized in that the laser optical system member is formed by using a high-purity synthetic quartz glass material having an OH group concentration of at least 100 ppm or more, preferably about 300 ppm or more as a starting base material.

It is a technique that the present inventor has previously found in the first prior art to improve ultraviolet resistance characteristics such as fluorescence characteristics, refractive index, and transmittance by increasing the OH group concentration in the glass structure. Further, in the second prior application technology, the concentration of impurities contained in the quartz glass structure is highly purified, more specifically, by setting the metal element to 50 ppb or less and the OH group concentration to 100 ppm or more, 400 nm It is described that it is possible to improve the laser resistance when irradiated with ultraviolet rays in the wavelength range up to.

However, the laser beam optical system member including a general lens member does not cut and polish or coat the base material as it is to manufacture a desired lens, but heat-treats the base material once to eliminate internal strain or the like. Since the desired lens is manufactured by the above-mentioned processing means after the removal, even if the laser resistance is guaranteed on the base material side, the laser beam as a completed part is not produced due to a defect that occurs during the processing process. There is a case where the optical system member for use cannot obtain desired laser resistance.

In particular, in the case of a synthetic quartz glass material, there are a wide variety of synthesizing methods and raw materials used for the synthesizing. Therefore, for example, even if the heat treatment is performed under the same conditions, the glass structure formed is different, and In contrast, in the case of a quartz glass material, the annealing point is as high as 1120 ° C, and therefore the heat treatment temperature must be set to at least a high temperature around the annealing point and the heat treatment must be performed for these reasons. It is expected that various problems that reduce the laser resistance will occur due to the composition change caused by the treatment.

The first is the problem of oxygen defects.

When heat treatment is carried out at a high temperature as described above, oxygen defects are generated or remain due to the difference in the synthesis method and heating atmosphere conditions, and the present invention is represented by the following formula in the glass structure (SiO ₂ ) of the synthetic quartz glass. It has been found that the presence of the representative oxygen-deficient type defect shown or the representative oxygen-excessive type defect represented by the following formula is likely to cause deterioration of optical characteristics due to laser light irradiation. (See Japanese Patent Application No. 63-21361) Therefore, the second characteristic of the present invention is that the optical system member is a quartz glass material in which oxygen defects are substantially absent in the glass structure. That is, more specifically, when the starting base material has oxygen-excessive defects, heat treatment is performed in a reducing atmosphere, while when the starting base material has oxygen-deficient defects, heat treatment is performed in an oxidizing atmosphere. The oxygen deficiency concentration can be reduced, and the oxygen deficiency can be substantially removed.

The reason why the presence of oxygen defects adversely affects the optical characteristics is not critical, but it is presumed that the reason is as follows.

That is, in the glass structure, if oxygen defects are present in addition to impurities, the bond between the elements constituting the glass structure becomes weaker than the bond between the elements of the ideal quartz gas, and the energy of the laser light causes The bond is easily broken,
It is presumed that the breaking of the bonds between the elements of the quartz glass causes a structural change and changes the refractive index. Similarly, the presence of impurities or oxygen vacancies serves as a precursor to form various color centers after laser light irradiation, resulting in a decrease in transmittance. Further, the presence of impurity elements and the formation of the color centers causes laser irradiation. It is considered that the fluorescence wavelength and intensity of the quartz glass inside are determined, and this facilitates the generation of fluorescence.

In the present invention, “substantially no oxygen deficiency”
And Shelby (1980.Journal of Applied Physics, Vol. 5
1, pp2589 to 2593) method, the measured value is below the detection limit when the deficient oxygen atom concentration and the excess oxygen atom concentration in the glass structure are measured, specifically, the ideal glass structure (SiO ₂ ) In contrast, if the number of oxygen atoms is insufficient or excessive, the glass
It is presumed that 1g or less is preferably 10 ¹⁷ or less. By the way, in the case of oxygen excess defects, excess oxygen atom concentration
10 ¹⁷ pieces (per 1 g of glass) correspond to about 3 ppm, and 10 ¹⁹ pieces correspond to about 300 ppm.

Here, the measurement of the excess oxygen concentration of the oxygen excess type defect by the Shelby (1980) method is to measure and quantify the infrared absorption of the OH group generated when reacting with hydrogen at high temperature, and the The deficient oxygen concentration is measured and measured by measuring the absorption peak at 7.6 eV (163 nm), which decreases when reacted with oxygen gas at high temperature.

The detection of oxygen-deficient defects is performed by Hosono et al. (1987), that is, a KrF excimer laser is applied to a quartz glass sample.
It is also possible by detecting the fluorescence of 4.3 eV (approx. 290 nm) generated when irradiated with (248 nm).

Furthermore, oxygen-rich defects can also be detected by the method of Nagasawa et al. (1988), that is, by measuring the ultraviolet transmittance of a quartz glass sample and detecting the presence of an absorption band of 3.8 eV (about 325 nm). .

Next, the second problem in heat treatment is the problem of degassing stored hydrogen.

That is, as described above, since the heat treatment temperature is high, the stored hydrogen in the quartz glass structure is degassed during the heat treatment, and due to the decrease in the hydrogen concentration, the hydrogen concentration is higher than a predetermined level before the heat treatment. It was confirmed that the laser resistance performance maintained at was reduced.

Therefore, a second feature of the present invention is that a high-concentration hydrogen gas is occluded in the optical system member by performing a heat treatment in a hydrogen gas-containing atmosphere, and more specifically, the optical system. The point is that hydrogen gas is contained so that the amount of hydrogen molecules released at a temperature of 1000 ° C. under vacuum of the member is at least 1 × 10 ²⁰ (molecules / m ² ).

The measurement of the amount of released hydrogen molecules is performed by, for example, the optical system member 40.
Prepare a sample cut into × 20 × t1mm and mirror-finished on both sides, place a vacuum atmosphere in the quartz chamber in which the sample is set, raise the temperature to 1000 ° C at 4 ° C / min, and then increase the temperature to 1000 ° C. Hold for 2 hours. The various gases released at that time may be introduced into a quadrupole mass spectrometer to measure the released amount.

In addition, hydrogen gas is contained so that the amount of released hydrogen molecules at a temperature rise of 1000 ° C. under vacuum is at least 1 × 10 ²⁰ (molecules / m ² ) or more means that the hydrogen molecule concentration is at least 5 × 10 ¹⁶ This is equivalent to containing hydrogen gas so that the concentration is higher than (molecules / m ³ ) concentration.

The reason why the present invention is limited to the optical system member for laser light used for laser light of a specific wavelength range up to 400 nm is that the wavelength range is
This is because it is not necessary to consider the stability of optical characteristics because the photo energy is small above 400 nm.

[Examples] The process leading to the present invention will be described based on specific experimental examples.

Prepare high-purity silicon tetrachloride stored in a stainless steel container with Teflon running after removing the impurities by distilling the raw material silicon tetrachloride. By the melt synthesis method,
A plurality of high-purity silica glass ingots, each of which is free of striae in three directions and whose fluctuation range (Δn) in the light use region is set to 2 × 10 ^−6, were synthesized. Then, an ingot having an OH group content of 5 ppm or less and an OH group concentration of 150 ppm (soot method) and 400 ppm (direct method) was selected from the ingot group.

Next, the ingots having the respective OH group concentrations were set in a quartz glass chamber in an atmosphere heating furnace, and diluted with argon gas in the first ingot group (actual -1 to 2, ratio -1 to 3). In the oxygen gas atmosphere described above, or in the second ingot group (ratio -4), heat treatment is performed at a temperature of about 1000 ° C. in a hydrogen gas atmosphere.

Next, about (first -1 to 2, ratio -1) of the first ingot group, about 1 to ₂ respectively in a hydrogen gas H ₂ atmosphere containing HCl.
After holding at 600-700 ° C for a certain period of time, it was then gradually cooled by a certain program until the temperature fell below about 200 ° C, and then air cooling was performed.

Then, for each ingot subjected to the heat treatment, the content analysis of each element of alkali metal elements Li, Na, K, alkaline earth metal elements Mg, Ca and transition metal elements Ti, Cr, Fe, Ni, Cu will be performed. In each case, the alkali metal element is 0.05 ppm or less,
Alkaline earth metal element is around 0.01ppm, transition metal element is
High purity of 0.01ppm or less was maintained.

Then, by cutting a part of such an ingot to a desired size, the excess oxygen concentration and the deficient oxygen concentration are raised based on the Shelby (1980) method, and the temperature is raised to 1000 ° C. under vacuum based on the above-mentioned measurement method. The results of measuring the amount of released hydrogen molecules at that time are shown in the table below.

Next, 40 × 30 × t1 from each ingot as a pseudo optical member
An excimer laser irradiation test piece was cut to a desired size of 0 mm and mirror-finished on both sides, and a KrF excimer laser (248 nm) was used for the test piece to obtain an energy density per pulse of 100,200,400 (mJ / cm ² Pulse) and the number of irradiation pulses 1 × 10 ⁵ , 1 × 10 ⁶ , and 1 × 10 ⁷ (pulse).

Then, for each test piece after the end of irradiation, a refractive index distribution change with an interferometer, solarization with a transmittance meter,
The fluorescence intensity was measured with a fluorescence measuring instrument, and the results are shown in the table below.

As can be seen from the ratio-1 in the table below, the OH group content is 5
In the case of ppm or less, no oxygen defect is detected, and the laser resistance is below the average level even if the amount of released hydrogen gas is 1 × 10 ²⁰ (molecules / m ² ) or more. As shown in -3, even if the OH group content is 100 ppm or more and no oxygen defects are detected, the hydrogen gas release rate is 1 × 10 ²⁰ (mol
ecules / m ² ) or less, the laser resistance is still at an average level, and the OH group content is 100 ppm as shown in ratio -4.
Above, and the amount of hydrogen gas released is 1 × 10 ²⁰ (molecules / m ² )
Even in the above cases and when oxygen defects were detected, it was proved that the laser resistance was poor.

On the other hand, the laser resistance of the materials having an OH group content of 100 ppm or more, no oxygen defects detected, and a hydrogen gas release amount of 1 × 10 ²⁰ (molecules / m ² ) or more was extremely high.

From these experimental results, it can be seen that the effects of the present invention are smoothly achieved.

"Effects of the Invention" As described above, according to the present invention, excimer laser light is irradiated for a long time by removing oxygen defects and effectively mixing OH groups and stored hydrogen in a high-purity glass structure. Even in such a case, it is possible to obtain an optical system member for laser light which can further improve laser resistance without a decrease in transmittance and a change in refractive index distribution, and thus, the laser optical system member according to the present invention is formed by lithography. The present invention is sufficiently applicable not only to the apparatus and other highly integrated circuit manufacturing apparatus, but also to laser fusion apparatus and other laser optical system base materials used for high-power excimer lasers.

It has various remarkable effects.

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location // H01S 3/137 8934-4M

Claims (2)

[Claims] 1. An optical system member for laser light used for laser light in an ultraviolet wavelength range of about 400 nm or less, wherein the optical system member is formed of a high-purity synthetic quartz glass material containing an OH group concentration of at least 100 ppm or more. In addition, the optical system member is a quartz glass material containing substantially no oxygen defects in its glass structure and containing hydrogen gas. 2. The content of hydrogen gas in the quartz glass material is such that the amount of released hydrogen molecules when the glass material is heated at 1000 ° C. under vacuum is at least 1 × 10 ²⁰ (molecules / m ² ) or more. The optical system member for laser light according to claim 1), characterized in that