JP2566151B2 - Method for manufacturing laser optical system base material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION “Industrial Application Field” The present invention relates to a lens, a window member, a mirror, which is used in a laser stepper device, a laser oscillator, a laser fusion device, and the like.
The present invention relates to a method for manufacturing a laser optical system base material such as a prism and a filter, and particularly to a method for manufacturing a laser optical system base material capable of ensuring durability and high quality with respect to laser light having a high output and a short wavelength range.

"Prior art" In recent years, development has rapidly progressed also in lithography technology for drawing an integrated circuit pattern on a wafer. For example, a pattern line width corresponding to 1 Mbit DRAM is 1 μm, and further 4 M
Pattern widths corresponding to bit DRAMs of 0.8 μm and technologies capable of drawing finer line widths are being developed. These fine line width drawing technologies are used for relatively high-intensity light sources, high-sensitivity resists, and stable It is generally performed by optical lithography because it has various conditions necessary for performing ultra-fine line width drawing such as complete optical materials. Therefore, there is a problem that the resolution is limited.

In order to solve such a problem, it is conceivable to increase the NA (numerical aperture) of the lens and other optical systems used for the photolithography and shorten the wavelength of light.

The high NA of the optical system has entered the era of exceeding NA (numerical aperture) 0.4, and a lens with NA 0.6 has been developed as a prototype. However, as the NA increases, the depth of focus becomes shallower. There is a limit to increasing the NA to improve the resolution.

 Therefore, the shortening of the wavelength of light will be considered next.

However, in order to shorten the wavelength of light, for example, approximately 400
When ultraviolet rays of μm or less are used, the light transmittance of the lens using the conventional optical glass decreases sharply from around the wavelength of 365 nm (i-line). In other words, light absorption and heat generation due to the light absorption. Occurs, and the focal position and other characteristics of the lens are disturbed.

It has been proposed to replace the lens material with quartz glass or fluorite in order to eliminate such drawbacks. However, when quartz glass or the like is used, it is very difficult to correct chromatic aberration due to the wide optical spectrum width of ordinary ultraviolet light. Is.

For this reason, it is considered to use a laser beam having a narrow spectrum width in combination with the quartz glass or the like, particularly an excimer laser which is a high-power pulse laser that oscillates mainly in the ultraviolet region, and such an excimer laser is advantageous in terms of oscillation efficiency and gas life. KrF (248 nm), XeCl (308 nm), ArF (193 nm) are preferred.

However, all the excimer lasers have a wavelength of 35
Because of the short wavelength of 0 nm or less, the uniformity of the refractive index of these optical materials is higher by one digit or more than that in the case of the g-line (436 nm) or i-line (365 nm), which is the wavelength of ultraviolet rays used in conventional mercury lamps. Δn≈1 × 10 ⁻⁷ to 1 × 10 ⁻⁶ , Δn: Refractive index fluctuation range) are required. Among the lens materials, for fluorite, the uniformity of the refractive index and the maximum dimension, There are still many problems in hygroscopicity and mechanical strength during processing.Therefore, as a laser optical system base material that can guarantee durability and high quality for laser light in the short wavelength range, other than quartz glass I can't find it.

[Problems to be Solved by the Invention] However, when a laser light source in the short wavelength range as described above is used, even if a laser optical system is manufactured using, for example, quartz glass, a high output pulsed light such as an excimer laser is used. Is irradiated for a long time, the quartz glass lens is damaged with time, distortion occurs and not only birefringence occurs, but also long-term irradiation with the laser beam causes a decrease in transmittance and an increase in absolute refractive index, This causes a problem that the refractive index distribution fluctuates and finally cracks occur. Particularly, for a quartz glass lens for lithography using an excimer laser, Δn of the refractive index distribution is 1
It is said that ^{x10 -6} or less is preferable, and when the optical properties of the quartz glass are changed as described above, the optical axis and the focus position of the lens are changed, and it becomes extremely difficult to form a fine and clear pattern.

Further, particularly when irradiated with short-wavelength laser light of 300 nm or less, fluorescence is generated even in silica glass having higher optical stability than conventional optical glass, and particularly in a projection exposure type apparatus such as an excimer laser stepper, Fluorescence from the lens and other optical systems reacts with the laser light on the photoresist on the wafer, making it difficult to form a clear pattern.

On the other hand, the change in optical properties such as a decrease in transmittance generated by the irradiation of laser light as described above and the generation of fluorescence are generally Li, Na, Mg, Al, K, C existing in the quartz glass structure.
a, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, etc., and is attributed to the impurity metal elements. Therefore, in the development of the photolithography, highly purified silicon compounds such as SiCl ₄ are used. By using the soot method or plasma method to form a synthetic quartz glass of extremely high purity while eliminating the mixture of metal elements as much as possible, and using the quartz glass material as a base material, the lens for short wavelength laser light, etc. is manufactured. However, although the above-mentioned drawbacks have been solved, satisfactory results have not been obtained as an optical system for high-power short-wavelength laser light.

In view of the above-mentioned drawbacks of the prior art, the present invention performs high-purity quartz glass material while subjecting the quartz glass material to a predetermined heat treatment, so that it has high durability and high durability against laser light of high output and short wavelength range. An object of the present invention is to provide a method for manufacturing a laser optical system base material capable of guaranteeing quality.

[Means for Solving the Problems] In order to achieve the above technical problems, the present invention provides, for example,
After synthesizing quartz glass by a soot method or a plasma method to form a high-purity quartz glass ingot, the high-purity quartz glass ingot is oxidized or reduced in one or more selected atmospheres. , A plurality of types of laser optical system preforms are formed by heat-treating the quartz glass block, and the energy density (J / cm ²
ulse) and short wavelength excimer laser light of the same wavelength range (248 nm) in which the total irradiation pulse number (pulse) is changed, and the fluorescence characteristics, transmittance, change in refractive index, and the presence of cracks are investigated. As a result, in the test piece,
It can reduce the fluorescence generation of the laser optical system used for short wavelength laser light and improve the stability of refractive index, transmittance, etc., and it is extremely useful as an optical system base material for high power lasers of 300 nm or less. Good results have been obtained.

Therefore, when the analysis was performed on the laser optical system base material from which the preferable result was obtained and the base material other than that, the base material from which the preferable result was obtained was quartz in comparison with the base material before the heat treatment. Oxygen defects existing in the glass structure (SiO ₂ ), specifically, in the synthetic quartz glass block synthesized by the soot remelting method, oxygen deficiency type defects represented by the following formula It was confirmed that oxygen excess defects represented by the following formula were substantially removed from the glass gob.

Here, the synthetic quartz glass block synthesized by the soot remelting method is, for example, SiCl ₄ or other high-purity silicon compound,
After producing a porous silica glass body (called silica glass soot body or simply soot body) by oxyhydrogen hydrolysis in a flame, the porous silica glass body is melted and solidified at a temperature of about 1500 to 1700 ° C. On the other hand, the synthetic quartz glass material synthesized by the plasma synthesis method means a synthetic quartz glass material obtained by decomposing and oxidizing a high-purity silicon compound in a plasma flame.

Therefore, the present invention has been made on the basis of the above-mentioned findings and experimental results, and is characterized by the fact that after forming a high-purity quartz glass ingot to be a base material of a laser optical system, it is oxidized or reduced. The point is that the quartz glass ingot is subjected to heat treatment in any one or a plurality of selected atmospheres to substantially remove oxygen defects existing in the quartz glass ingot structure.

That is, specifically, in the first invention, after a high-purity quartz glass ingot to be a base material of a laser optical system is synthesized by a plasma synthesis method, the synthetic quartz glass ingot is heat-treated in a reducing atmosphere. By this, the oxygen defects existing in the quartz glass ingot structure are substantially removed.

In the second invention, after the high-purity quartz glass ingot to be the base material of the laser optical system is synthesized by the soot remelting method, the synthetic quartz glass ingot is heat-treated in an oxidizing atmosphere to obtain the quartz glass. It is characterized in that the oxygen defects existing in the lump structure are substantially removed.

In the third invention, after a high-purity quartz glass ingot to be a base material of a laser optical system is synthesized by a soot remelting method, the synthetic quartz glass ingot is heat-treated in an oxidizing atmosphere, and then in a reducing atmosphere. It is characterized in that the oxygen deficiency existing in the quartz glass lump structure is substantially removed by the heat treatment in step 1.

That is, according to the third aspect of the present invention, most of the excess oxygen is converted to OH groups by subjecting oxygen-deficient quartz glass to oxidative heat treatment, changing it to quartz glass having some oxygen-excessive defects, and then heat-treating it in a hydrogen atmosphere. Thus, the excimer laser resistance is improved.

It is desirable to arbitrarily select the temperature of heat treatment between 500 and 1500 ° C, and it becomes difficult to remove oxygen defects at a temperature lower than 500 ° C, while the effect is further improved if the temperature is higher than 1500 ° C. However, the disadvantage of deforming the glass lumps occurs.

The reason why the presence of oxygen defects adversely affects the optical properties 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 glass, and the energy of the laser light causes It is presumed that the bonds are easily broken, and the bonds between the elements of the quartz glass are broken, causing a density change and changing 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.

And in order to achieve the present invention smoothly, the preferred degree of reduction of oxygen defects is based on the experimental results described in the following examples, the concentration of deficient oxygen atoms in the glass structure with reference to the following Shelby (1980) method. And, when the concentration of excess oxygen atoms is measured, the measured value is below the detection limit, specifically, the number of oxygen atoms insufficient or excessive with respect to the ideal glass structure (SiO ₂ ) is
It is presumed that it is preferable that there are approximately 10 ¹⁷ or less in 1 g of glass.

By the way, in the case of oxygen excess type defects, excess oxygen atom concentration 10
¹⁷ pieces (per 1g of glass) correspond to about 3ppm, and this is 10 ¹⁹
When it is an individual, it corresponds to about 300 ppm.

Oxygen-excessive defects can be detected by measuring the increase in the infrared absorption peak of the OH group generated when they are reacted with hydrogen at a high temperature, and oxygen-deficient defects can be detected in the vacuum ultraviolet region of 7.6 eV (163
(nm) absorption peak and the 7.6 eV absorption peak that decreases when reacted with oxygen at high temperature can be detected.

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

After the raw material silicon tetrachloride is distilled to remove impurities, high-purity silicon tetrachloride stored in a stainless steel container with Teflon lining is prepared, and an argon plasma synthesis method is performed using the high-purity silicon tetrachloride raw material. By using the CVD soot remelting synthesis method, two kinds of high-purity quartz glass lumps are respectively synthesized. In this case, in the quartz glass block,
The content of metallic element impurities such as Fe, Mg, Al, Li, Na, K, Ca, Ti, Cr and Cu was below the detection limit in both soot method and plasma method.

Next, among the plurality of quartz glass ingots of each of the above-mentioned two types, some quartz glass ingots are left and other quartz glass ingots are sequentially installed in a quartz glass chamber in an atmosphere heating furnace, and an argon gas or a chip is used. While selectively changing the concentration of oxygen gas or hydrogen gas diluted with elementary gas and the heat treatment temperature, some quartz glass ingots were heat-treated at about 1000 ° C. in an oxidizing atmosphere, or similarly heat-treated. The quartz glass ingot of this part was heat-treated in a reducing atmosphere by heat treatment at about 1000 ° C.

Then, the synthetic quartz glass block after the heat treatment and before the heat treatment thus formed was cut into a size of 30 × 20 × 10 mm, and double-sided mirror finishing was performed to obtain test pieces for excimer laser irradiation experiment 9 times each. Create one.

Then, for each of these 9 test pieces, 248 nm (KrF)
Irradiation conditions consisting of energy density per pulse of 200,400,600 (mJ / cm ² · pulse) and number of irradiation pulses 1 × 10 ⁴ , 1 × 10 ⁵ , 1 × 10 ⁶ (pulse) Irradiation was performed.

Then, for each test piece after the end of irradiation, the refractive index distribution change with an interferometer, the solarization with a transmittance meter, the fluorescence intensity was measured with a fluorescence measuring device, and the results are shown in the following experimental result list. Show.

As can be understood from the table below, the quartz glass ingots synthesized by the plasma method had oxygen-excessive defects, and the excimer laser resistance was not favorable, but after that, the quartz glass ingots were reduced. It was possible to significantly improve the excimer laser resistance by carrying out heat treatment in a strong atmosphere and substantially removing the oxygen defect concentration below the detection limit. However, the sample heat treated in an oxidizing atmosphere,
It was revealed that the oxygen defect concentration was increased and the excimer laser resistance was significantly deteriorated (Experimental Examples 1 to 4)).

Next, in the quartz glass ingot synthesized by the soot remelting method, oxygen deficiency type defects were present, and the excimer laser resistance was not favorable, but after that, the quartz glass ingot was exposed to an oxidizing atmosphere. It was possible to significantly improve the excimer laser resistance by heat treatment to substantially remove the oxygen defect concentration. However, it was revealed that, in the sample heat-treated in the reducing atmosphere, the oxygen defect concentration was increased and the excimer laser resistance was deteriorated (Experimental Example 5).
7)).

Further, as shown in Experimental Example 8), the quartz glass ingot synthesized by the soot remelting method was heat-treated once in an oxidizing atmosphere and then in a reducing atmosphere to reduce the oxygen defect concentration to below the detection limit. It was confirmed that the excimer laser resistance is improved by the effective removal.

This is presumed as follows. That is, the oxygen-deficient quartz glass is subjected to an oxidative heat treatment to change it to a quartz glass having some oxygen-excessive defects, and then heat-treated in a hydrogen atmosphere to change most of the excess oxygen into an OH group so that the excimer resistance is high. It is considered that the laser property is improved.

"Effects of the Invention" As described above, according to the present invention, by using a high-purity silica glass material and performing a special treatment on the silica glass material, a laser beam of high output and in a short wavelength range can be obtained. It is possible to provide a method for manufacturing a laser optical system base material capable of guaranteeing durability and high quality, and a laser optical system base material manufactured according to the present invention can be used only in a lithography apparatus or other highly integrated circuit manufacturing apparatus. Of course, the present invention is sufficiently applicable to laser fusion devices and other laser optical system base materials used in high-power excimer lasers.

It has various remarkable effects.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References Japanese Patent Publication No. 42-22634 (JP, B2) Japanese Patent Publication No. 61-45824 (JP, B2) Institute of Electrical Engineers of Japan Material Optical and Quantum Device Study Group (1985-7- 22) OQD-85-48, P. 15-19 "Glass Handbook" First Edition (Sep. 50, September 30) Asakura Shoten P.P. 1005

Claims (3)

(57) [Claims] 1. A method of manufacturing a laser optical system base material used for laser light in a specific wavelength range of about 400 nm or less, wherein a high-purity quartz glass ingot to be a base material of the laser optical system is synthesized by a plasma synthesis method. Then, the synthetic quartz glass gob is subjected to a heat treatment in a reducing atmosphere to substantially remove oxygen defects existing in the structure of the quartz glass gob, and a method for producing a laser optical system preform. 2. A method of manufacturing a laser optical system base material used for laser light in a specific wavelength range of about 400 nm or less, wherein a high-purity silica glass block to be a base material of the laser optical system is synthesized by a soot remelting method. After that, the synthetic quartz glass ingot is subjected to heat treatment in an oxidizing atmosphere to substantially remove oxygen defects existing in the structure of the quartz glass ingot. Method 3. A method of manufacturing a laser optical system base material used for laser light in a specific wavelength range of about 400 nm or less, wherein a high-purity quartz glass block to be a base material of the laser optical system is synthesized by a soot remelting method. After that, the synthetic quartz glass ingot is heat-treated in an oxidizing atmosphere, and then further heat-treated in a reducing atmosphere to substantially remove oxygen defects existing in the structure of the quartz glass ingot. For manufacturing laser optical system base material