CAI, Yanmin (No. 1525 Zhangdong Road, Zhangjiang High-Tech Park, Shanghai 3, 201203, CN)
LIU, Guogan (No. 1525 Zhangdong Road, Zhangjiang High-Tech Park, Shanghai 3, 201203, CN)
CAI, Yanmin (No. 1525 Zhangdong Road, Zhangjiang High-Tech Park, Shanghai 3, 201203, CN)
| What is claimed is:
1. An optical system comprising along an optical axis: a concave spherical mirror; a lens group with positive refracting power arranged adjacent the mirror with an airspace there between, the lens group comprising in order farthest from, to closest to, the mirror: a pair of prisms each having respective first and second flat surfaces, wherein the second flat surfaces are arranged adjacent the flat surface of the plano-convex lens element on opposite sides of the optical axis, and wherein the first flat surfaces are arranged adjacent object plane and image plane, respectively; a positive lens group comprising a plano-convex lens element having a flat surface facing away from the mirror and a meniscus lens element, wherein the plano-convex lens element and the meniscus lens element have respective convex surfaces facing the mirror, and; a telescopic lens group comprising a double-convex positive lens element, a concave-convex lens element and a double-concave negative lens element.
2. The optical system of claim 1, having: at least one of a square field having a size of at least 50.5 mm x 50.5 mm, and a rectangular field having a size of at least 44 mm x 54.4 mm; a numerical aperture of 0.18 or greater; and
. at least one illuminating wavelength in a spectral region that contains at least the g-line, h-line and i-line wavelength.
3. The optical system of claim 1, having a numerical aperture of between 0.16 and 0.20, inclusive.
4. The optical system of claim 1, wherein the object plane and the image plane are separated by the pairs of prisms.
5. The optical system of claim 1, wherein the positive lens element is formed from the glass with high refractive index and high abbe number glass. 6. The optical system of claim 1, wherein all the surfaces of the optical system are spherical or flat shape.
7. The optical system of claim 1, wherein the telescopic lens group is used for enlarged the working distance and reducing the overall length of the optical system.
8. The optical system of claim 1, wherein the working distance is more than 23 mm, and the overall length is less than 780 mm. |
LARGE WORKING DISTANCE UNIT-MAGNIFICATION PROJECTION
OPTICAL SYSTEM
Technical Field
The present invention relates to projection optical system and in particular to a large working distance and unit-magnification projection optical system.
Background of the invention
The minimum line width of integrated circuits (ICs) is becoming smaller with the development of photolithography technology, such as the MPU physical gate length and DRAM half pitch are presently less than 100 nanometers, chips' critical dimensions have attained to the range from 90 nanometers to 65 nanometers according to the direction of ITRS (International Technology Roadmap for Semiconductors). In the same way, photolithography is also employed in gold-bumping, solder-bumping, and other wafer-level IC packaging technologies that require relatively low (i.e., a few microns) resolution, a large depth of focus, and a high throughput. Accordingly, there is an increasing demand for relatively low-resolution in high throughput projection photolithography systems. An optical system with a micron dimension resolution is presently employed to this kind of photolithography, i.e. bumping-lithography.
The present invention described in the "Detailed Description of the Invention" section below, is related to an optical system described in U.S. Pat. No. 6,879,383 (hereinafter, "the '383 patent") issued on April 12, 2005 to Mercado and assigned to Ultratech, Inc.
FIG. 1 is a cross-sectional diagram of an example prior art projection optical system according to the '383 patent. The optical system described in the '383 patent and illustrated in FIG. 1 here is a large field, broad-spectral band, color-corrected, anastigmatic projection optical system that projects an image of a pattern formed on a reticle onto a substrate (wafer) at substantially unit magnification.
The optical system described in the '383 patent includes, along an optical axis, a concave spherical mirror and a positive lens group. An aperture stop of said optical system locates at said mirror. The said positive lens group includes three lens elements, which form a co-axis spherical optical system with the said concave mirror.
The optical system is symmetrical relative to the aperture stop so that the system is initially corrected for coma, distortion, and lateral color. All of the spherical surfaces in optical system are nearly concentric. In addition, the concave mirror can save half of lens element so as to reduce the cost of the optical system. Adjacent lens group is a first prism PA with surfaces SlA and S2A, and a second prism PB with surfaces SlB and S2B. The surface SlA faces an object plane OP2 and the surface SlB faces an image plane IP2. The object plane OP2 and the image plane IP2 are spaced apart from respective flat surfaces SlA and SlB by respective gaps WDA and WDB representing working distances. There is complete symmetry with respect to the aperture stop AS2, WDA is equal to WDB.
However, the optical system described in the '383 patent is employed to bumping-lithography that require a large working distance provided by said optical system. The working distance described in the '383 patent is 7.55 millimeters ~ 11 millimeters so that it is very difficult for motion positioning, measurement and transmission of the reticle and wafer with the increasing demand of techniques. A large working distance can facilitate motion design of wafer stage and reticle stage and transfer system design of a reticle and a wafer. Further, the overall length of the optical system described in the '383 patent is more than 1150 millimeters ~ 1200 millimeters, which has some difficult to environment control requirement of the exposure system.
Summary of the Invention
It is an aim of the present invention to provide an optical system that has large working distances for facilitating the reticle stage and wafer stage design.
In order to achieve the aforementioned aim, the present invention is set forth as follows. The optical system includes, along an optical axis, a concave spherical mirror and a lens group with positive refractive power. The lens group includes, in order towards the mirror, a plano-convex lens element having a convex surface facing the mirror, a negative meniscus lens element and a positive meniscus lens element with their convex surfaces facing the mirror, and a double-convex lens element and a double-concave lens element arranged adjacent the mirror. It is set forth a telescopic lens group that includes the said positive meniscus lens element, said double-convex lens element, and said double-concave lens element. The said telescopic lens group is
used for the aim that enlarged the working distance of said optical system and reduced the overall length of said optical system.
The optical system further includes a pair of prisms each having respective first and second flat surfaces wherein the second flat surfaces are arranged adjacent the flat surface of the plano-convex lens element on opposite sides of the optical axis, and wherein the first flat surfaces are arranged adjacent object plane and image plane, respectively. Each element in the lens group is formed from a different glass type such that the lens group and first and second prisms provide chromatic aberration correction at g, h and i-line wavelengths, and over a square field size of about 50.5 mm x 50.5 mm or greater, or over a rectangular field size of about 44 mm x 54.4 mm or greater, and at numerical aperture of 0.18 or greater. The working distance of said optical system is more than 23 mm, and overall optical length is less than 780 mm.
A second aspect of the invention is a photolithography projection system that includes the projection optical system of the present invention.
Brief description of the drawings
FIG. 1 is a schematic view of an example prior art projection optical system according to the '383 patent;
FIG. 2 is a schematic view of an example embodiment of the projection optical system of the present invention;
FIG. 3 is a schematic view of an another example embodiment of the projection optical system of the present invention, and
FIG. 4 is a schematic diagram of a photolithography system employing the projection optical system of the present invention.
Detailed Description of the Preferred Embodiment
The present invention will be described in detail by reference to the drawings and the preferred embodiment.
The projection optical system is a modified Wynne-Dyson system. The optical projection system further includes a telescopic lens group with positive and negative refractive power for reducing the overall length of the optical system and increasing the working distance of the optical system.
The present invention includes a concave mirror L7 and a lens group with positive refractive power. A positive field curvature of the concave mirror L7 is compensated for a negative field curvature of the lens group. The optical path passes twice through the lens group. FIG. 2 is schematic view of an example embodiment of the present invention.
This optical system includes, along an optical axis, a first lens group Ll and a concave mirror L7 with an airspace there between. The optical system further includes, in order from farthest the first lens group Ll to closest to the mirror L7, a plano-convex lens L2 and a meniscus lens L3, a positive meniscus lens L4, a double-convex lens L5 and a double-concave lens L6.
FIG. 3 is schematic view of another example embodiment of the present invention. The optical system includes, along an optical axis, a first lens group, a second lens group and a concave mirror L7. The first lens group includes a prisms group Ll, a positive plano-convex lens L2 having a flat surface and a convex surface facing the mirror L7, and a meniscus lens L3 having a convex surface facing the mirror L7. The positive lens L2 are arranged adjacent the meniscus lens L3 with small airspace there between for reducing high order spherical aberration of said optical system.
The second lens group includes a positive meniscus lens L4, a double-convex lens L5 and a double-concave lens L6. The second lens group forms a telescopic lens group with enlarged the working distance of said optical system and shortened the overall length of said optical system further reduces the residual aberrations and particularly the chromatic variations.
The prisms group Ll comprises a fist prism LIl having a surface Al and a surface A2, and a second prism L12 having a surface Bl and a surface B2. The surface A2 of the first prism LIl and the surface B2 of the second prism L12 are closely contact to the flat surface of the plano-convex lens L2. The surface Al of the first prism LIl faces an object plane OP and spaces apart from the object plane OP by a spacing WDl representing working distance at object side. The surface Bl of the second prism L12 faces an image plane IP and spaces apart from the image plane IP by a spacing WD2 representing working distance at image side. The WDl and WD2 is at least 23 mm, respectively, and WDl is equal WD2. These prisms LIl and L12
play a role in the aberration correction, including chromatic aberration correction.
The meniscus lens L3 is formed with the high refractive index and high abbe number glass that not only is useful for correcting chromatic aberration, but also is useful for correcting field curvature of said optical system.
Therefore, the first lens group plays a role in correcting chromatic, aberration, spherical aberration, astigmatism, and field curvature of said optical system of present invention.
The positive meniscus lens L4 is concave-convex type, with a convex surface facing the mirror L7. The positive lens L5 is double-convex type and the negative lens L6 is double-concave type. The negative lens L6 is spaced apart from the positive lenses L4 and L5. The negative lens L6 induces the mount of positive spherical aberration for compensating for negative spherical aberration and negative astigmatism that is generated by the positive lenses L4 and L5.
All the surfaces of the lens elements of said optical system are spherical surfaces or flat surfaces.
The design data of the example embodiments of projection optical system are provided in Table 1.
Table 1
Fig. 4 is a schematic diagram of a photolithography system employing the projection optical system 4 of the present invention. The photolithography system has an optical axis and includes a reticle stage 3 adapted to support a reticle 2 at the object plane OP. The reticle 2 has a pattern formed on a reticle surface 21. An illuminator 1 is arranged adjacent reticle stage 3 opposite optical . system 4 and is adapted to illuminate reticle 2.
The photolithography system also includes a wafer stage 6 adapted to movably support a wafer 5 at the image plane IP. In operation, illuminator 1 illuminates reticle 2 so that the pattern is imaged at the wafer 5 by the optical system 4. The result, is an exposure field that occupies a portion of the wafer 5. Wafer stage 6 then moves wafer 5 in a given direction by a given increment, and the exposure process is repeated. This step-and-exposure process is repeated until a desired number of exposure field are formed on wafer 5. In an example embodiment of the present invention, the maximum numerical aperture attains to 0.18, the maximum field height is 70 mm, and the maximum optical resolution is 0.5 microns. In addition, the maximum field of present invention is 70 mm which is used for wafer-level packaging applications, such as bumping lithography and the like. The many features and advantages of the present invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the described apparatus that follow the true spirit and scope of the invention. Furthermore, since numerous modifications and changes will readily occur to those of skill in the art, it is not desired to limit the invention to the exact construction and operation described herein. Accordingly, other embodiments are within the scope of the appended claim.