Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2002-07-10
2004-09-07
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C359S355000
Reexamination Certificate
active
06788389
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to projection optical systems, production methods of the projection optical systems, and exposure apparatus including the projection optical system, in particular, projection optical systems which are suited for exposure apparatus to be used to manufacture microdevices such as semiconductor devices and liquid crystal display devices using a photolithography process.
2. Description of Related Art
It is known to use a method in which a pattern of a photomask (also called a reticle), after being reduction magnified, 4-5 fold, is formed onto a photosensitive substrate (an exposed substrate) such as a wafer to form an electronic device (microdevice) such as a semiconductor integrated circuit or a liquid crystal display. In this type of projection exposure apparatus, the exposure wavelength continues to shift towards a shorter wavelength in order to cope with the trend toward finer semiconductor integrated circuits.
Currently, a KrF excimer laser having an exposure wavelength of 248 nm is mainly used, but the ArF excimer laser with a shorter wavelength of 193 nm is beginning to be commercialized. In addition, a projection exposure apparatus using a light source which provides a beam in the wavelength band of the vacuum ultraviolet region such as an F
2
laser with 157 nm wavelength, a Kr
2
laser of 146 nm wavelength and an Ar
2
laser of 126 nm wavelength is being considered. High resolution through a larger numerical aperture (NA) of a projection optical system is being achieved, and the development of a projection optical system having a larger numerical aperture, in addition to the development of a shorter wavelength for the exposure wavelength is ongoing.
Availability of optical material (lens material) having an excellent transmissiveness (transmittance) and uniform property for the exposure beam of a short wavelength in the vacuum ultraviolet region is limited. In a projection optical system with the ArF excimer laser as a light source, synthetic silica glass may be used as the lens material. However, with only one type of lens material, correction of chromatic aberrations 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 that can be used.
SUMMARY OF THE INVENTION
Recently, the existence of intrinsic birefringence in a cubic (isometric) system calcium fluoride crystal (fluorite) for such vacuum ultraviolet rays with a short wavelength has been reported. In a super high precision optical system such as a projection optical system used in the manufacturing of electronic devices, the aberration generated in conjunction with birefringence of the lens material is fatal, and the use of a lens composition and lens design to substantially avoid the impact of birefringence is crucial.
Considering the aforementioned problems, aspects of the present invention aim to assure excellent optical performance substantially without receiving the impact of birefringence even if a crystal material with intrinsic birefringence, such as fluorite, is used.
In order to achieve the aforementioned objective, a first aspect of the invention relates to a production method of a projection optical system, of the type which projects an image of a first surface onto a second surface based on light having a predetermined wavelength and which includes at least one refractive member made of an isometric system crystal material that transmits the light having the predetermined wavelength. This aspect of the invention comprises a design step, including a sub-step of determining the direction of a crystal axis of the refractive member, made of at least one of the isometric system crystal material, while evaluating the light of a first polarization component and a second polarization component differing from the first polarization component, for obtaining the predetermined design data. This aspect of the invention also includes a crystal material preparation step of preparing the isometric system crystal material; a crystal axis measurement step of measuring the crystal axis of the isometric system crystal material; a refractive member formation step of forming a refractive member with a predetermined shape from the isometric system crystal material based on the design data in the design step; and an assembly step of arranging the refractive member based on the direction of the crystal axis of the refractive member obtained in the design step.
This aspect of the invention assures excellent optical performance because it enables the determination of the installation angle of the crystal axis of the refractive member made of the isometric system crystal material in such a manner that the impact of the birefringence is minimized while evaluating the impact of the birefringence caused by the isometric system crystal material in terms of a plurality of polarization components.
This aspect of the present invention also may include another step of preparing at least one refractive member with a predetermined birefringence distribution wherein the birefringence distribution is determined based on the design data in the design step. This aspect of the invention assures even more excellent optical performance because it enables the correction of the residual effect of birefringence which is reduced by optimization of the installation of the isometric system crystal material with the refractive member having the predetermined birefringence distribution.
The predetermined birefringence distribution may be at least one of a predetermined birefringence stress distribution of the refractive member and a birefringence distribution caused by a thin film provided on the refractive member.
The refractive member having the predetermined birefringence distribution may be made of silica or fluoride doped silica.
Preferably, the relationship, 0.6<&phgr;p/&phgr;c≦1, is satisfied, where, &phgr;c is a clear aperture of the refractive member with the birefringence stress distribution, and &phgr;p is the light beam aperture of the light beam emitted from a predetermined point on the first surface when the light beam passes through the refractive member with the birefringence stress distribution. This aspect of the invention assures the position of the refractive member having the birefringence stress distribution to be the optimal position for correcting the birefringence caused by the isometric system crystal material using the refractive member having the birefringence stress distribution, namely, the position where the birefringence correction capability of the refractive member having the birefringence stress distribution is utilized as much as possible.
This aspect of the present invention may further comprise an aspherical surface creation step of forming the surface shape of at least one optical member in the projection optical system in an aspherical shape, wherein the aspherical shape is determined by the design data in the design step. This aspect of the invention enables the correction of a scalar component aberration (the aberration that does not depend on polarization direction) out of all the aberrations generated by the intrinsic birefringence of the isometric system crystal material using the aspherical surface.
The aspherical surface shape may include an asymmetric aspherical shape relative to the optical axis of the optical member.
The assembly step may comprise an optical performance measurement sub-step of measuring the optical performance of the assembled projection optical system, an optical member correction sub-step of changing the position and/or posture of at least one optical member in the projection optical system in order to make the measured optical performance be a predetermined optical performance, and an aspherical surface processing sub-step of forming the surface shape of at least one optical member in the
Fujishima Youhei
Ikezawa Hironori
Omura Yasuhiro
Ozawa Toshihiko
Suzuki Takeshi
Nguyen Henry Hung
Nikon Corporation
Oliff & Berridg,e PLC
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