Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2001-06-19
2004-06-29
Font, Frank G. (Department: 2877)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S055000, C355S077000, C250S492200, C250S492220, C359S507000, C359S512000, C359S820000, C359S196100, C359S364000, C359S365000, C359S379000, C359S380000, C353S037000, C353S072000
Reexamination Certificate
active
06757051
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection optical system, for example, provided in a projection exposure apparatus that is used on occasion of photolithography when manufacturing semiconductor elements, display elements such as liquid crystal display and plasma display, and micromachine etc. such as microdevices. It also relates to a manufacturing method of the projection optical system and to a projection exposure apparatus,
2. Related Background Art
In a photolithographic process used for manufacturing semiconductor elements and the like, projection exposure apparatus is used to expose, through a projection optical system, the image of the pattern of the reticle as mask onto a substrate, i.e. a wafer (or glass plate, etch) on which is applied a photo resist. As the degree of integration of semiconductor elements and so on continue to improve, demand for a resolving power of the projection optical system in a projection exposure apparatus becomes even greater. In order to meet the demand, shortening the wavelength of the illuminating light (exposure light) for exposure and increasing the numerical aperture (NA) of the projection optical system have been performed. However, when exposure a light wavelengths are shortened, varieties of glass material can be used practically are becoming fewer, due to the light absorption. In particular, when the exposure light wavelength becomes vacuum ultraviolet VUV), approximately 200 nm or shorter, the glass materials that can be used under current conditions becomes limited to, for example, synthetic silica, fluorite (CaF
2
), and magnesium fluoride (MgF
2
). The problem of how to compensate chromatic aberration therefore happens.
Even so, there has been progress made in the practical application of the refractive system in projection optical systems using an ArF excimer laser (wavelength 193 nm) as exposure light. However, when the exposure light wavelength fall to 180 nm or shorter as with an F
2
laser (wavelength 157 nm), it is difficult to put projection optical systems into practical use with a refractive system, but expectations are rising for catadioptric systems providing a reflective member as means for compensating chromatic aberration. In this regard, since it is known that fluorite has sufficient transmissivity up to approximately 100 nm, it may be used as a refractive member up to this range. A catadioptric optical system is realized by assembling a refractive member made of fluorite (calcium fluoride) and a reflective member.
A number of types of catadioptric optical systems have already been proposed. Of these, optical systems that have the center portion of the aperture stop shielded are considered most influential since, by using a reflective surface having two or more faces, all of the optical elements can be disposed in an upright cylindrical shape along a single the optical axis without requiring an optical path deflection member. Moreover, since the image of an object in the optical axis can be formed on an image plane, they also include the advantage of chromatic aberration being able to be compensated in a wide exposure field with only a relatively low number of optical elements. Of these, optical systems that form an intermediate image partway through the optical system, for example, such as the optical system disclosed in U.S. Pat. No. 5,650,877, are quite effective in terms of throughput and ease in manufacturing in comparison with optical systems that have an optical member such as a semi-transparent mirror disposed partway, and therefore have no intermediate image formed, greatly reduced light intensity, and increased danger of large-scale flares developing. This type or conventional technology is cited in, for example, U.S. Pat. Nos. 5,717,518 and 5,488,229.
In addition, slice the optical absorption rate of the exposure light increases due to oxygen and carbon dioxide when exposure light is approximately 200 nm or shorter, in order to increase the illuminance on the wafer, it is necessary to replace the air within the lens barrel of the projection optical system with a purge gas such as nitrogen gas (N
2
) or helium gas (He), which have high transmissivity for wavelengths of approximately 200 nm or shorter. To this end, various purge gas supply mechanisms have conventionally been proposed.
SUMMARY OF THE INVENTION
The present invention was devised as a result of the following findings by the inventors. As shown with the above, in cases where an exposure light belonging to vacuum ultraviolet region is used, a catadioptric optical system is an excellent projection optical system. However, with the catadioptric optical system, even if a virtually upright cylinder, the entire length is quite long in comparison with a refractive system, and becomes difficult to support with a single lens barrel. Furthermore, with the catadioptric optical system in U.S. Pat. No. 5,717,518, as it can be seen from the fact that the material for the reflective members is large compared to the glass used for the refractive members, with the catadioptric optical systems it is often the case that the reflective members (material) rust be made larger than the refractive members (glass). In such cases, if a single lens-barrel with a relatively small refractive member and a relatively large reflective member are supported as a single body, the configuration of that lens barrel becomes complicated, and the stable support of that lens barrel becomes difficult. Moreover, there are problems with production costs becoming high with projection optical systems since it takes time to perform the assembly adjustment of that lens barrel and all of the optical elements.
The method for holding the optical element group that comprises catadioptric optical system not by a single lens-barrel, but divided between a plurality of divided lens-barrels is also considered. However, in cases where these optical elements are simply divided into a plurality of groups and held by a plurality of divided lens-barrels, there require the divided lens-barrels with partially extremely complex structures, and also, tears are entertained as to complex processes of assembly adjustment such as aligning the respective axes of each divided lens-barrel to each other.
Furthermore, since there is a danger of the positional relationship between optical elements that are held by different divided lens-barrels fluctuating relative to set values by simply holding a plurality of divided lens-barrels, there is a danger of an increase in contributing factors for the development of aberrations. In cases where a plurality of divided lens-barrels are used, there require positioning, for example, that suppresses the development of aberrations
Moreover, in the projection exposure apparatus using vacuum ultraviolet light having wavelengths of approximately 200 nm or less as exposure light, when a purge gas is flowed through the optical path inside of projection optical systems, if those lens barrels are configured from a plurality of divided lens-barrels, then stagnancy develops in the flow of purge gas at the boundaries of the divided lens-barrels, and there is a danger that the density of light absorbing substances, such as oxygen, may not sufficiently decrease. In particular, in a projection exposure apparatus using light of a wavelength of 170 nm or shorter, for example F
2
laser light (157 nm wavelength) as exposure light, since the permissible value of the residual density of the light absorbing substances is lowered, if a divided lens-barrel type is simply adopted, there is a danger that the residual density of the light absorbing substances may not fall below the permissible value,
An object of the present invention is to provide a projection optical system capable of obtaining high optical performance, a manufacturing method thereof, and a projection exposure apparatus in which it is included.
Another object of the present invention is to provide, in addition to the above objects, a projection optical system that includes a c
Nishikawa Jin
Omura Yasuhiro
Takahashi Tetsuo
Brown Khaled
Font Frank G.
Nikon Corporation
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