Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2002-10-11
2004-06-22
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S067000
Reexamination Certificate
active
06753948
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure apparatus which is used in the photolithography process for manufacturing microdevices, such as semiconductor elements, or liquid crystal elements, image pick-up elements (CCD), thin film magnetic heads, opto-magnetic discs, etc., and in particular, to a scanning exposure apparatus of the so-called slit scan method, the step and scan method, or the like, which scans a mask and a photosensitive substrate in synchronization so as to perform exposure of said substrate with a pattern image of said mask.
2. Related Background Art
In producing semiconductor elements or liquid crystal display elements by the photolithography process, a projection exposure apparatus is used for projecting an image of a pattern on a photomask or a reticle (the both will be hereinafter referred to as “reticle”) through a projection optical system onto a substrate (a wafer, a glass plate, or the like) on which a coating of photoresist or the like is given, and thereby effecting exposure of the image on the substrate. Such a projection exposure apparatus is required to perform such accurate overlay exposure that a presently exposed reticle pattern is accurately overlaid across the entire exposure field over a chip pattern on the substrate such as a wafer, which was formed by previous exposure and process treatments. Namely, it is necessary that the exposure is conducted while keeping high overlay accuracy between the pattern formed on the substrate and the pattern of the reticle.
A semiconductor element or the like is normally constructed in lamination of many layers of circuit patterns overlaid on a substrate. For example, if the circuit patterns in the layers are formed using different projection exposure apparatus and if there is a difference exceeding a predetermined acceptable value between a magnification error of a projection exposure apparatus which performed previous exposure of a circuit pattern in a previous layer and a magnification error of a projection exposure apparatus which is to perform present exposure of a circuit pattern in a next layer, the overlay accuracy is degraded so as to lower the yield of semiconductor elements. Also, an overlay error occurs if there is a difference in distortion of projected image of mask pattern between two projection exposure apparatus. There is also a case in which a substrate is distorted by heating due to various process treatments after exposure, which results in distorting a previously exposed pattern. The distortion of pattern in this case will eventually be similar to the distortion of projected image caused by a previously used projection exposure apparatus.
Concerning this, conventional projection exposure apparatus were generally apparatus (steppers) of the full exposure method (or “full field method”) which projected a reduced image of reticle pattern over the entire exposure field on a photosensitive substrate by one operation. In applications with such projection exposure apparatus of the full exposure method, imaging properties of a projection optical system are actively adjusted by driving some lens elements in the projection optical system or a reticle along the optical axis or inclining it with respect to the optical axis for example to change a projection magnification of projected pattern or to distort the projected pattern in the form of trapezoid or barrel. As described, there are suggestions for the method in which exposure is effected while the distortion of projected image of presently exposed pattern is kept correspondent to that of previously exposed pattern (see U.S. Pat. Nos. 4,734,746 and 5,117,255).
The suggestions for improvement in overlay accuracy as described above were made under a premise of use of the full exposure method (full field method). However, a recent trend is to increase the size of a chip pattern of semiconductor element, which requires the projection exposure apparatus to increase an exposure area, for permitting a pattern with a larger area on the reticle to be exposed on the photosensitive substrate. To handle the larger area exposure of transferred pattern and the limit of exposure field of projection optical system, there are suggested projection exposure apparatus of the so-called slit scan exposure method, which moves the reticle and the photosensitive substrate in synchronization relative to a rectangular, arcuate or hexagonal illumination area (hereinafter referred to as “slit illumination area”), for example, whereby patterns with a larger area than the slit illumination area on the reticle are successively projected and exposed on the substrate.
The scanning exposure method has such advantages that illumination uniformity on the reticle (or a wafer) is improved, a distortion, a curvature of the image field, astigmatism, and the like of the projection optical system are reduced, and the uniformity of focusing position in the exposure field is improved, because the illumination area on the reticle in the slit scan exposure method is smaller than that in the full exposure method. Also, the slit scan exposure method has another advantage that large area exposure is possible in the scanning directions of the reticle and the substrate without being affected by the limit of field size of projection optical system.
The conventional projection exposure apparatus of the slit scan exposure method as described above, however, had such a disadvantage that if the magnification error was simply corrected only by the projection optical system, the magnification error could be corrected in the non-scanning direction perpendicular to the scanning direction, but the magnification error could not be fully corrected in the scanning direction.
In addition, since the conventional projection exposure apparatus of the slit scan exposure method have the width of slit illumination area in the scanning direction different from that in the non-scanning direction, they are likely to have a difference between the magnification error in the scanning direction and the magnification error in the non-scanning direction in each shot area because of a bias of heat distribution caused by absorption of exposure light in the projection optical system and because of a difference between a shot size in the scanning direction and a shot size in the non-scanning direction on the substrate. Accordingly, it is to be desired that the apparatus are arranged to correct the magnification error in the scanning direction and the magnification error in the non-scanning direction independently of each other in particular.
Further, since the conventional slit scan exposure method uses only a part of field of the projection optical system and a same pattern passes through a plurality of portions in the field of projection optical system upon scanning exposure, it cannot be possible to distort the projected image as a whole in the trapezoidal shape or in the barrel shape by simply inclining a lens element in the projection optical system.
The method for improving the overlay accuracy in the slit scan exposure method as described above is disclosed for example in Japanese Patent Publication No. 5-29129. The projection optical system in the exposure apparatus as disclosed in the publication is of the reflection type, using only mirrors such as a concave mirror and a convex mirror. Also, the mask and the substrate are held on a single scanning member (scanning frame) so that they are moved together in a same direction.
An amount of positional deviation between the mask and the substrate is measured at a plurality of portions during movement of the mask and the substrate. A fine feed mechanism for adjusting the position of wafer by a fine amount is driven based on the detection result to change a relative position between the mask and the substrate.
The above conventional technology, however, simply adjusts the position of wafer by a fine amount and has a disadvantage that, for example, if a shot area on the substrate is distorted in a trapezoidal shape or in
Adams Russell
Esplin D. Ben
Miles & Stockbridge P.C.
Nikon Corporation
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