Optical: systems and elements – Lens – With field curvature shaping
Reexamination Certificate
1998-07-16
2001-03-06
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With field curvature shaping
C359S650000, C359S651000, C359S740000, C359S766000, C355S067000, C355S053000
Reexamination Certificate
active
06198576
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a projection optical system for projecting a pattern on a first object onto a second object. More particularly, the invention concerns a projection optical system for use in a lithographic system for projecting a pattern formed on a reticle (or mask) onto a substrate (e.g., silicon wafer, glass plate, etc.).
BACKGROUND OF THE INVENTION
As the resolution for integrated circuit patterns increases, higher performance is being demanded of projection optical systems that are used to project an image from a reticle or mask onto a semiconductor wafer or substrate. In current projection optical systems, in order to meet such demands, the resolving power of the projection optical system can conceivably be improved by increasing the numerical aperture (NA) of the projection optical system. In addition, such optical systems are normally double telecentric to avoid magnification errors and have the capability to vary the numerical aperture to obtain the appropriate conditions for imaging the integrated circuit patterns.
Referring to
FIG. 1A
, one problem associated with current projection optical systems is the existence of a certain amount of field curvature of pupil at the pupil location. This occurs because a mechanical aperture stop is in a plane, and when the numerical aperture is changed, phenomena such as asymmetrical vignetting occurs. As seen in
FIG. 1A
, a projection optical system may be broken down into two parts, one part on a first object side of the aperture stop having a first Petzval sum (ptz(
1
)), and another part on a second object side of the aperture stop having a second Petzval sum (ptz(
2
)). Referring to
FIG. 1B
, when the NA is decreased, asymmetrical vignetting occurs, resulting in a degradation of the imaging performance of the projection optical system. Phenomena such as asymmetrical vignetting cause degradation in the imaging performance of the projection optical system as a whole. Due to the current resolution limits required in projection optical systems, these degradations could be ignored. However, as systems seek to improve resolution and utilize projection optical systems having a higher NA (e.g., NA>0.63), field curvature of pupil at the aperture stop and other asymmetrical phenomena present a much more serious problem and cannot be ignored.
Therefore, what is needed are projection optical systems having high numerical apertures and minimum field curvature of pupil in the aperture stop plane.
SUMMARY OF THE INVENTION
The present invention overcomes the above and other problems associated with prior art projection optical systems. More specifically, the present invention reduces the field curvature of pupil in the aperture stop plane while maintaining imaging performance. Further, the present invention avoids degradation in the imaging performance when the numerical aperture is changed.
One embodiment of the present invention which achieves the above and other objects of the present invention includes a projection optical system having a numerical aperture of 0.63 or greater and satisfies the following conditions:
−0.000005
<PTZ
(
1
)+
PTZ
(
2
)<0.000005 (1)
−0.005<
PTZ
(
1
)<0.005 (2)
where,
PTZ(
1
) is the amount of Petzval sum between first object and the aperture stop; and
PTZ(
2
) is the amount of Petzval sum between the aperture stop and the second object.
The projection optical system according to the present invention can be utilized with various exposure systems that perform a one-shot exposure method, such as the step and repeat exposure systems, or with systems which perform a scanning exposure method.
An exposure apparatus utilizing the projection optical system of the present invention includes a first stage capable of holding a wafer with a photosensitive substrate on the main surface thereof. A second stage is included for holding a mask thereon. An illumination optical system is provided for illuminating a pattern on the mask to form an image. A projection optical system is disposed between the mask and the substrate to project the image formed by the illumination system onto the substrate.
A projection optical system in accordance with the present invention for use in the above-mentioned exposure apparatus includes, from the reticle side thereof,
a first lens group having a positive refractive power,
a second lens group having a negative refractive power,
a third lens group having a positive refractive power,
a fourth lens group having a negative refractive power,
a fifth lens group having a positive refractive power and an
aperture stop placed within the fifth lens group.
These and other advantages of the present invention will become more apparent upon a reading of the detailed description of the preferred embodiments which follows, when considered in conjunction with the drawings of which the following is a brief description. It should be clear that the drawings are merely illustrative of the currently preferred embodiments of the present invention, and that the invention is in no way limited to the illustrated embodiments. The present invention is best defined by the claims appended to this specification.
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Hecht, Eugene, “Optics: Second Edition,” Addison-Wesley Publishing Company, Reading, Massachusetts, pp. 228-229, Apr. 1988.
Beyer Weaver & Thomas LLP
Epps Georgia
Lester Evelyn A.
Nikon Corporation
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