Optical: systems and elements – Lens – With reflecting element
Reexamination Certificate
2003-01-02
2004-11-02
Spector, David N. (Department: 2873)
Optical: systems and elements
Lens
With reflecting element
C355S053000
Reexamination Certificate
active
06813098
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical projection systems, and in particular to a variable numerical aperture, large-field unit-magnification projection optical system.
2. Description of the Prior Art
Photolithography is presently employed not only in sub-micron resolution integrated circuit (IC) manufacturing, but also to an increasing degree in advanced wafer-level IC packaging as well as in semiconductor, microelectromechanical systems (MEMS), nanotechnology (i.e., forming nanoscale structures and devices), and other applications. These applications require multiple imaging capabilities from relative low (i.e., a few microns) resolution with large depth of focus, to sub-micron resolution and a high throughput.
The present invention, as described in the Detailed Description of the Invention section below, is related to the optical system described in U.S. Pat. No. 4,391,494 (hereinafter, “the '494 patent”) issued on Jul. 5, 1983 to Ronald S. Hershel and assigned to General Signal Corporation, which patent is hereby incorporated as reference.
FIG. 1
is a cross-sectional diagram of an example prior art optical system
8
according to the '494 patent. The optical system described in the '494 patent and illustrated in
FIG. 1
is a unit-magnification, catadioptric, achromatic and anastigmatic, optical projection system that uses both reflective and refractive elements in a complementary fashion to achieve large field sizes and high numerical apertures (NAs). The system is basically symmetrical relative to an aperture stop located at the mirror, thus eliminating odd order aberrations such as coma, distortion and lateral color. All of the spherical surfaces are nearly concentric, with the centers of curvature located close to where the focal plane would be located were the system not folded. Thus, the resultant system is essentially independent of the index of refraction of the air in the lens, making pressure compensation unnecessary.
Optical system
8
includes a concave spherical mirror
10
, an aperture stop AS
1
located at the mirror, and a composite, achromatic plano-convex doublet lens-prism assembly
12
. Mirror
10
and assembly
12
are disposed symmetrically about an optical axis
14
. Optical system
8
is essentially symmetrical relative to an aperture stop AS
1
located at mirror
10
so that the system is initially corrected for coma, distortion, and lateral color. All of the spherical surfaces in optical system
8
are nearly concentric.
In optical system
8
, doublet-prism assembly
12
includes a meniscus lens
13
A, a piano-convex lens
13
B and symmetric fold prisms
15
A and
15
B. In conjunction with mirror
10
, assembly
12
corrects the remaining optical aberrations, which include axial color, astigmatism, petzval, and spherical aberration. Symmetric fold prisms
15
A and
15
B are used to attain sufficient working space for movement of a reticle
16
and a wafer
18
.
Optical system
8
also includes an object plane OP
1
and an image plane IP
1
, which are separated via folding prisms
15
A and
15
B. The cost of this gain in working space is the reduction of available field size to about 25% to 35% of the total potential field. In the past, this reduction in field size has not been critical since it has been possible to obtain both acceptable field size and the degree of resolution required for the state-of-the-art circuits.
However, most present-day (and anticipated future) high-technology micro-fabrication processes (e.g., for wafer-level IC packaging, semiconductor fabrication, forming MEMS and nano-structures, etc.) include performing a large number of exposure steps using 200-mm and 300-mm wafers. Further, the exposures must be performed in a manner that provides a large throughput so that the fabrication process is economically feasible.
Unfortunately, the optical system of the '494 patent is not capable of providing high-quality imaging at large field sizes (e.g., from three to six 34×26 mm step-and scan fields) with minimum resolution ranging from 0.75 micron to 1.4 microns. Such performance is necessary for, among other things, so-called “mix-and-match” applications, wherein different masks requiring different resolutions (which, in turn typically requires different photolithographic systems) are used in the microdevice fabrication process.
SUMMARY OF THE INVENTION
The present invention is a variable NA, unit-magnification projection optical system. In one embodiment, the system is generally capable of imaging at least six 34 mm×26 mm step-and-scan fields in a low NA configuration (i.e., NA of 0.16 or less), and at least three 34×26 mm step-and-scan fields in a high NA configuration (i.e., NA of 0.34 or greater), or four to five 34 mm×26 mm step-and-scan fields in a mid-range NA configuration (i.e., NA between 0.16 and 0.34).
A first aspect of the invention is an optical system that includes along an optical axis a concave spherical mirror. A variable aperture stop, located at the mirror, determines the NA of the system. A main lens group with positive refracting power is arranged adjacent the mirror and is spaced apart therefrom. The main lens group consists of, in order towards the mirror, a first subgroup having positive refractive power and a second subgroup having negative refractive power. The second subgroup is spaced apart from the first subgroup by an air space. The system also includes first and second prisms each having respective first and second flat surfaces. The second flat surfaces are arranged adjacent the positive subgroup on opposite sides of the optical axis. The first flat surfaces are arranged adjacent the object and image planes, respectively. The system also has a field size at the image plane that is adjustable by adjusting the variable aperture stop. The adjustable field size capable of imaging two or more 34 mm×26 mm step-and-scan fields.
A second aspect of the invention is a photolithography system that includes the projection optical system of the present invention.
REFERENCES:
patent: 4103989 (1978-08-01), Rosin
patent: 4171870 (1979-10-01), Bruning et al.
patent: 4391494 (1983-07-01), Hershel
patent: 4425037 (1984-01-01), Hershel et al.
patent: 4964705 (1990-10-01), Markle
patent: 5031977 (1991-07-01), Gibson
patent: 5040882 (1991-08-01), Markle
patent: 5161062 (1992-11-01), Shafer et al.
patent: 5559629 (1996-09-01), Sheets et al.
patent: 5805356 (1998-09-01), Chiba
patent: 6142641 (2000-11-01), Cohen et al.
patent: 6312134 (2001-11-01), Jain et al.
patent: 6381077 (2002-04-01), Jeong et al.
Jones Allston L.
Spector David N.
Ultratech, Inc.
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