Photocopying – Projection printing and copying cameras – Focus or magnification control
Reexamination Certificate
2002-09-20
2003-04-15
Nguyen, Hung Henry (Department: 2851)
Photocopying
Projection printing and copying cameras
Focus or magnification control
C355S052000, C355S053000
Reexamination Certificate
active
06549271
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a projection exposure apparatus and method. The apparatus and method may be used, for example, in a photolithographic process which forms a part of a fabrication process of semiconductor devices, liquid crystal displays or other products, in exposing a mask pattern on a substrate, such as a wafer.
In a photolithographic process for fabricating semiconductor devices or other products, there have been used various projection exposure apparatus including stepping projection exposure apparatus called stepping projection aligners or steppers, as well as scanning projection exposure apparatus called scanning projection aligners or step-and-scan projection aligners. These projection exposure apparatus use a projection optical system (or projection lens) which has to provide an extremely high resolution approaching the theoretical resolution limit. In order to support such a high resolution, many projection optical systems have certain mechanisms for measuring various factors in the resolution (such as, the atmospheric pressure and the ambient temperature) and then correcting the image formation characteristics of the projection optical system depending on the result of such measurement. For higher resolution, projection optical systems are designed to have a large numerical aperture. As a result, the depth of focus is very small. Thus, many projection optical systems have a autofocus mechanism which may comprise an oblique-incidence focus position detection system (or AF sensor). The AF sensor serves to measure the focus position (or the position in the direction along the optical axis of the projection optical system) of the surface of a wafer (or substrate) which typically has some irregularities. The autofocus mechanism brings the surface of the wafer into a position at which it will be coincident with the image plane of the projection optical system, based on the result of such measurement.
In recent years, image formation errors caused by deformation of a mask or reticle have become a problem. If substantially all the pattern-bearing surface area of a reticle is deformed down toward the projection optical system, the average position of the image plane (or image surface) of the pattern-bearing surface is displaced downward, so that the focus position of a wafer could suffer from defocusing if it were not adjusted. Further, when the pattern-bearing surface of a reticle is deformed, the positions of the pattern, which are perpendicular to the optical axis of the projection optical system, on the pattern-bearing surface may also is displaced. Such lateral displacements (or displacements in the direction perpendicular to the optical axis of the projection optical system) may cause distortion errors.
Further, regarding the demagnification projection optical system used in various demagnification projection exposure apparatus including those of non-scan-type (such as, steppers) and those of scan-type (such as, step-and-scan projection aligners), there is an urgent need for improvement in characteristics relating to a reduction in lens aberrations. In the state of the art, almost all lens aberrations could be highly suppressed if lenses were manufactured accurately to design specifications. However, in fact, due to accumulation of tolerances and allowances necessary for fabricating the lenses, the image formation characteristics of the finished lenses are limited and include errors.
There have been many proposals to reduce lens aberrations contributed to by factors involved in the fabrication process. One such technique uses one or more aberration correction plates disposed between the projection optical system and the wafer (or glass substrate), for canceling out the aberrations. Specifically, any residual aberrations which remain after final adjustment of a lens are canceled out by the reverse aberrations intentionally produced by the aberration correction plates so as to minimize the resultant lens aberrations. The residual aberrations may be determined by making and analyzing a test. print using that lens together with a test reticle having evaluation patterns formed thereon, or by measuring the position of aerial images of evaluation patterns of a test reticle formed by that lens by means of a measuring photodetector.
There have been further factors affecting the final image formation characteristics of a lens, which relate to the accuracy (or drawing errors) in the evaluation patterns of a test reticle. In particular, the patterning accuracy (including the linewidth accuracy) of a test reticle is of great concern. In order to correct errors arising from insufficient patterning accuracy, a system has been used in which the pattern positions (or the distances between the patterns) of a finished test reticle are measured and stored using a precision coordinate measurement device, and the positions of the projected images are corrected by an utilization of the stored pattern positions.
As described above, when the image formation characteristics of a projection optical system are evaluated, measurement errors due to insufficient patterning accuracy of an evaluation reticle may be corrected by an utilization of the pattern positions which are measured and stored in advance. However, there are further structural factors affecting the final or total image formation characteristics of the projection exposure apparatus, including the flatness of the exposure area of an evaluation reticle, flatness of the area outside the exposure area of a reticle, and the deflection of a reticle when it is loaded on the projection exposure apparatus. In the past when requirements for the image formation characteristics were less severe, the need for various accuracies relating to these structural factors did not arise. However, in recent years, requirements for the patterning accuracy of semiconductor chips and the registration accuracy between layers of the semiconductor chips have become very severe, so that the accuracies relating to the above structural factors have become of more significance in order to meet such requirements.
One of the structural factors, the deflection of a reticle when it is loaded on the projection exposure apparatus, is caused by gravity. Typically, a reticle is supported at three or four points over its peripheral area and secured by means of vacuum suction, which invariably causes some deflection. When the reticle is curved by deflection, the features of the pattern on the reticle are laterally displaced thereby, the projected images of the features are laterally displaced from their desired positions on the wafer. Apart from the deflection, since each reticles is different in its flatness, if there is a poor flatness in the reticles, the images of the features may be projected features off their desired positions on the wafer. Further, there are sometimes big difference in its flatness between the exposure area and the peripheral area outside the exposure area, and in particular, the peripheral area may typically have a poor flatness. In such a case, since the reticle is supported and secured at the peripheral area, the flatness of the reticle may be possibly deteriorated. This results in larger lateral displacements of the projected images depending on the matching between the contact surfaces of the reticle and the projection exposure apparatus.
FIG.
30
(A) shows how a reticle or mask
900
is supported at its peripheral area by a mask holder
902
. The contact surfaces of the mask holder
902
in contact with the mask
900
have holes or grooves
904
for securing the mask
900
onto the mask holder
902
. When the holes or grooves
904
are in communication with a vacuum source, the mask
900
is secured t the onto the mask holder
902
by vacuum suction.
However, holding a mask on a mask holder in this manner suffers from a problem that the mask
900
is deflected by the gravity as shown in FIG.
30
(A). This results in the lateral displacements of the features of the pattern formed on the mask
900
as we
Ishii Yuuki
Nei Masahiro
Taniguchi Tetsuo
Yasuda Masahiko
Armstrong Westerman & Hattori, LLP
Nguyen Hung Henry
Nikon Corporation
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