Optical: systems and elements – Mirror – Plural mirrors or reflecting surfaces
Reexamination Certificate
2001-01-17
2001-10-16
Sikder, Mohammad (Department: 2872)
Optical: systems and elements
Mirror
Plural mirrors or reflecting surfaces
C359S859000, C359S861000, C359S365000, C359S366000
Reexamination Certificate
active
06302548
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to photolithography, and in particular to catoptric reductions projection optical systems, and exposure apparatus and exposure methods for fabricating semiconductor and like devices using same.
BACKGROUND OF THE INVENTION
With the advance of micro-miniaturization in recent years in the manufacture of semiconductor and like devices, and semiconductor chip mounting boards, there has been a demand for higher resolving power in the exposure apparatus that prints the patterns therefor. To satisfy this demand, the wavelength of the light source of the exposure apparatus must be shortened and the NA (i.e., numerical aperture of the projection optical system) must be increased. Nevertheless, if the wavelength is shortened, optical glass that can withstand practical use is limited due to the absorption of light. Furthermore, when it comes to the short wavelengths of ultraviolet rays and X-rays, usable optical glass materials are nonexistent. In this case, it is completely impossible to construct a reduction projection optical system as a dioptric optical system or a catoptric optical system.
A catoptric reduction projection optical system is disclosed in, for example, Japanese Patent Application Kokai No. Hei 9-211332. The projection optical system disclosed therein comprises two sets of optical systems comprising reflective surfaces with a concave-convex-concave configuration, and forms an intermediate image between these two sets of reduction projection systems. The system has a total of six reflective surfaces, and so has a large number of degrees of freedom for correcting aberrations. However, it has the problem of a large loss in the quantity of light reaching the image plane due to the excessive number of reflective surfaces. Furthermore, the imaging performance of the overall projection optical system declines due to aberrations generated by manufacturing errors in each reflective surface. Consequently, the system is difficult to manufacture since the tolerance of each reflective surface must be kept extremely tight. Accordingly, although imaging performance from the standpoint of optical design is excellent, there is a risk that the imaging performance of an actually manufactured system will be insufficient.
U.S. Pat. No. 5,153,898 (the '898 patent) discloses another type of catoptric reduction projection optical system wherein X-rays from a reticle are sequentially reflected by a first concave mirror, a convex mirror and a second concave mirror. In addition, the same patent discloses a constitution that prevents interference between the wafer and the X-rays during scanning by arranging a plane mirror between the reticle and the first concave mirror, between the second concave mirror and the wafer, or between both. In the '898 patent, the wafer interferes with rays in the synchronous scanning of the reticle and wafer, inviting the eclipsing of the rays. Furthermore, since the angle at which the X-rays intersect on the plane mirror is on the order of 45°, and since the X-ray reflective mirror is generally constituted by layering a multilayer film, phase shifts arise due to differences in the incident angles, thereby generating aberrations.
U.S. Pat. No. 5,917,879 (the '879 patent) discloses an X-ray lithographic catoptric reduction imaging optical system comprising, objectwise to imagewise, a first concave mirror, a plane mirror, a convex mirror and a second concave mirror arranged coaxially. Each concave mirror and convex mirror are formed as an aspherical surface. The convex mirror is arranged in the pupil plane, and the system is imagewise telecentric. The '879 patent avoids eclipsing of the light beam that occurs when constituting a catoptric optical system with a plurality of mirror systems. However, to do so, the NA on the wafer side is required to be small (0.06). Accordingly, sufficient resolution cannot be obtained.
SUMMARY OF THE INVENTION
The present invention relates to photolithography, and in particular to catoptric reduction projection optical systems, and exposure apparatus and exposure methods for fabricating semiconductor and like devices using same.
Accordingly, the present invention has the objective to achieve extremely excellent imaging performance while keeping the number of reflective surfaces to a relatively small number, and has a further objective to improve the imaging performance of an actually manufactured system.
Accordingly, a first aspect of the invention is a catoptric reduction projection optical system that forms a reduced image of an object present in a first (object) plane onto a third (image) plane. The system comprises, along an optical axis, first and second catoptric optical systems. The first catoptric optical system has two mirrors (a first mirror pair), and forms an intermediate image of the object onto a second plane. The second catoptric optical system has a second mirror pair comprising a convex mirror and a concave mirror, and images the intermediate image onto the third (image) plane. The first and second mirror pairs are positioned so that light from the first plane travels through the first mirror pair, forms the intermediate image in the second plane, then travels through the second mirror pair in the order of the convex mirror and the concave mirror, and then proceeds to form a reduced image of the object in the third plane.
A second aspect of the invention is catoptric reduction imaging optical system capable of forming a reduced image of an object present in a first plane onto a third plane. The system comprises, along an optical axis proceeding in order along an optical path from the first plane to the third plane, a first catoptric reduction imaging system that converges light from the object to form an intermediate image in a second intermediate image plane. The system also includes a second catoptric reduction imaging system that converges light from the intermediate image to form an image in the third plane. The following condition is also preferable satisfied:
0.8<|&bgr;1/&bgr;|<2
wherein &bgr;1 is a reduction magnification of the first catoptric reduction imaging system, and &bgr; is a combined reduction magnification of the first catoptric reduction imaging system and the second catoptric reduction imaging system.
A third aspect of the invention is a method of exposing a pattern present on a reticle onto a photosensitive substrate. The method comprises the steps of first, illuminating a portion of the reticle with light of a predetermined wavelength, then guiding light from the reticle to a catoptric reduction projection exposure system of the present invention, as described briefly above and in more detail below, so as to form an image of the reticle pattern portion onto the photosensitive substrate, and then scanning the reticle and the photosensitive substrate relative to the catoptric reduction projection exposure system so as to form an image of the entire reticle pattern onto the photosensitive substrate.
REFERENCES:
patent: 5153898 (1992-10-01), Suzuki et al.
patent: 5173801 (1992-12-01), Cook
patent: 5686728 (1997-11-01), Shafer
patent: 5805365 (1998-09-01), Sweatt
patent: 5815310 (1998-09-01), Williamson
patent: 5917879 (1999-06-01), Mashima
Suenaga Yutaka
Takahashi Tomowaki
Nikon Corporation
Oliff & Berridg,e PLC
Sikder Mohammad
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