Optical: systems and elements – Polarization without modulation – By relatively adjustable superimposed or in series polarizers
Reexamination Certificate
2002-07-10
2004-08-10
Epps, Georgia (Department: 2873)
Optical: systems and elements
Polarization without modulation
By relatively adjustable superimposed or in series polarizers
C359S490020, C359S352000, C355S067000, C355S053000
Reexamination Certificate
active
06775063
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to an optical system and exposure apparatus having the optical system, and in particular to a projection optical system which is suitable for an exposure apparatus to be used to manufacture microdevices such as semiconductor devices and liquid crystal display devices using photolithography techniques.
2. Description of Related Art
It is known to use a method in which a pattern of a photomask (also called a reticle), which is etched therein by proportionally magnifying, 4-5 fold, the pattern to be formed on an electronic device (a microdevice) such as a semiconductor integrated circuit or a liquid crystal display, is reduced, exposed and formed onto a photosensitive substrate (an exposed substrate) such as a wafer. In this type of projection exposure apparatus, the exposure wavelength continues to shift towards shorter wavelengths in order to cope with the trend toward forming finer semiconductor integrated circuits.
Currently, a KrF excimer laser having an exposure wavelength of 248 nm is mainly used, but ArF excimer lasers with a shorter wavelength of 193 nm are beginning to be commercialized. Moreover, projection exposure apparatus using a light source which provides a beam in the wavelength band of the so-called vacuum ultraviolet region such as F
2
laser with 157 nm wavelength, Kr
2
laser with 146 nm wavelength and Ar
2
laser having 126 nm wavelength are being proposed. Moreover, high resolution through larger numerical aperture (NA) of a projection optical system is being achieved, and development of an optical projection system having a larger numerical aperture, in addition to development of shorter wavelength for exposure, is being conducted.
The availability of optical material (lens material) having an excellent transmission rate and uniform property for exposure beam of short wavelength in the ultraviolet region is limited. In a projection optical system with an ArF excimer laser as a light source, synthetic silica glass may be used as a lens material, but with only one type of lens material, correction of chromatic aberration cannot be achieved sufficiently. Hence, calcium fluoride crystal (fluorite) is used for some of the lenses. On the other hand, in a projection optical system using an F
2
laser as a light source, in reality, calcium fluoride crystal (fluorite) is the only lens material suitable for use.
Recently, the existence of intrinsic birefringence in cubic (isometric) system calcium fluoride crystal (fluorite) for such ultraviolet light with short wavelength has been reported. In a super high precision optical system such as a projection optical system used in manufacturing of electronic devices, aberration generated in conjunction with birefringence of the lens material is fatal, and the use of a lens composition and lens design substantially avoiding the effect of birefringence is crucial.
SUMMARY OF THE INVENTION
Considering the aforementioned problems, the present invention aims to assure excellent optical performance substantially without suffering the effect of birefringence even if a crystal material with intrinsic birefringence such as fluorite is used. In addition, the present invention aims to provide a microdevice production method enabling production of high performance microdevices, based on high resolution exposure technology, using an exposure apparatus in which a projection optical system having excellent optical performance utilizing crystal material is provided.
In order to address the aforementioned problems, a first aspect of the present invention provides an optical system having a plurality of crystal optical elements formed with cubic system crystals, wherein the plurality of crystal optical elements comprise first crystal optical elements having a first crystal axis that substantially coincides with an optical axis of the optical system, and second crystal optical elements having a second crystal axis that is different from the first crystal axis, and that is disposed to substantially coincide with the optical axis. The plurality of crystal optical elements Gj are arranged in such a manner that a direction of a predetermined crystal axis in a surface perpendicular to the optical axis is rotated &rgr;j around the optical axis relative to the direction of a predetermined axis in the surface. For a specific light beam passing through the plurality of crystal optical elements Gj, a first evaluation amount Rj for a first predetermined polarization and a second evaluation amount Sj for a second predetermined polarization which are determined by a material constant a of the crystal, the crystal axis substantially coinciding with the optical axis, an angle &rgr;j, an angle &thgr;j, an angle &PHgr;j and an optical path length Lj are established, where &thgr;j is an angle between the specific light beam and the direction of the optical axis, &PHgr;j is an angle between the specific light beam and the direction of the predetermined axis, and Lj is the optical path length of the specific optical path. In addition, a first sum of evaluation amounts &Sgr;Rj which is a sum of the first evaluation amount Rj for the plurality of crystal optical elements and a second sum of evaluation amounts &Sgr;Sj which is a sum of the second evaluation amount Sj for the plurality of crystal optical elements have a predetermined relationship for light beams in imaging beams converged on at least one arbitrary point on an image plane or an object plane of the optical system.
In a preferred embodiment of the first aspect of the invention, the first evaluation amount Rj is information concerning a change in optical path length for the first predetermined polarization, and the second evaluation amount Sj is information concerning a change in optical path length for the second predetermined polarization. In addition, the first predetermined polarization is preferably R polarization having a polarization direction in the radial direction with the center at the optical axis, and the second predetermined polarization is preferably &thgr; polarization having a polarization direction in the tangential direction with the center at the optical axis. Furthermore, the predetermined relationship preferably includes a relationship in which the first sum of evaluation amounts Rj is substantially equal for light beams in imaging beams converged on at least one arbitrary point on the image plane or the object plane of the optical system, the second sum of evaluation amounts &Sgr;Sj is substantially equal for light beams in imaging beams converged on at least one arbitrary point on the image plane or the object plane of the optical system, and the first sum of the evaluation amounts Rj and the second sum of the evaluation amounts Sj are substantially equal to each other for light beams in imaging beams converged on at least one arbitrary point on the image plane or the object plane of the optical system.
In a preferred embodiment of the first aspect of the invention, the first evaluation amount Rj and the second evaluation amount Sj are represented by the following equations, and the crystal optical elements Gj are set in such a manner that the optical axis substantially coincides with the crystal axis [111] or a crystal axis optically equivalent thereto, and the predetermined crystal axis is the crystal axis [−110] or a crystal axis optically equivalent thereto:
[Equations 3]
Rj=&agr;×Lj×[
56×{1−cos(4&thgr;
j
)}−322×sin(4&thgr;
j
)×sin(3&ohgr;
j
)]/192
Sj=&agr;×Lj×[
32×{1−cos(2&thgr;
j
)}+642×sin(2&thgr;
j
)×sin(3&ohgr;
j
)]/192,
where &ohgr;j=&PHgr;j−&rgr;j.
In a preferred embodiment of the first aspect of the invention, the first evaluation amount Rj and the second evaluation amount Sj are represented by the following equations, and the crystal optical elements Gj are disposed in such a manner that the optical
Epps Georgia
Hasan M.
Nikon Corporation
Oliff & Berridg,e PLC
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