Optical: systems and elements – Lens – With field curvature shaping
Reexamination Certificate
2000-03-20
2003-03-25
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With field curvature shaping
C359S642000, C359S754000, C359S763000, C359S766000
Reexamination Certificate
active
06538821
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection optical system which is employed when a pattern such as an electric circuit pattern drawn on a projection original plate including a reticle or a mask is transferred onto a photosensitive substrate such as a semiconductor wafer or a glass plate coated with photosensitive material by projection photolithography.
2. Related Background Art
Recently, a projection exposure method is fairly generally employed for transferring a necessary pattern onto an integrated circuit such as an IC or a LSI etc., a flat display of such as a liquid crystal etc.
Especially, for manufacturing a semiconductor integrated circuit or a substrate packaging a semiconductor chip therein, a pattern thereof is increasingly miniaturized and a wider projection area is required for a flat display for liquid crystals, or the like. Consequently, an exposure apparatus, especially a projection optical system thereof, for printing such patterns is required to have a higher resolving power and a wider exposure area.
However, any of projection optical systems conventionally employed in an exposure apparatus does not fully satisfy both of such requirements, i.e., a higher resolving power and a wider exposure area.
More specifically, for obtaining a higher resolving power, it is required to enlarge the numerical aperture of the optical system, which inevitably results in an enlarged lens size. In the same manner, in order to obtain a wider exposure area, the lens size is still enlarged since a flat object is to be projected on a flat surface. If the lens size is enlarged, a glass material for the lens is required to have a larger size. However, it becomes difficult to prepare a glass material having a lager size than the current one in terms of the homogeneity of the material, or the like. An enlarged lens size makes another difficulty in a step of polishing the glass material, and it becomes impossible to polish a lens having a larger size than the present one. Under such circumstances, it is an important object to be achieved up to now to reduce the maximum effective diameter of lens of the optical system, while securing a large numerical aperture.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a projection optical system which has a large numerical aperture and a satisfactorily reduced maximum effective diameter of lens of the optical system.
The present invention has been contrived to solve the above-mentioned problem. According to the present invention, there is provided a projection optical system for projecting an image on a first surface onto a second surface, comprises first lens group G
1
of positive refracting power including two or more positive lenses, a second lens group G
2
of negative refracting power including two or more negative lenses, a third lens group G
3
of positive refracting power including three or more positive lenses, a fourth lens group G
4
of negative refracting power including two or more negative lenses, and a fifth lens group G
5
of positive refracting power including at least six or more consecutive positive lenses, in the named order from the first surface side to the second surface side, wherein either one of the fourth lens group G
4
and the fifth lens group G
5
has one aspherical surface, the fifth lens group G
5
has an aperture stop inside thereof, a portion at which a light flux is diverged right before the aperture stop has a first air lens LA of negative refracting power, the radius of curvature rA
1
of the lens surface on the first surface side of the first air lens LA is positive, and a portion at which the light flux is converged at the rear of the aperture stop has a second air lens LB of negative refracting power.
In the projection optical system of the present invention, an aspherical surface is introduced to correct a spherical aberration which is generated due to an enlargement of the numerical aperture. This aspherical surface is to be applied to a portion at which mainly spherical aberration is generated and, more naturally, to a portion at which the spherical aberration can be easily corrected. Consequently, the aspherical surface is to be applied to a portion in the vicinity of the aperture stop. According to the present invention, since the aperture stop is provided in the fifth lens group G
5
, the aspherical surface is to be applied to the fifth lens group G
5
which has this aperture stop therein or to the fourth lens group G
4
near the aperture stop.
However, it is more preferable that the aspherical surface should be applied to the portion at which the light flux is converged, in order to avoid the aspherical lens surface from being enlarged. Consequently, in first and second embodiments described below, one surface out of the fourth lens group G
4
is selected as the aspherical surface.
Since one of the surfaces is used as the aspherical surface, it is possible to correct a spherical aberration. However, it is also required to correct other aberrations which may be generated due to an enlargement of the numerical aperture.
The projection optical system has not only a high numerical aperture but also a large field size, so that there are generated larger aberrations around the image field. Especially, a coma aberration around the image field is generated to be larger due to the enlargement of the numerical aperture. Further, an amount of a generated coma is normally different depending of an amount of enlargement of the field angle (i.e., increase of the image height). This is called a field angle fluctuation component of a coma aberration.
In order to remove this field angle fluctuation component of the coma aberration, a more complicated correction is required to be conducted. In general, at least another two aspherical surfaces are required for correcting an upper coma and a lower coma, respectively. However, employment of a large number of aspherical surfaces brings about an increase in the cost undesirably. Then, according to the present invention, the field angle fluctuation component of the coma aberration is corrected by a spherical surface, a method of which will be described below.
Generally, in an optical system, the smaller an angle at which a light beam is incident onto each lens surface is, the less aberrations are generated and the more loosen a tolerance or the like becomes, appropriately. Especially, such tendency is strong in an optical system which pursuits the extreme performance of a projection optical system or the like.
However, according to the present invention, there are provided surfaces acting against the light beam to make an angle of incidence large, conversely. These surfaces are a lens surface rA
1
on the first surface side of the first air lens LA and a lens surface rB
2
on the second surface side of the second air lens LB. Since these air lenses LA, LB are provided in the portions in which the light flux is diverged and the light flux is converged to sandwich the aperture stop therebetween, so that the field angle fluctuation components of the upper coma and the lower coma can be corrected.
Though a considerable amount of aberrations is normally generated on such surfaces oriented to act against light fluxes, converse aberrations are caused by the curved surfaces existing in front or rear thereof and having a similar curvature, that is, the lens surface rA
2
on the second surface side of the first air lens LA and the lens surface rB
1
on the first surface side of the second air lens LB, so that high order aberrations are corrected by a difference therebetween.
With the above arrangement, the field angle fluctuation component of coma is corrected. As a result, it becomes possible to reduce the maximum effective diameter of lens.
Next, according to the present invention, it is preferable to satisfy the following conditions:
0.1
<D/L
<0.3; (1)
|
PA−PB|×L
<1.0; (2)
0.2
<|PA|×L
<2.0; (3)
0.2
&
Choi William
Epps Georgia
Miles & Stockbridge P.C.
Nikon Corporation
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