Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-10-12
2001-02-27
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06194104
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a method of Optical Proximity Correction for photolithographic patterns and more specifically to the use of mathematical modeling based on curve fitting to achieve Optical Proximity Correction.
(2) Description of the Related Art
Photolithography is critical to the fabrication of integrated circuit wafers and utilizes masks to transfer images, such as line/space patterns, to the wafer. As circuit densities increase critical dimensions decrease and line/space patterns become more and more dense.
FIGS. 1A and 1B
show an example of line distortion that can occur for small critical dimensions, such as in the sub-micron range.
FIG. 1A
shows an intended pattern element
10
which is to be transferred to a layer of resist material on an integrated circuit wafer.
FIG. 1B
shows the optical effects of the small dimensions in the patterns
11
actually transferred to the layer of resist. As shown in
FIG. 1B
the corners of the pattern are rounded resulting in rounded ends
12
of the pattern element and a shortened line. The dashed lines
14
indicate the intended limits of the pattern element.
FIGS. 2A and 2B
show an example of corner rounding that can occur in pattern segments of larger rectangular regions.
FIG. 2A
shows an intended pattern of a larger rectangular pattern segment
16
which is to be transferred to a layer of resist material on an integrated circuit wafer.
FIG. 2B
shows the optical proximity effects on the corners of the larger rectangular pattern segment
17
actually transferred to the layer of resist. As shown in
FIG. 2B
the corners of the pattern segment are distorted resulting in rounded corners
19
. The dashed lines
18
indicate the intended limits of the pattern segment.
This optical distortion shown in
FIGS. 1A-2B
is the result of Optical Proximity Effect. Optical Proximity Effect is a form of optical distortion associated with the formation of photolithographic images. Diffraction effects occurring on both sides of a sharp pattern edge become important as the critical dimensions of pattern features decreases. Optical Proximity Correction is employed to compensate for the Optical Proximity Effect.
U.S. Pat. No. 5,553,273 to Liebman describes a method which first identifies a plurality of design shapes in computer aided design data for a lithography mask. The design shapes are sorted according to geometric type. The design shapes are them sorted and a bias is applied to the sorted design shapes.
U.S. Pat. No. 5,182,718 to Harafuji et al. describes a method of correcting design patterns in cells having hierarchial structure and corresponding to exposure patterns.
U.S. Pat. No. 5,432,714 to Chung et al. describes a system and method for preparing shape data for proximity correction.
U.S. Pat. No. 5,827,623 to Ishida et al. describes a method of forming an improved halftone type phase shift mask having an Optical Proximity Correction function.
U.S. Pat. No. 5,682,323 to Pasch et al. describes a system and method of performing optical proximity correction on an integrated circuit by performing optical proximity correction on a library of cells.
U.S. Pat. No. 5,723,233 to Garza et al. describes a photolithography Optical Proximity Correction method for mask layouts.
A paper by O. W. Otto et al. “Automated optical proximity correction—a rules-based approach,” Optical/Laser Microlithography VII, Proc. SPIE (2197) 1994, pages 278-293 describes a rules based approach for optical proximity correction.
A paper by J. Stirniman and M. Rieger “Optimizing proximity correction for wafer fabrication process,” 14th Annual BACUS Symposium on Photomask Technology and Management, Proc. SPIE (2322) 1994, pages 239-246 describes a methodology for characterizing proximity effects from measurements taken on a processed wafer.
A paper by S. Shioiri and H. Tanabe “Fast Optical Proximity Correction: Analytical Method,” Optical/Laser Microlithography VIII, Proc. SPIE (2440) 1995, pages 261-269 describes a method for calculating proximity corrected features analytically.
SUMMARY OF THE INVENTION
Optical Proximity Correction often adds many small patterns around the original design. These small patterns can add significantly to the time needed to form the mask using an electron beam and in many cases do not add significantly to improvement of the pattern.
It is a primary objective of this invention to provide a method of performing Optical Proximity Correction on pattern segments which will not add excessively to the time required to form masks having the optical proximity corrected pattern using an electron beam.
It is another primary objective of this invention to provide a method of forming masks using Optical Proximity Correction on pattern segments which will not add excessively to the time required to form the mask using an electron beam.
These objectives are achieved by using curve fitting to approximate pattern edges and line ends of segments of patterns distorted by optical proximity effects such as line shortening and corner rounding. A unit normal vector and an area vector is determined for these curves. The unit normal vector is a unit vector perpendicular to the curves used to represent the pattern edges and directed toward the undistorted pattern edge. The area vector has the same direction as the unit normal vector and a magnitude determined by the distance between the curve used to represent the distorted pattern edge and the undistorted pattern edge.
An Optical Proximity Correction vector is then determined which is the sum of a first scaler function multiplied by the unit normal vector and a second scaler function multiplied by the area vector, wherein said first scaler function and said second scaler function are functions of position on the curve used to represent the distorted pattern edge. The curves used to represent the distorted pattern edge are then moved a distance and direction equal to the Optical Proximity Correction vector to define an optimum Optical Proximity Correction curve thereby defining a new pattern periphery.
The Optical Proximity Correction curve is very time consuming to duplicate in a mask using electron beam methods so regular geometric shapes; such as squares, rectangles, and triangles; are used to approximate the optimum Optical Proximity Correction curve thereby defining a new pattern periphery which achieves adequate Optical Proximity Correction. The new pattern periphery, defined using regular geometric shapes, can be formed in a mask using electron beam methods without requiring excessive time in the mask formation.
REFERENCES:
patent: 5182718 (1993-01-01), Harafuji et al.
patent: 5432714 (1995-07-01), Chung et al.
patent: 5553273 (1996-09-01), Liebmann
patent: 5682323 (1997-10-01), Pasch et al.
patent: 5723233 (1998-03-01), Garza et al.
patent: 5827623 (1998-10-01), Ishida et al.
patent: 6064485 (2000-05-01), Lin et al.
O.W. Otto et al., “Automated Optical Proximity Correction—A Rules-Based Approach”, Optical/Laser Micrlithography VII, Proc. SPIE (2197), 1994, pp. 278-293.
J. Stirniman et al., “Optimizing Proximity Correction for Wafer Fabrication Processes”, 14th Annual BACUS Symposium on Photomask Technology & Management, Proc. SPIE (2322) 1994, pp. 239-246.
S. Shioiri et al., “Fast Optical Proximity Correction: Analytical Method”, Optical/Laser Microlithography VIII, Proc. SPIE (2440) 1995, pp. 261-269.
Ackerman Stephen B.
Prescott Larry J.
Rosasco S.
Saile George O.
Taiwan Semiconductor Manufacturing Company
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