Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
1999-12-23
2001-12-04
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S055000, C355S077000
Reexamination Certificate
active
06327023
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical lithographic technique used in the formation of integrated circuits(ICs) on a wafer; and, more particularly, to an improved method for scanning a reticle having a plurality of lines thereon by keeping the direction of scanning in the direction that minimizes pattern distortion.
DESCRIPTION OF THE PRIOR ART
As is well known, photo lithography methods have been used for patterning a resist layer on a semiconductor wafer to form integrated circuits(ICs) such as processors, ASICs and Dynamic Random Access Memory(DRAM). As the ICs on the semiconductor wafer becomes smaller in dimensions, the photo lithography method is gaining more importance.
Presently, one of the most conventional photo lithography methods used nowadays is a stepper, so called as a step-and-repeater, which moves and aligns a wafer based on alignment marks on a reticle containing an image such that desired patterns on the wafer are exposed based on the image. The stepper includes a radiation lamp for generating radiation, an imaging lens, a reticle stage for mounting and moving the reticle and a wafer stage for loading the wafer. In the stepper, after the wafer is loaded on the wafer stage, the reticle stage searches and moves to a predetermined position to form a chip, thereby implementing alignment process. Thereafter, amounts of misalignment errors such as a X and a Y misregistration, rotation and orthogonal errors are measured to accurately position the reticle stage at the predetermined position. In the next step, the radiation from the radiation lamp travels to the imaging lens after passing through the reticle. The wafer is exposed by the radiation from the imaging lens by moving the wafer stage until the entire of the wafer is scanned. A number of successive steps of photo lithography, film growth, deposition and implantation of impurities create a complete IC with many identical copies on the same wafer. Each copy is known as a chip.
There is shown in
FIG. 1
a schematic diagram of a relationship between a radiation lamp
16
and a reticle
10
for use in a conventional scanning method. The radiation lamp
16
is imaged through a slot
18
in such a way that the slot
18
is arranged in the form of a straight stick. The reticle
10
has a number of lines
12
and spaces
14
. In the method, the radiation lamp
16
scans the reticle
10
along the direction I which is parallel to the lines
12
.
One of problems associated with the above-described conventional scanning method is the lens aberrations, causing across chip linewidth variation and image placement errors. These errors are becoming a larger fraction of the total device error budget as feature sizes shrink.
SUMMARY OF THE INVENTION
It is, therefore, a primary object of the present invention to provide an improved method for scanning a reticle pattern having a plurality of lines thereon capable of reducing across chip linewidth variation and image placement errors.
In accordance with the invention, a process of rotating a reticle pattern as part of device flow for critical dimension and overlay optimization is provided.
A further object of the invention is to provide a process to determine interaction between reticle pattern orientation, lens quality, illumination condition and device layout, and to provide a process to optimize reticle pattern orientations level by level.
Further, a method for exposure tool implementation, e.g., software and hardware to support wafer rotations within device flow, is provided.
In accordance with one aspect of the present invention, there is provided a method for patterning a reticle pattern on a wafer loaded on a wafer stage, wherein the reticle pattern has a plurality of lines, the method comprising the step of: loading the reticle pattern on a reticle stage; performing simulations with appropriate lens aberration set, taking into account multiple reticle orientations; determining an optimum reticle pattern orientation for the loaded reticle based on the simulation results; scanning the reticle pattern while keeping the optimum reticle pattern orientation as the reticle pattern is imaged by a radiation source.
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patent: 5945239 (1999-08-01), Taniguchi
patent: 6011611 (2000-01-01), Nomura et al.
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patent: 6077631 (2000-01-01), Unno
Bukofsky Scott J.
Progler Christopher J.
Adams Russell
International Business Machines - Corporation
Nguyen Henry Hung
Pepper Margaret A.
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