JPH02238401A - Achromatic lens for ultraviolet ray - Google Patents

Abstract

PURPOSE:To prevent the decrease of transmittance and the generation of fluorescence by constituting the above lens of two kinds of glass; high-purity quartz glass having >=99.9% purity and quartz glass contg. <=50wt.% aluminum oxide. CONSTITUTION:A triplet lens is produced by using the high-purity quartz glass having >=99.99% purity for convex lenses 1, 3 and quartz glass contg. <=50wt.% aluminum oxide for a concave lens 2. The achromatic lens formed by using the quartz glass added with the aluminum is smaller in the value of chromatic aberration and has a better chromatic aberration correcting effect as compared to a monochromatic lens system. Working is easy and polishing with high accuracy is possible. The optical uniformity is enhanced and the durability is improved. The formation of the lens to larger aperture diameters is possible.

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention is applicable to a reduction projection exposure apparatus (stepper) used in ultraviolet lithography, etc., such as XcCI, Kr
This invention relates to optical achromatic lenses used in excimer laser steppers such as P. [Prior Art] Steppers using ultra-high pressure mercury lamps as light sources have already been used for photolithography in semiconductor manufacturing. However, in recent years, as LSIs have become more highly integrated, the resolution of the G-line (436rv) and i-line (3C5nge) of the ultra-high pressure mercury lamp used in conventional steppers has become insufficient. Therefore, in order to increase the resolution, XcCl, which has a shorter wavelength,
Progress is being made in the development of steppers that use excimer laser light as a light source. However, in order to obtain the desired resolution with these steppers, it is necessary to remove chromatic aberration. Currently, two methods have been proposed for removing this chromatic aberration in excimer lasers. One method is to narrow the band of the optical certificate's laser light to keep chromatic aberration within acceptable limits, and the other method is to
This is a method of correcting chromatic aberration by using an achromatic lens in the optical system. In the case of the first method of narrowing the half-width (linewidth) of the laser beam, so-called band narrowing, to suppress chromatic aberration within acceptable limits, etalon. Using elements and methods such as prisms and injection locking, the line width of the laser beam can be reduced to 0.
003~0. The band is narrowed to 0 0 5 +1-. However, narrowing the band causes some problems. For example, in order to compensate for the decrease in laser output due to losses caused by the elements, the output of the laser must be increased, and therefore the laser generator itself must also be increased in size. Further, problems such as speckle patterns are likely to occur and it is difficult to increase the irradiation area occur. Another method of correcting chromatic aberration by using an achromatic lens in the optical system has the problem that materials that can efficiently transmit excimer laser light are limited. To make an achromatic lens, two types of optical materials with different dispersions are required, and a combination of high-purity silica glass and calcium fluoride single crystal (fluorite) has been proposed. In addition, oxides of transition elements such as titanium and iron,
An achromatic lens has been proposed by combining a synthetic silica glass lens doped with oxides of rare earth elements such as lanthanum, cerium, and europium, and synthetic silica glass without additives, taking advantage of the difference in dispersion power between the two. (Tokkai Sho (i3-
8512, etc.). [Problem to be Solved by the Invention'XJ] However, when attempting to use fluorite as an optical polishing material such as a lens, it has various disadvantages compared to glass. First of all, fluorite has a hardness of at <<<, so it is easily scratched, and optical polishing is not easy. In addition, it has some solubility in water and is inferior in durability. Furthermore, it is difficult to obtain high transmittance for light in the ultraviolet region, and the mechanical strength is also poor. Furthermore, it is difficult to increase the diameter. As described above, fluorite has a number of drawbacks when considered as an optical material, so it is difficult to use it to fabricate large-diameter optical lenses with a diameter of 100 cm or more. That is extremely difficult. For these reasons, excimer laser steppers equipped with an achromatic lens in their optical systems have not yet reached the practical stage. On the other hand, when using a lens made of synthetic silica glass containing oxides of transition elements or rare earth elements, these additives not only do not provide a sufficiently large change in the refractive index, but also act as a source of ultraviolet absorption. This causes a decrease in transmittance and generation of fluorescence. For this reason, it is inconvenient as an achromatic lens for ultraviolet rays, and it is preferable to remove it as much as possible. Therefore, an object of the present invention is to provide a novel achromatic lens for ultraviolet rays that does not have the above-mentioned problems. [Means for Solving the Problems] In view of the above problems, as a result of intensive research, the present inventors have discovered that it is possible to freely change the refractive index and dispersion of aluminum-containing quartz glass by adjusting the amount of aluminum oxide added. We discovered that not only is this possible, but it also does not have the above-mentioned drawbacks found in silica glass, which contains fluorite and transition element oxides as optical materials, making it possible to produce large-diameter, high-precision achromatic lenses. However, we have arrived at the present invention. That is, the present invention is characterized in that it is composed of two types of glass: (^) high-purity quartz glass with a purity of 99.9% or more and (11) quartz glass containing 50% by weight or less of aluminum oxide. Found in achromatic lenses for ultraviolet rays. Also,
The achromatic lens for ultraviolet rays of the present invention is characterized in that it can efficiently transmit ultraviolet rays with a wavelength shorter than 300I1-, and can be used in the optical system of an excimer laser reduction projection exposure apparatus. The present invention will be explained in detail below. The first feature of the present invention is to provide a novel achromatic lens for ultraviolet rays, which is constructed from lenses made of silica glass doped with aluminum oxide and silica glass without dopant. Generally, in order to construct an achromatic lens, two lenses with different dispersion are required.
It is necessary to combine lenses made of different materials. The present inventors have discovered that aluminum oxide-doped silica glass is suitable as a material to be combined with quartz glass. In silica glass systems, the addition of aluminum oxide increases both the refractive index and dispersion. For example, when 5% by weight of aluminum oxide is added to silica glass, the refractive index for light with a wavelength of 630 nm increases by approximately 0.5% compared to that without addition, and the dispersion value ratio with that of silica glass without addition is ΔAl-8102. /ΔStO is approximately 1.0
It becomes 7. Therefore, additive-free quartz glass is used as a convex lens,
An achromatic lens as shown in FIG. 1 can be made using aluminum-doped quartz glass as a concave lens. Furthermore, the change in refractive index due to the addition of aluminum oxide is greater than that due to the addition of phosphorus oxide, and the effect is greater. Furthermore, it does not cause ultraviolet absorption, and there is almost no decrease in ultraviolet transmittance. The effect can be seen even if the amount of aluminum oxide added is 11, but in general, the larger the dispersion ratio, the more achromatization becomes possible over a wider wavelength range.
It is desirable that the amount of aluminum oxide added be as large as possible. However, if aluminum oxide is added in excess, a problem arises in that it becomes opaque without vitrification. therefore,
The amount of aluminum oxide added is 50% by weight or less. The preferred amount of aluminum oxide added is 2ffl! m% or more and 20 weight SI 6 or less. There are various methods for obtaining aluminum oxide-containing quartz glass. For example, in the process of vapor phase synthesis of glass, ^1 is added to the glass by the action of an aluminum compound gas, or a glass particle deposit (soot matrix) is created using a flame hydrolysis method. A method of obtaining a glass base material doped with soot by heat-treating it in an atmosphere containing an aluminum compound gas, and a method of obtaining an AI-doped glass by a sol-gel method using aluminum alkoxide are known. ing. The aluminum compound to be used may be one that turns into aluminum oxide in glass, and in the case of vapor phase synthesis method or sol-gel method, ^I013 or the like is used. Additionally, aluminum oxide itself can be mixed in to vitrify it. Although it is theoretically possible to use the aluminum-containing quartz glass obtained by the above method as the achromatic lens for ultraviolet rays of the present invention, the above method is mainly used to obtain a base material for optical fibers. This method is not necessarily suitable as a method for obtaining optical glass, especially large-diameter lens glass. The present inventors have discovered a method for producing aluminum-doped silica glass for the purpose of obtaining optical glass. That is, a silicon and aluminum alkoxide such as silicon tetraethoxide or aluminum tributoxide is used as a starting material, and this is hydrolyzed to produce aluminum-containing silica glass powder. 1. After molding this powder with a mold press or cold isostatic press, it is completely or partially vitrified by sintering in a helium gas atmosphere, and then hot press or hot isostatic press. This is a method of creating completely bubble-free glass under high temperature and pressure. A suitable sintering temperature is 1200 to 1400°C. In addition, the temperature and pressure of high-temperature and high-pressure processing are, in the case of hot press, a temperature of 1300°C or higher and a pressure of 10 MPa or higher.
In the case of hot isostatic pressing, the temperature is 1300℃ or higher and the pressure is 5.
0 MPa or more is preferable. According to this method, it is not only possible to make the glass larger, but also the material is free from veins and distortion, which are important problems for optical glasses, and has excellent optical homogeneity. Note that the lens combined with the lens made of aluminum-containing quartz glass must be made of high-purity quartz glass, but in order to achieve the purpose of the present invention, the purity must be 99%.
．． It needs to be 9% or more. In particular, in order to avoid absorption of ultraviolet rays, impurities of transition elements and rare earth elements should be kept below several hundred I) pI, preferably several tens of pp.
Must be kept below m. The second feature of the present invention is that an achromatic lens for ultraviolet light using aluminum-doped silica glass is used in the reduction projection optical system of an excimer laser stepper. For example, K
The laser light emitted from the rl' excimer laser is usually 0.
．． 7 Set the half width (line width) of rv to H. When a current stepper performs reduction projection exposure using a laser beam with this line width and an optical system lens made only of quartz glass, the focal length due to chromatic aberration @4) can reach several tens of micrometers.
It is not possible to obtain a clear image. For this reason, it may be possible to suppress chromatic aberration by narrowing the band of laser light, but this poses the problems described above. By using the achromatic lens of the present invention, such problems can be essentially solved. That is, since chromatic aberration can be corrected with the lens of the optical system, it is not necessary to make the laser linewidth extremely narrow. Further, processing of the glass material of the present invention into an optical lens is much easier than that of fluorite. In other words, it is much more li than fluorite! ! Because the polishing properties are high and scratches are less likely to occur during polishing, polishing can be carried out as usual using abrasive sand for polishing optical lenses, and the dimensional accuracy can also be improved. .

【implementation ? I1] The present invention will be explained in detail by the following examples. However, the present invention is not limited to these examples. Figure 1 shows an achromatic lens using three lenses, but the shape is different. combination. Of course, there is no limit to the number of sheets. Example 1 High-purity quartz glass is made by hydrolyzing silicon alkoxide, and aluminum-containing quartz glass is made by hydrolyzing silicon and aluminum alkoxides such as silicon tetraethoxide and aluminum tributoxide. It was obtained by sintering at a temperature of 1500°C in an atmosphere and annealing at 1000°C for 24 hours in an oxygen atmosphere. High-purity quartz glass (99.99% or more) and aluminum-containing 6-quartz glass (aluminum oxide content 11% by weight), respectively, with a diameter of 13Qts and a thickness of 30mm.
The optical homogeneity of was measured using a laser interferometer. In addition, prisms were made from each type of glass, and the refractive index in the ultraviolet region was measured using a precision spectrophotometer. Furthermore, the dispersion ratio at 250 ns was measured. These results are shown in Table 1. Δn, which is a value representing the optical homogeneity of high-purity quartz glass and aluminum-doped silica glass, is Δn − 3.
0.times.IG-' or less, which was found to be a satisfactory characteristic as an optical lens. Furthermore, it was found that both the refractive index and dispersion of the aluminum-doped silica glass increased with the addition of aluminum. Example 2, Comparative Example 1 Each obtained in the same manner as Example 1, with a diameter of 130 m.
High purity quartz glass (99.9mm)
9% or more) and aluminum-containing quartz glass (aluminum oxide content: 96%), high-purity quartz glass is used for convex lenses 1 and 3 shown in FIG. 1, and aluminum oxide-added silica glass is used for concave lens 2. make use of,
A triplet lens was made. The lens was manufactured by first finishing it into the specified size and shape using a rough polishing plate and polishing sand, then applying sand pressure using fine polishing sand, and then precision polishing with cerium oxide. . Thereafter, using a centering machine, the position of the optical axis was determined and centering work was performed. The dimensions were inspected to be within the tolerance range, and the lens was used as an optical component. This work is similar to the normal optical lens manufacturing procedure,
It does not require as complex a process as processing fluorite, and the parts did not show any residual polishing scratches that are often seen in fluorite. Using this achromatic lens, the longitudinal chromatic aberration that occurs when the excimer laser beams having different line widths shown in Table 2 are focused is calculated.The results are shown in Table 2. As a comparative example, a similar 1 lens made of additive-free silica glass was used.
・It also shows the chromatic aberration of the intermediate color lens system that made up the libretto lens. Furthermore, the dispersion ratio at 250 Ilm is also shown. From the results in Table 2, it can be seen that an achromatic lens using silica glass doped with aluminum has a smaller chromatic aberration value than a monochromatic lens system, and has an excellent chromatic aberration correction effect. [Effect of Yumei] As is clear from the above explanation, the achromatic lens for ultraviolet rays of the present invention is easier to process than those using fluorite, and can be polished with high precision. It has many advantages such as high optical homogeneity, excellent durability, and the ability to increase the diameter. Therefore, it can be widely used in optical instruments that utilize ultraviolet rays, such as optical systems for excimer laser steppers.

[Brief explanation of drawings]

FIG. 1 is a sectional view of an achromatic lens manufactured according to Example 2 of the present invention and Comparative Example 1. 1.3...Convex lens 2...Concave lens

Claims (2)

[Claims] (1) An achromatic lens for ultraviolet rays characterized by being composed of two types of glasses A and B below. (A) High purity quartz glass with purity of 99.9% or more (B) 5
Quartz glass containing 0% by weight or less of aluminum oxide (2) In the achromatic lens for ultraviolet rays according to claim 1,
An achromatic lens capable of efficiently transmitting ultraviolet light having a wavelength shorter than 300 nm, and capable of being used in an optical system of an excimer laser reduction projection exposure apparatus.