JPH02248911A - Achromatic lens for UV rays - Google Patents

Abstract

PURPOSE:To facilitate working and to enhance fluorite hardness and dimensional accuracy by combining specific SiO2-B2O3 two-component glass and synthetic quartz glass or fluorine-added low-dispersion quartz glass. CONSTITUTION:This lens is constituted of two kinds of the glass; the SiO2-B2O3 two-component glass expressed by (1-x)SiO2.xBO1.5:x=0.05 to 0.8 and the synthetic quartz glass or the quartz glass contg. fluorine. Namely, both the refractive index and dispersion are lowered by the addition of the fluorine in the quartz glass system but the refractive index is lowered and the dispersion is increased by the addition of the boron. The effective achromatic lens is, therefore, constituted by forming the synthetic quartz glass or the fluorine-added quartz glass as convex lenses 1, 3 and the SiO2-B2O3 two-component glass as a concave lens 2. The working is facilitated in this way and the highly accurate polishing is executed. The optical properties and durability are thus improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a reduction projection exposure apparatus (stepper) used in ultraviolet lithography, etc.
This invention relates to optical achromatic lenses used in excimer laser steppers such as the following.

[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 LSI integration has progressed, the resolution of the g-line (430 ns) and i-line (ff135 ns) of the ultra-high pressure mercury lamp used in conventional steppers has become insufficient.

Therefore, in order to increase the resolution, XeC1, which has a shorter wavelength,
, Krl', and other steppers that use excimer laser light as a light source are progressing. 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 laser light from the light source 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.

However, in the case of a method that narrows the band of laser light and suppresses chromatic aberration within acceptable limits, etalon.

By using elements and methods such as brism and injectin locking, the half-width of laser light can be adjusted to o, ooa ~ 0.0.
The band is narrowed to 0.05 nm. However, narrowing the band causes some problems. For example, l? In order to compensate for the decrease in laser output, the laser has to be increased in output, which means that the laser generator itself has to be larger, speckle patterns are more likely to occur, and the irradiation area has to be larger. Problems arise, such as difficulty in

In the case of a method of correcting chromatic aberration by using an achromatic lens in an optical system, there is a 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 small calcium fluoride crystals (fluorite) has been proposed. In addition, oxides of transition elements such as titanium and iron, cerium,
An achromatic lens has been proposed by combining a synthetic silica glass lens doped with oxides of rare earth elements such as europium, aluminum oxide, etc., and synthetic silica glass without additives, taking advantage of the difference in dispersion power between the two ( Tokukai Sho (i3-6
512, etc.).

[Problems to be Solved by the Invention] However, when attempting to use fluorite as an optical material such as a lens, it has various drawbacks compared to glass. First, fluorite has low hardness and is easily scratched, making optical polishing difficult.

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 extremely difficult to use it to fabricate a large-diameter optical lens with a diameter of 1001 or more.

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 cause ultraviolet absorption, reducing the amount of light transmitted. This results in a decrease in rate and generation of fluorescence. For this reason, it is inconvenient as an achromatic lens for ultraviolet rays, and it is preferable to remove the additive 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 to solve the problem]

As a result of intensive research in view of the above problems, the present inventors have found that Sl 0
2-B203-based two-component glass exhibits optical properties of lower refractive index and higher dispersion than additive-free silica glass, is transparent to ultraviolet rays, contains a large amount of boron, and is suitable for other optical materials. It does not have the above-mentioned drawbacks found in quartz glass containing fluorite or transition element oxides, and can be combined with synthetic quartz glass or fluorine-doped low-dispersion silica glass to create a large-diameter, high-precision achromatic lens. We have discovered that it is possible to do this, and have arrived at the present invention.

That is, the achromatic lens for ultraviolet rays of the present invention has (^) (1
-x) S102@1f801.5:X=0.05~
8102-L O3-based binary glass expressed by 0.8,
and (1B) synthetic quartz glass or fluorine-containing quartz glass.

Further, the achromatic lens for ultraviolet rays of the present invention is characterized in that it can efficiently transmit ultraviolet rays having a wavelength shorter than 300+u+, 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 uninvention is that Sj 02-11203 series 2
It is an object of the present invention to provide a novel achromatic lens for ultraviolet rays made from a component glass and a lens made of synthetic quartz glass or fluorine-doped quartz glass.

Generally, in order to construct an achromatic lens, two lenses with different dispersion are required.
It is necessary to combine lenses made of Pfi type materials. The present inventors have developed a combination of SIO2-1320s binary glass and synthetic quartz glass or fluorine-doped silica glass.
It has been discovered that an effective achromatic effect can be produced by combining the two.

In silica glass systems, the addition of fluorine lowers both the refractive index and dispersion, but the addition of boron lowers the refractive index and increases dispersion (however, BO+,
When the s molar ratio exceeds 30%, the refractive index increases again).

For example, when 2% by weight of fluorine is added to quartz glass, the refractive index for light with a wavelength of 830 nm is lowered by 0.5% than that without additives, and the dispersion value ratio with silica glass without additives is Δ
5102/Δ1-8102 is! , becomes 10. Also,
8102-B203 containing 30 mol% BO+,s
In the system binary glass, the refractive index for light at a wavelength (i30 nm) is 0.2% lower than that of synthetic silica glass, and the dispersion value ratio with synthetic silica glass is ΔSIO2-L 03 /Δ
St, , becomes 1.11+. Therefore, synthetic quartz glass or fluorine-doped quartz glass is used as a convex lens, 5102
- An effective achromatic lens can be constructed by using the B203 series binary component glass as a concave lens. Furthermore, in these glasses, fluorine and boron do not cause ultraviolet absorption, and there is almost no decrease in ultraviolet transmittance.

Generally, the larger the dispersion ratio, the more achromatization becomes possible over a wider wavelength range, so it is more desirable to add a larger amount of boron oxide. However, adding too much boron oxide causes the problem that the glass becomes water-soluble and becomes unsuitable as a lens material. Therefore, the amount of boron oxide added should be 80% or less in molar ratio of BO+ and s. The preferred content of BOIl is 30 to 60 mol%.

In order to obtain SIO□-B20 v-based two-component glass,
There are various methods. For example, 5102 powder and B20. There are methods for mixing powder in a ball mill and melting it, methods for adding boron to glass by applying a boron compound gas during the process of vapor phase synthesis of glass, and methods for adding boron to glass using flame hydrolysis. A method for obtaining a glass base material with boron added in the soot by producing a soot base material and heat-treating it in an atmosphere containing a boron compound gas, and a sol-gel method using boron alkoxide. A method for obtaining SIO2-1320s binary component glass is known.

The color lr for ultraviolet rays of the present invention! 5IO2' that makes up the lens
-1) Although it is possible in principle to use the 2Oi-based two-component glass obtained by the above method,
The above method is mainly aimed at obtaining a base material for optical fibers, and is not necessarily preferred as a method for obtaining optical glass, especially glass for large diameter lenses.

The present inventors have discovered that S
We have discovered a method for producing iO□-B203-based two-component glass.

- For example, silicon tetraethoxide and trimethylboric acid are used as starting materials. First, silicon tetraethoxide is partially hydrolyzed, and then trimethylboric acid is added to bond silica and boron oxide with a co-H bond. After that, add enough water to completely hydrolyze the mixture.
Make the solution alkaline! Adjustments are made to promote dehydration polymerization to obtain SIO2-LO3 based binary glass powder. This powder is molded using a mold press or cold isostatic press, and then completely or partially vitrified by sintering in a helium gas atmosphere. This is a method of creating completely bubble-free glass under high temperature and pressure. Sintering temperature is 1200~14
00°C is appropriate. In addition, the temperature and pressure of the high-temperature and high-pressure treatment are as follows: in the case of hot pressing, the temperature is 10°C or higher and the pressure is 10 MPa or higher; in the case of hot isostatic pressing, the temperature and pressure are 10 MPa or higher.
Preferably, the temperature is 300° C. or higher and the pressure is 50 Mh or higher. According to this method, it is not only possible to increase the size of the glass, but also
The material is free from striae and distortion, which are important problems for optical glasses, and has excellent optical homogeneity.

In addition, 5I02-B, 0. A lens to be combined with a lens made of two-component glass must be made of high-purity quartz glass or fluorine-doped quartz glass. High purity quartz glass needs to have a purity of 99.9% or more in order to achieve the purpose of the present invention. In particular, in order to avoid absorption of ultraviolet rays, impurities of transition elements and rare earth elements are
It must be kept below pm, preferably below several tens of ppm.

Fluorine-containing glass is produced by using high-purity silica fine powder as a starting material, first molding it into a powder compact using a method such as a rubber press, and then sintering it in a fluoride-based gas atmosphere such as %5IF4. It can be manufactured by a method of vitrifying the glass or partially vitrifying it, and then converting it into completely bubble-free glass under high temperature and pressure using a hot press or hot isostatic press equipment (Japanese Patent Application No. 88592/1982). . The fluorine content of fluoridated glass is
From the viewpoint of refractive index, it is preferable to have a larger amount, but it is preferable that the amount does not adversely affect transmittance.

The second feature of the present invention is that an achromatic lens for ultraviolet light using SI O2-BzOl binary glass and synthetic silica glass or fluorine-doped silica glass is used in a reduction projection optical system of an excimer laser stepper. There is a particular thing. For example, laser light emitted from an rP excimer laser typically has a half width (line width) of 0.7 ns+. When a current stepper uses a laser beam with this line width and performs reduction projection using an optical system lens made only of quartz glass, the focal length shift due to chromatic aberration reaches tens of microns, making it difficult to obtain a clear image. Can not. 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 extracted by the lens of the optical system, it is not necessary to extremely narrow the laser line width.

The 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 harder than fluorite and scratches occur during polishing.
Polishing can be carried out using conventional abrasive sand for polishing optical lenses. In addition, dimensional accuracy can also be improved at this time.

〔Example] The invention will be explained in detail by the following examples.

However, the present invention is not limited to these examples. Although FIG. 1 shows an achromatic lens using three lenses, it goes without saying that there is no limit to the number of lenses in eight combinations of shapes.

Example 1 High-purity quartz glass was obtained by selecting appropriate areas from an ingot produced by the Bertai method. Further, fluorine-doped silica glass was obtained by sintering a molded body of glass powder at 1500° C. in a silicon tetrafluoride gas atmosphere. Furthermore, quartz glass containing boron is produced by sintering powder obtained by hydrolyzing silicon and boron alkoxides, such as tetraethoxysilane and trimethoxyborane, at 1500°C in a helium gas atmosphere, and then heating to 1000°C. It was obtained by annealing for 24 hours.

Each glass lump obtained by the above method has a diameter of 130 m.
5 and high purity synthetic quartz glass (99,9
9% or more) and fluorine-doped quartz glass (fluorine concentration 2.0
Weight 26) and optical homogeneity of 8102-B20s binary glass (BO+, q content 30 mol% and 50 mol%) were all determined 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. The results are shown in Table 1.

Δn, which is a chain representing the optical homogeneity of high-purity synthetic quartz glass, fluorine-doped silica glass, and S102-B203 binary glass, is Δn = 3. OX 1G-' or less, which was found to be a satisfactory characteristic as an optical lens. Furthermore, in fluorine-doped silica glass, both the refractive index and dispersion decreased with the addition of fluorine. 9102
-820, the dispersion of the binary glass increased with the addition of boron, but the refractive index decreased at 30 mol%, and the refractive index decreased with the addition of boron.
It became high at 0 mol 26.

Example 2. Comparative Example 1 Each obtained in the same manner as Example 1, with a diameter of 1301 mm.
- and j17I 30a+m high purity quartz glass (9
9,995 or higher), fluorine-doped quartz glass (fluorine concentration 2.6% by weight) and Sl 02-13203 second component glass (BO+, s content 30 mol% and 50 mol%), as shown in Figure 1. The convex lens 1.3 shown is made of high-purity quartz glass or fluorine-doped quartz glass, and the concave lens 2 is made of S.
A triplet lens was made using IO2-11203 second component glass. The lens was manufactured by first finishing it into a predetermined size and shape using a rough grinding plate and abrasive sand, then sanding with fine abrasive 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,
Machining of fluorite does not require complicated processes, and the parts did not show any of the polishing scratches that are often seen in fluorite.

Krl' with different line widths using this triplet lens.
The longitudinal chromatic aberration that occurs when excimer laser light is focused has been reduced. The results are shown in Table 2. As a comparative example, the chromatic aberration of a monochromatic lens system made of a similar triplet lens made of additive-free silica glass is also shown.

From the results in Table 2, 8102-1320 *Achromatic lens using a combination of second component glass and synthetic silica glass, and color using a combination of SIO□-1hoi two-component glass and fluorine-doped silica glass. It can be seen that the vanishing lens has a smaller chromatic aberration value than a monochromatic lens system, and has an excellent chromatic aberration correction effect.

〔Effect of the invention〕

As is clear from the above description, the achromatic lens for ultraviolet rays of the present invention is easier to process, can be polished with high precision, and has high optical homogeneity compared to those using fluorite. It has many advantages, such as being highly durable, capable of large apertures, and having excellent chromatic aberration correction effects. 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 showing an achromatic lens constructed according to an example of the present invention or a comparative example.

Claims (2)

[Claims] (1) An achromatic lens for ultraviolet rays characterized by being composed of two types of glass, A and B below. (A) (1-x)SiO_2・xBO_1_. SiO_2-B_2O_3 binary component glass (B) synthetic quartz glass or fluorine-containing silica glass, represented by _5 (where x = 0.05 to 0.8) (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.