Optical: systems and elements – Having significant infrared or ultraviolet property – Multilayer filter or multilayer reflector
Reexamination Certificate
2001-05-30
2003-06-03
Juba, Jr., John (Department: 2872)
Optical: systems and elements
Having significant infrared or ultraviolet property
Multilayer filter or multilayer reflector
C359S355000, C359S586000, C355S067000, C355S071000
Reexamination Certificate
active
06574039
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical element such as a lens, a prism, and a reflecting mirror provided with a multilayered optical thin film on its surface. The present invention also relates to an exposure apparatus provided with the optical element as described above.
BACKGROUND ART
A variety of optical thin films such as reflection films and anti-reflection films are applied to the optical element for constructing an optical system such as a lens, a prism, and a reflecting mirror. For example, the anti-reflection film is applied in order to reduce undesirable reflection. On the other hand, the reflection film is applied to the surface of the optical element in order to efficiently reflect the incident light on the surface of the reflection film.
Such an optical thin film is generally produced in accordance with the dry process. The dry process includes the vacuum deposition, the sputtering, CVD (Chemical Vapor Deposition). The dry process is described, for example, in Joy George, Preparation of Thin Films (Marcel Dekker, Inc., New York, 1992) and Francois R. Flory, Thin Films for Optical Systems (Marcel Dekker, Inc., New York, 1995).
The anti-reflection film is required to have such performance that the reflectance is low over a wide range of angle of incidence. The reflection film is required to have such performance that the reflectance is high with satisfactory angle-dependent characteristics over a wide range of wavelength. In order to respond to the request for the performance as described above, it is known that a multilayered film is formed in a well-suited manner by combining a plurality of coating materials having different refractive indexes. Further, as for the multilayered film, it is known that the larger the difference in refractive index among a variety of coating materials to be used is, and the lower the minimum refractive index of those of the variety of coating materials is, the more the optical performance of the multilayered film is improved. Further, it is also known that the number of coating layers can be decreased by using coating materials which are greatly different in refractive index in combination, and using a coating material which has an extremely low refractive index. As a result, an optical thin film, which has high performance in relation to the light beam in the visible region, is obtained.
The integration is highly advanced and the function is highly progressive for ULSI in the exposure apparatus for semiconductors. An optical system such as a projection lens thereof is required to have a high resolution and a deep depth of focus in order to successfully obtain a machining line width of 0.18 &mgr;m. The projection lens is used to project a device pattern on a photomask onto a wafer so that the wafer is exposed therewith. The resolution and the depth of focus of the projection lens are determined by the wavelength of the light used for the exposure and N.A. (numerical aperture) of the projection lens.
In general, as for the device pattern on the photomask, the higher the definition is, the larger the angle of diffraction of the diffracted light is. Therefore, in order to perform the exposure with such a pattern, the diffracted light may be fetched by using a projection lens having large N.A. Further, the angle of diffraction of the diffracted light from the pattern is decreased when the light has a shorter wavelength &lgr;. Therefore, it is also advantageous to use the light beam having a short wavelength for the exposure of the pattern having such a definition as described above.
The resolution and the depth of focus are represented by the following expressions respectively.
Resolution=
k
1
(
&lgr;/N.A.
) (1)
Depth of focus=
k
2
{&lgr;/(
N.A.
)
2
} (2)
(In the expressions, k
1
and k
2
are proportional constants.)
Therefore, in order to improve the resolution (decrease the value), N.A. may be increased, or &lgr; may be shortened. However, if N.A. is increased, the depth of focus is shortened, as appreciated from the expression of the depth of focus. When the depth of focus of the optical element such as the projection lens is shortened in the semiconductor exposure apparatus, the throughput is affected thereby. Therefore, in order to improve the resolution, it is more preferred to shorten &lgr; rather than N.A. is increased. From such a viewpoint, the wavelength of the exposure light beam is progressively shortened, from the g-ray (436 nm) to the i-ray (365 nm) and further to the excimer laser beams such as KrF (248 nm) and ArF (193 nm).
In spite of the trend to realize the exposure with the short wavelength as described above, it has been hitherto extremely difficult to obtain a high performance optical thin film for an ultraviolet light source, for example, those used in the vicinity of 200 nm, unlike those obtained in the visible region, because of the following reason. That is, many coating materials absorb the light in this wavelength region, resulting in light loss. The coating material, which can be used in the ultraviolet region in the vicinity of 200 nm as described above, is extremely restricted. Therefore, it is difficult to sufficiently increase the difference in refractive index between the coating materials as described above, and it is difficult to extremely decrease the minimum refractive index of those of various coating materials. Therefore, it has been hitherto extremely difficult to design and produce a high performance optical thin film to be used in such a wavelength region.
At present, it is possible to use a variety of anti-reflection film materials in order to produce a typical anti-reflection film to be used for the light in the visible region by means of the dry process. In general, in the visible region, TiO
2
(n=2.4 to 2.7 at 500 nm) is used as the maximum refractive index material, and MgF
2
(n=1.38 at 500 nm) is used as the minimum refractive index material (n represents the refractive index). However, only a few coating materials are usable for the ultraviolet light having the wavelength in the vicinity of 200 nm. In general, the refractive index n is about 1.7 (n=about 1.7) in relation to the wavelength of 200 nm for any one of LaF
2
, NdF
3
, and GdF
3
. These materials are usable coating materials having the maximum refractive index. The refractive index n is 1.36 (n=1.36) in relation to the wavelength of 200 nm for Na
3
AlF
6
. This material is a usable coating material having the minimum refractive index. Therefore, the difference in refractive index among a plurality of coating materials used for the light at the wavelength of 200 nm is by far smaller than the difference in refractive index among a plurality of coating materials used for the light in the visible region.
The coating material, which is usable in the ultraviolet region, is extremely limited as described above. Therefore, those skilled in the art will understand the fact that the design and the production of the optical thin film are more difficult in the ultraviolet region than in the visible region.
It is known that the optical thin film is produced in accordance with the wet process. For example, a thin film can be produced by means of hydrolysis and polymerization with a metal alkoxide solution, i.e., a liquid. The wet process is called “sol-gel process”. As well-known in the art, for example, SiO
2
, ZrO
2
, HfO
2
, TiO
2
, and Al
2
O
3
can be produced not only by the dry process but also by the sol-gel process. The method is disclosed, for example, in Ian M. Thomas, Applied Optics Vol. 26, No. 21 (1987) pp. 4688-4691 and Ian M. Thomas, SPIE Vol. 2288 Sol Gel Optics III (1994) pp. 50-55. In the case of the SiO
2
film formed by the sol-gel process, a colloidal SiO
2
suspension, which is appropriate to manufacture the SiO
2
film, is usually prepared by means of hydrolysis of silicon alkoxide in base alcohol as a solvent. Hydrolysis of tetraethyl silicate in ethanol can be represented, for example, by the following
Ishizawa Hitoshi
Murata Tsuyoshi
Jr. John Juba
Nikon Corporation
Oliff & Berridg,e PLC
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