Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2000-11-28
2003-04-08
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S030000
Reexamination Certificate
active
06545746
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a projection exposure apparatus used to transfer a mask pattern onto a photosensitive substrate in a photolithography process for producing, for example, semiconductor devices, image pickup devices (CCDs, etc.), liquid-crystal display devices, or thin-film magnetic heads.
To produce semiconductor devices, for example, a projection exposure apparatus is used to transfer a pattern formed on a reticle as a mask onto each shot area on a wafer coated with a photoresist. Hitherto, a step-and-repeat type (one-shot exposure type) reduction projection exposure apparatus (stepper) has frequently been used as a projection exposure apparatus for the pattern transfer process. On the other hand, to meet the demand that the area of a pattern to be transferred should be increased without substantially increasing the load on the projection optical system, attention has recently been paid to a step-and-scan type projection exposure apparatus wherein a reticle and a wafer are synchronously scanned relative to a projection optical system in a state where a part of a pattern on the reticle is projected as a reduced (demagnified) image on the wafer, thereby sequentially transferring the demagnified image of the reticle pattern onto each shot area on the wafer. The step-and-scan method has been developed by combining the advantage of the transfer method of the aligner (slit scan method) that transfers a pattern on the whole surface of a reticle onto the whole surface of a wafer in the magnification ratio of 1:1 by one scanning exposure with the advantage of the transfer method of the stepper.
In general, it is a requirement that resolution of projection exposure apparatuses be increased. One approach to increasing resolution is to use a light beam of a shorter wavelength as an illuminating light for exposure. Accordingly, use has recently been made of excimer laser light in the ultraviolet and far-ultraviolet regions as illuminating light for exposure, such as KrF excimer laser light (wavelength: 248 nm) or ArF excimer laser light (wavelength: 193 nm). The use of metal vapor laser light, higher harmonics of YAG laser light, etc. has also been examined.
When excimer laser light is used as illuminating light for exposure, for example, broad band laser light sources and narrow band laser light sources are available as excimer laser light sources. The term “narrow band laser light source” means a laser light source in which the spectral half-width of laser light is not more than 2 pm to 3 pm. The term “broad band laser light source” means a laser light source in which the spectral half-width of laser light is not less than 100 pm. When illuminating light of a short wavelength in the ultraviolet region or shorter wavelength region, such as excimer laser light, is used, vitreous materials usable for refracting lenses of projection optical systems are limited to materials such as quartz and fluorite. Therefore, as the wavelength of illuminating light used shortens as described above, it becomes more difficult to achromatize the projection optical system. Accordingly, it is desirable to use a narrow band laser light source in order to facilitate achromatization of the projection optical system.
However, the band of excimer laser light is originally broad. Therefore, in narrow band laser light sources, the oscillation spectral width of excimer laser light is narrowed by injection locking or the like. For this reason, the laser output of narrow band laser light sources is lower than that of broad band laser light sources. Further, narrow band laser light sources are inferior to broad band laser light sources in terms of lifetime and production cost. Therefore, in terms of the laser output, lifetime and production cost, broad band laser light sources are more advantageous than narrow band laser light sources. Accordingly, attempts have recently been made to use a broad band laser light source in a projection exposure apparatus having a projection optical system structured such that achromatization can be readily achieved.
Incidentally, projection optical systems usable in scanning exposure type projection exposure apparatuses (scanning projection exposure apparatuses) such as step-and-scan type projection exposure apparatuses include a catadioptric system that uses a concave mirror, and a refracting optical system formed from a combination of refracting lenses only, as disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. 6-132191. When such a catadioptric system is used, achromatization can be readily achieved by disposing a concave mirror in a group of refracting lenses because concave mirrors are free from chromatic aberrations. Consequently, it becomes possible to use a broad band laser light source, which is advantageous in terms of laser output, lifetime, etc.
Even in the case of using the second-mentioned refracting optical system, it is possible to use a broad band laser light source because the range of achromatization can be widened by increasing the proportion of fluorite in the entire refracting lens system.
In the above-described prior art, when the second-mentioned refracting optical system is used as a projection optical system, it is necessary to use fluorite for ten-odd lens elements in a total of twenty-odd lens elements, for example, in order to achieve achromatization over a wavelength width of the order of 100 pm to use a broad band laser light source. However, fluorite has the following properties: it is difficult to machine; the yield after the machining is unfavorably low; the change of refractive index with temperature is large; and the coefficient of thermal expansion is high, so that deformation occurs to a considerable extent in response to changes in temperature. Therefore, if many fluorite lenses are used, the temperature dependence of the image-forming characteristics of the projection optical system becomes unfavorably high.
When the first-mentioned catadioptric system is used as a projection optical system, it is possible to achieve achromatization over a wavelength width of the order of 100 pm by disposing a concave mirror in a predetermined position in a group of a predetermined number of refracting lenses, for example, because concave mirrors are free from chromatic aberrations. However, it is necessary in a scanning projection exposure apparatus to set the demagnification ratio for a pattern transferred from a reticle to a wafer on the order of from 1/4 to 1/5, for example, and if a concave mirror is merely disposed in such a group of refracting lenses, the range in which favorable image-forming characteristics can be obtained becomes an arcuate area Since the pattern areas on reticles have a rectangular external shape, if scanning exposure is carried out with such an arcuate area, the reticle scanning distance must be set to use considerably greater than the width of the pattern area. This causes the reticle-side stage to increase in size, unfavorably.
Further, when such an arcuate area is used, it is also necessary to use a lens that is not axially symmetric, and it is not easy to machine a non-axially symmetric lens with the desired accuracy.
Moreover, when a catadioptric system is used as a projection optical system, the projection optical system becomes large in size and complicated in arrangement because of routing of the image-forming light beam. Therefore, it is desirable to form the whole projection exposure apparatus in as compact a structure as possible by taking into consideration the reticle scanning direction, etc.
Further, it has been pointed out that when ultraviolet light such as excimer laser light is used as illuminating light for exposure, it is necessary to circulate nitrogen (N
2
) gas or a gas (e.g. air) having ozone removed therefrom in the projection exposure apparatus in consideration for the absorption of ultraviolet light by ozone and also the properties of photoresist. However, if all the gas in a chamber in which the projection exposure apparatus is installed is
Armstrong Westerman & Hattori, LLP
Nguyen Henry Hung
Nikon Corporation
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