Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2002-09-30
2004-04-27
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S067000, C355S072000
Reexamination Certificate
active
06727978
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a projection exposure apparatus for using a circular or elliptical mask on which a rectangular pattern area is formed.
DESCRIPTION OF THE RELATED ART
A projection exposure apparatus is used for transferring a circuit pattern of a semiconductor integrated circuit or a liquid crystal substrate onto a photosensitive substrate under a lithography technique. Such projection exposure apparatuses are generally classified based on their exposure systems into two known types, namely a mirror projection aligner and a reduction projection exposure apparatus called “stepper”. In the mirror projection aligner, a mask and a wafer are simultaneously scanned with respect to the projection optical system interposed therebetween such that a pattern on the mask is transferred onto the wafer with equivalent magnification.
FIG. 10A
shows a typical example of the mask used in the mirror projection aligner. The aligner transfers an entire pattern on the mask onto the wafer by one time of scanning. Accordingly, the pattern having the same dimension as that of the wafer has been also formed on the mask. Since the wafer generally has a circular shape, exposure shot areas are designed so as to form as many semiconducter chips as possible on a wafer surface without wasting wafer area. Therefore, the mask has had the pattern as shown in
FIG. 10A
, and a contour of the mask has been also circular in conformity with the wafer. Such a mask has been secured to a mask stage by a vacuum chuck or the like by using portions
52
A to
52
D on which no pattern is formed.
On the other hand, the stepper is an apparatus for trasferring a reticle (mask) image onto a wafer by projection through a reduction projection optical system while reducing the image. In recent years, although the pattern for a circuit becomes finer and finer, it is difficult to obtain sufficient resolution for the finer pattern by the mirror projection system described above. For this reason, the reduction projection exposure system using the stepper becomes dominant. The stepper successively exposes a plurality of chips (shot areas) into which a wafer is divided. The stepper is classified to two systems, namely the step-and-repeat system in which only a wafer is step-driven with respect to a projection optical system while a reticle is fixed, and the step-and-scan system in which a reticle and a wafer are relatively scanned with respect to a projection optical system.
FIG. 10B
shows a plan view of a reticle used for such reduction projection exposure systems. The reticle
53
has a rectangular or square pattern section
54
which has a shape equivalent to the shape of chips formed on the wafer. In general, the pattern section
54
is covered with a pellicle for preventing a pattern from adhesion of contaminants. A pellicle frame
55
for supporting the pellicle is provided outside the pattern section
54
. Attracting sections
56
A to
56
D for securing the reticle
53
to a reticle stage are provided at four corners outside the pellicle frame
55
so that they extend in a scanning direction for the reticle.
In any type of the projection exposure apparatuses described above, the contour of the reticle (mask) has been determined in conformity with a shape of an exposure field usable for exposure at one time. Namely, a circular mask has been used for the projection exposure apparatus of the mirror projection system, while a rectangular reticle has been used for the reduction type projection exposure apparatus of. However, as the development of fine patterns and the development of large chips are advancing in recent years, the rectangular or square shape of the reticle, which has been used in the reduction projection exposure apparatus, gives rise to an inconvenience from the viewpoint of rigidity and processing accuracy. In general, the reticle is made of a material of quartz glass from which a circular reticle would be produced easier. Therefore, it is advantageous for reticle production to produce reticles having circular contours. If the reticle has a circular shape, such a reticle is not subject to partial damage, because the flexure against external force or the like is liable to act isotropically on a plane of such a reticle. Accordingly, it is desired to use a circular reticle also in the reduction projection exposure apparatus. However, there has been hitherto no case in which any circular reticle is used in the reduction type projection exposure apparatus. Various problems must be solved upon the use of a circular reticle. For example, installation of a circular reticle to a reticle stage would require accurate positional adjustment, especially adjustment for arrangement in a rotational direction (orientation). In addition, it would be also necessary to investigate arrangement of a pattern and attracting sections in a circular reticle, installation of a pellicle, and positions for attracting the reticle on a reticle stage in conformity therewith.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a novel projection exposure apparatus including a mechanism for performing positional adjustment for a circular reticle on a reticle stage efficiently and highly accurately.
Another object of the present invention is to provide a projection exposure apparatus including a prealignment stage which is capable of previously adjusting a rotational error before arranging a circular reticle on a reticle stage.
Still another object of the present invention is to provide a method which makes it possible to efficiently perform various calibration processes such as adjustment for image formation characteristics of a projection optical system, and positional adjustment for a reticle stage and a photosensitive substrate stage during exchange of a photosensitive substrate in a series of exposure operations for the photosensitive substrate by using a projection exposure apparatus of the step-and-scan system.
Still another object of the present invention is to provide a projection exposure method comprising a step which makes it possible to efficiently perform various calibration processes such as adjustment for image formation characteristics of a projection optical system, and positional adjustment for a reticle stage and a photosensitive substrate stage during exchange of a photosensitive substrate in a series of exposure operations for the photosensitive substrate by using a projection exposure apparatus of the step-and-scan system.
According to a first aspect of the present invention, there is provided a projection exposure apparatus for exposing a photosensitive substrate with a pattern formed on a circular mask having a rectangular pattern area by projection through a projection optical system, comprising:
an illumination optical system for illuminating the circular mask;
a mask stage;
a stage for moving the photosensitive substrate two-dimensionally;
a prealignment stage for pre-aligning the circular mask for its orientation, the prealignment stage comprising a detecting means for detecting an orientational error of the circular mask from a predetermined orientation on the prealignment stage, a rotatable means for rotating the circular mask on the prealignment stage, and a control means for controlling the rotatable means on the basis of the orientational error so that the circular mask has the predetermined orientation; and
a transport means for transporting the circular mask from the prealignment stage to the mask stage while maintaining a pre-aligned orientation of the circular mask. The detecting means may comprise two optical sensors for detecting marks formed at least at two positions on the circular mask respectively, a moving unit for relatively moving the prealignment stage with respect to the optical sensors so that each of the marks on the circular mask relatively passes across each of optical paths of the optical sensors, and a calculating means for calculating the orientational error on the basis of a difference between detected positions of the marks detect
Nguyen Henry Hung
Oliff & Berridg,e PLC
LandOfFree
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