Optical: systems and elements – Lens – With reflecting element
Reexamination Certificate
2000-08-24
2002-10-29
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With reflecting element
C359S364000
Reexamination Certificate
active
06473243
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a catadioptric imaging system which is preferably used in projection exposure for producing, for example, a semiconductor device, a liquid crystal display device, or the like, by photolithography, as well as a projection exposure apparatus and an exposure method using such catadioptric imaging system, and more particularly, to a catadioptric imaging system, or the like, which attains a resolution of 0.1 &mgr;m or lower in the ultraviolet region by using a reflection system as a factor of an imaging optical system inside a catadioptric imaging system.
BRIEF DESCRIPTION OF THE PRIOR ART
In a process of photolithography for producing a semiconductor device, or the like, there is employed a projection exposure apparatus which performs projection exposure of a pattern image formed on a photo mask or a reticle (hereinafter collectively called the “reticle”) on a wafer or a glass plate with photo resist coated thereon through a projection optical system. Then, with enhancement of the degree of integration of the semiconductor device, or the like, a resolving power required for the projection optical system used in the projection exposure apparatus is gradually increasing. To satisfy this requirement, it is needed to reduce the wavelength of illumination radiation (exposure light) and to enlarge the numerical aperture (NA) of the projection optical system. For example, in case that the wavelength of the illumination radiation is not more than 180 nm, a high resolution of 0.1 &mgr;m or less can be achieved.
However, if the wavelength of the illumination radiation is reduced, light absorption increases and the kinds of practicable glass materials are limited. Particularly, if the wavelength becomes 180 nm or less, only fluorite is practicable as the glass material. For this reason, in a projection optical system which is constituted only by a refractive lens system, or only by lens components containing no reflecting mirror with a refracting power (a concave reflecting mirror or a convex reflecting mirror), it becomes impossible to correct a chromatic aberration.
Also, since an optical performance required for a projection optical system is extremely high, it is required to correct the aberrations to be substantially zero. However, in order to achieve a desired optical performance in a dioptric projection system, a large number of lens components is required, so that it is inevitable to reduce the transmittance or to increase the production cost.
In contrast, a catadioptric system using a power (the refracting power) of a concave reflecting mirror, or the like, that is, an optical system which contains a reflecting mirror having a refracting power without containing a lens component generates no chromatic aberration and shows a contribution having a sign reverse to that of a lens component with respect to the Petzval sum. Accordingly, in an optical system which is a combination of a catoptric system and a dioptric system, or a so-called optical system of catadioptric type (hereinafter called the “catadioptric imaging system”), various aberrations including a chromatic aberration can be satisfactorily corrected to be substantially zero without increasing the number of lenses. In this case, the catadioptric imaging system is an optical system which contains at least one lens component and at least one reflecting mirror having a refracting power. In this respect, there is no need to say that a plane parallel plate or a plane reflecting mirror for deflecting an optical path may be provided if needed.
However, if a concave reflecting mirror is used in an optical path of a projection optical system of a projection exposure apparatus, light incident on this concave reflecting mirror from the reticle side is reflected to move back to the reticle side again. For this reason, there are conventionally proposed various technologies for separating an optical path for the light incident on the concave reflecting mirror from an optical path for the light reflected by the concave reflecting mirror so as to lead the reflected light from the concave mirror toward the wafer, that is, the technologies for constituting a projection optical system by a catadioptric imaging system.
As a representative method for separating optical paths from each other, a method for separating optical paths from each other by using a transmission reflecting surface such as a half mirror or a polarizing beam splitter is proposed in Japanese Patent Publication No. HEI 7-117648. Also, in U.S. Pat. No. 4,779,966, there is proposed a method for separating optical paths from each other by forming an intermediate image by using an off-axis optical path and providing a plane mirror for bending optical paths in the vicinity of the forming position of the intermediate image. Further, in U.S. Pat. No. 5,031,976, there is proposed a method for separating optical paths from each other by using two reflecting mirrors each having an opening at the center thereof, and providing the two reflecting mirrors so that a beam is reflected when the section of the beam is large in the vicinity of the pupil of the optical system and the beam passes through the central openings when the section of the beam is small in the vicinity of the image plane.
However, since the optical path separation methods disclosed in Japanese Patent Publication No. HEI 7-114648 and U.S. Pat. No. 4,779,966 employ a plane mirror which is provided in an inclined manner with respect to the optical axis for the optical path separation, an optical system is required to have a plurality of optical axes. In a projection optical system which requires adjustment of the optical components with high precision, a highly sophisticated technology is required for positioning the plurality of optical axes with precision and to dispose the optical components at desired positions with respect to the respective optical axes in the order of microns. As a result, it is inevitable to increase the cost for producing the optical system.
On the other hand, according to the optical path separation method disclosed in U.S. Pat. No. 5,031,976, all of the optical elements for constituting the optical system can be disposed along the single optical axis. As a result, it is possible to produce the optical system with precision in accordance with the adjusting method of the optical components which is conventionally used in a projection optical system. Optical systems employing such optical path separation method are disclosed in U.S. Pat. Nos. 5,488,229, 5,650,877, 5,717,518, and the like, in addition to U.S. Pat. No. 5,031,976.
However, in the optical system disclosed in U.S. Pat. No. 5,031,976, or the like, it is required to shield a part of the beams centering the optical axis, out of imaging beams, in order to prevent stray light from being generated, which passes, without being reflected by two reflecting mirrors at all, through the central openings thereof to reach the image plane. As a result, due to this central shielding of the imaging beams, the image forming characteristic of the optical system is degraded. Accordingly, in order to apply the optical path separation method disclosed in U.S. Pat. No. 5,031,976 to a projection optical system, it is essential to suppress the rate of central shielding of the imaging beams (hereinafter simply called the “central shielding rate”) to the minimum, so as to obtain a sufficient optical characteristic.
In the optical system disclosed in U.S. Pat. No. 5,650,877, a half mirror is arranged to be close to an object plane (a plane corresponding to a mask plane) while a reflecting mirror having an opening at the center thereof is arranged to be close to an image plane (a plane corresponding to a wafer plane), without forming an intermediate image. In this manner, the central shielding rate is suppressed to some extent. That is, in this optical system, it is inevitable to employ a half mirror. However, when this optical system is applied to a projection optical system which uses exposure light having the w
Miles & Stockbridge P.C.
Nikon Corporation
Schwartz Jordan M.
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