Optics: measuring and testing – By alignment in lateral direction – With registration indicia
Reexamination Certificate
2000-09-22
2002-06-25
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
With registration indicia
C250S548000
Reexamination Certificate
active
06411387
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a stage apparatus, a projection optical apparatus and an exposure method. More particularly, the present invention relates to a stage apparatus including a material support capable of positioning a material piece carried thereon with at least three degrees of freedom, a projection optical apparatus including such a stage apparatus used as a positioning stage for a material piece, and an exposure method for transferring a pattern formed on a mask through a projection optical system by exposure to a photosensitive substrate loaded on a movable stage. One representative example of the above-mentioned projection optical apparatus is a projection exposure apparatus, such as a stepper, for use in a photolithographic process forming a part of the fabrication process of semiconductor devices, liquid crystal displays or other products. Another example is lens inspection equipment for measuring various aberrations associated with a projection optical system (or projection lens) equipped for such a projection exposure apparatus.
In photolithographic process for fabricating semiconductor devices, liquid crystal displays or other products, there have been used various projection exposure machines in which an exposure light beam is used to illuminate a mask or a reticle (a generic term “reticle” is used to mean either hereinafter) so as to transfer a pattern formed on the reticle through a projection optical system onto a photosensitive substrate. Further, in order to measure aberrations associated with a projection optical system equipped for such a projection exposure machine, there has been used lens inspection equipment in which various aberrations with the projection optical system are measured by making a projection exposure so as to transfer an image of a reticle pattern onto a photosensitive substrate and analyzing the exposure result (or the resulting pattern image formed on the photosensitive substrate.)
With these machines and equipment, a stage apparatus is typically used for positioning a material piece or substrate having a photosensitive layer coated on its surface. The substrate may typically comprise a silicon wafer or a glass plate (referred to simply as a “wafer” hereinafter). One common stage apparatus comprises an XY-stage capable of two-dimensional movement in a plane (typically, an XY-plane) and a material support (or Z-stage) for supporting a material piece or wafer thereon, the material support being mounted on the XY-stage and capable of displacing in the Z-direction perpendicular to the XY-plane.
By way of example, consider a demagnification projection exposure apparatus of the step-and-repeat type (commonly referred to as a “stepper”). In an exposure apparatus of this type, after the exposure of a shot-area, the wafer is moved or stepped to the next shot-area and the process is repeated until the entire surface of the wafer (or every shot-area on the wafer) has been processed to have reticle patterns transferred onto it. The stepping movement of the wafer is produced by the two-dimensional movement or translation of the XY-stage supporting that wafer.
With recent progress in the microminiaturization of circuit patterns exposure apparatuses are required to provide higher performance including an improved overlay accuracy as well as a higher resolution. Because a typical demagnification exposure apparatus uses a projection lens having a large numerical aperture (NA), its depth of focus is so small that a certain, appropriate mechanism must be provided for bringing with precision the surface of the wafer to a position at which it will be coincident with the image plane of the reticle pattern defined by the projection lens (such a mechanism may be, for example, an auto-focusing mechanism or a leveling mechanism). Also, it is desired to measure minute irregularities in the nominally flat surface of a wafer, or variation in height of the surface of a wafer placed on the stage among different points on the wafer surface. That is, wafer-flatness measurement is desired. Further, it is also desired to measure with accuracy various aberrations (such as, variation in depth of focus, curvature of image plane, tilt of image plane and others) associated with a projection optical system.
Wafer-flatness measurement, as well as the measurement of aberrations with the projection lens by lens inspection equipment, have been typically performed by using the positions of the moving XY-stage as the reference for the measurement. Specifically, in the case of wafer-flatness measurement, the XY-stage is driven so as to sequentially move the wafer into a number of predetermined measurement points arranged in a rectangular array or matrix, and the height (or the position in the Z-direction) of the surface of the wafer is measured at each measurement point. The Z-direction is perpendicular to the plane in which the XY-stage moves and is usually defined as the direction along the optical axis of the projection optical system.
Unfortunately, the surface of the wafer or material piece carried by the XY-stage may rise and sink (or displace in the Z-direction) while the XY-stage is driven (or more exactly an X-stage and/or a Y-stage, together composing the XY-stage, are driven) to move the wafer in the XY-plane. In the case where the drive mechanism for the stage comprises a feed screw in threading engagement with the stage and an electric motor for rotating the feed screw, such displacement in the Z-direction may be possibly caused by various factors, including minute irregularities in the nominally flat bearing surface along which the stage is guided, an inevitable play between the feed screw and the stage engaging with each other, some distortion induced in the feed screw, and others.
Therefore, the rise/sink of the stage produced when the stage is moved into different positions, or the displacement characteristic of the stage, often causes errors in the results of the wafer-flatness measurement and the lens-aberration measurement. Further, unlike the yaw of wafers which may be separately measured with ease by a yaw-measuring interferometer, it has been impossible to separately measure the displacement characteristic of the stage (or the rise/sink of the stage), with the result that the wafer-flatness measurement and the lens-aberration measurement have been performed so far, allowing for such errors.
Nevertheless, with the continuing increase in the complexity of present-day integrated circuits (ICs), the measurement errors caused by an undesirable displacement characteristic of the stage are becoming unacceptable, and it is almost certain that such measurement errors are not acceptable for the fabrication of the next generation of 64-megabit dynamic random access memories (DRAMs) any longer.
On the other hand, with respect to exposure apparatuses used in producing, for example, semiconductors, in order to transfer a pattern on a mask through a projection optical system to a photosensitive substrate with high accuracy, it is important to correct an amount of tilt of the photosensitive substrate which is provided on a movable stage, relative to an image plane with respect to the projection optical system. In a conventional exposure apparatus, in order to suppress variations in an amount of tilt of the movable stage during movement thereof, a stage-driving surface on which the movable stage is moved is machined with high precision according to the type of exposure apparatus, thereby flattening the stage-driving surface. However, in the above-mentioned conventional exposure apparatus, there is a disadvantage, such that correction of the amount of tilt of the movable stage is dependent on the degree of precision in machining the stage-driving surface, so that an improvement in accuracy of correction of the amount of tilt of the movable stage is limited. Further, due to the need for machining with high precision, production costs of stages become undesirably high.
In order to obviate such disadvantages, various countermeasures have been
Kaneko Kenichiro
Yamamoto Naoyuki
Armstrong Westerman & Hattori, LLP
Font Frank G.
Lauchman Layla
Nikon Corporation
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