Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2001-01-10
2003-09-16
Matthews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C359S364000, C359S726000
Reexamination Certificate
active
06621557
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to projection exposure apparatus and exposure methods, for example, for fabricating semiconductor devices, liquid-crystal display devices, etc. by photolithography and, more particularly, to projection exposure apparatus provided with a projection optical system comprised of catadioptric optics or catoptric optics, and exposure methods using such apparatus.
2. Related Background Art
In the photography step for fabrication of the semiconductor devices and other devices, the projection exposure systems are used for printing a pattern image of a photomask or a reticle (which will be referred to hereinafter together as “reticle”) through a projection optical system on a wafer (or a glass plate or the like) coated with a photoresist or the like.
The resolving power required of the projection optical system used in such projection exposure systems has been becoming higher and higher with increase in integration of the semiconductor devices and other devices. For meeting this demand, there arise the needs for decrease in the wavelength of illumination light and for increase in the numerical aperture (N.A.) of the projection optical system.
However, the decrease in the wavelength of illumination light leads to limitation of kinds of glass materials applicable to practical use because of absorption of light. The glass materials practically applicable at present in the wavelength range of not more than 300 nm are only synthetic silica and fluorite, and for this reason, there are desires for use of catoptric optics in order to effect correction for chromatic aberration. Specifically, practical use has considerably been developed using dioptric optics with the KrF laser of 248 nm, but it is very difficult to realize practical use using dioptric optics in the wavelength range of not more than 200 nm. Therefore, expectations are rising for the catadioptric optics. Particularly, it is known that fluorite has sufficient transmittance even at 100 nm, and since it can be used as a refracting member above this range, the catadioptric optics can be constructed in the range of wavelengths 100 to 300 nm.
Several types of the catadioptric optics have been proposed heretofore. Among them, an optical system of a type wherein the central part of N.A. is shielded (which will be referred to hereinafter as a center shield type) is a promising type, because all optical elements can be assembled on the basis of one optical axis without inclusion of a path deflecting member by use of two or more reflective surfaces and because it has a merit of capability of imaging an object on the optical axis on the image plane and thus correcting for aberration in a wide exposure field by a small number of optical elements. Prior arts of this type include those disclosed, for example, in U.S. Pat. No. 5,717,518, No. 5,650,877, and so on.
In general, the illumination light radiates to the projection optical system during exposure in the projection exposure apparatus, so as to bring about absorption of the illumination light (or exposure light), and the optical members undergo, for example, asymmetric deformation, internal temperature distribution, etc., thus causing variation in aberration (see Japanese Patent Application Laid-Open No. H09-213611).
SUMMARY OF THE INVENTION
This aberration variation is considered to be not only due to absorption inside the glass materials of the projection optical system, but also due to absorption in thin films or the like on surfaces. Particularly, in the catadioptric optics, absorption at the reflective surfaces is considered to be especially larger than that inside the glass materials and in the surface thin films, and the aberration variation due to absorption can be more problematic than in the case of the dioptric optics.
An effective means for the projection optical systems in the extreme ultraviolet region is use of a back reflector that causes reflection on a back surface of a refracting member, for example, as described in aforementioned U.S. Pat. No. 5,717,518. In this case, materials used need to be glass materials capable of transmitting light in this wavelength range, e.g., silica, fluorite, BaF
2
, and LiF
2
. Since these glass materials have large dN/dT and expansion coefficients, if a refracting member has a reflective surface, as described above, relatively large quantity of heat generated at the reflective surface will propagate into the refractive member to produce a relatively large temperature distribution inside glass. It is thus considered that there are some measures necessary for suppressing the aberration variation. Under such circumstances that development of thin films is presently under way, particularly, for the exposure light of not more than 200 nm and in conjunction with the fact that the performance expected for the projection optical system itself is also of very high precision in this wavelength range, the aberration variation due to irradiation of the optical members including the reflective surfaces is a significant problem.
Further, in the case of the projection optical system in above-noted U.S. Pat. No. 5,717,518, the beams are considerably concentrated in the finally focused part near the reduced image plane, i.e., in the last optical path in the refracting and reflecting member. Such beams can pose a significant problem, because they produce a heterogeneous large temperature distribution in the optical member.
Even reflecting members sometimes need to be made of a substance with a large expansion coefficient, such as CaF
2
or BSC7, because of compatibility with a reflective coat. In such cases a problem will be posed by aberration caused by thermal expansion because of much greater absorptances than those of refracting members.
For avoiding this, it is conceivable to space this refracting and reflecting member apart from the image plane. However, taking it into consideration that the center shield part needs to be designed as small as possible, to space the refracting and reflecting member apart from the image plane will raise considerable difficulties in design. As countermeasures against the aberration due to such heterogeneous temperature distribution, for example, Japanese Patent Application Laid-Open No. H10-242048 notes that rotationally asymmetric aberration variation occurs during execution of scanning exposure or the like and proposes a method and the like for preliminarily fabricating optical means comprising aspherical surfaces in accordance with the aberration variation and moving it as occasion arises. For carrying out this method, however, it is necessary to make the rotationally asymmetric aspherical surfaces according to the aberration variation and to prepare a fabrication system dedicated therefor. In addition, a decentering adjusting mechanism with considerably high accuracy also needs to be incorporated in order to properly decenter the aspherical surfaces during irradiation, which makes the fabrication of the projection optical system difficult.
The aberration variation also occurs with change in the temperature of the entire projection optical system. However, as to the aberration due to deformation of the reflector among such aberration variation, since change of beams due to reflection is approximately four times greater than that due to refraction, influence of temperature change tends to raise a problem in the above-described catadioptric optics. For that reason, it can be said that it is desirable to design the optics, preliminarily taking the deformation of the reflector into consideration.
An example of the countermeasures in this case is the one disclosed in Japanese Patent Application Laid-Open No. H05-144701. This invention is to determine thermal shape changes of some lenses forming the projection optical system and implement optical adjustment with adjusting means, based thereon. It, however, seems considerably difficult in practice to measure the shape changes of the lenses during the exposure operation without negatively affe
Matthews Alan A.
Nikon Corporation
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