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Method and Apparatus of in Situ Measurement and Overlay Error Analysis for Correcting Step and Repeat Lithographic Cameras

IP.com Disclosure Number: IPCOM000041677D
Original Publication Date: 1984-Feb-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 4 page(s) / 54K

Publishing Venue

IBM

Related People

Herd, H: AUTHOR [+2]

Abstract

Overlay errors can be expressed as a 2-dimensional polynomial series in each of the x- and y-directions. It has been repeatedly observed that the major contributions to these errors in step-and-repeat machines can be described by the linear terms of such series, typically accounting for 80% to 95% of total overlay error, and it has been amply demonstrated in laboratory practice that these errors are indeed correctable by simple machine instructions. The linear overlay errors are, in order of importance: offsets, magnification or stepping, and rotations (including orthogonality). Lithographic cameras when tuned up to practical perfection do not reside long in that state. Errors return with time or use.

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Method and Apparatus of in Situ Measurement and Overlay Error Analysis for Correcting Step and Repeat Lithographic Cameras

Overlay errors can be expressed as a 2-dimensional polynomial series in each of the x- and y-directions. It has been repeatedly observed that the major contributions to these errors in step-and-repeat machines can be described by the linear terms of such series, typically accounting for 80% to 95% of total overlay error, and it has been amply demonstrated in laboratory practice that these errors are indeed correctable by simple machine instructions. The linear overlay errors are, in order of importance: offsets, magnification or stepping, and rotations (including orthogonality). Lithographic cameras when tuned up to practical perfection do not reside long in that state. Errors return with time or use. It has been found experimentally that for the maximum precision of overlay, the magnitude of these errors must be monitored with a frequent schedule. This monitoring is at present accomplished by having a human operator read a considerable number of verniers on a wafer, calculating the errors, and applying the appropriate machine corrections. This process is so slow and subject to human factors as to be impractical in a manufacturing environment. It is the purpose of this article to show how the same results can be achieved automatically and efficiently with minimum human intervention. Further disclosed is the means for an independent audit to verify that the machine executes commands and achieves desired, specified results on the wafer. Implementation of the general working concepts described here will permit an iteratively improving process to a predetermined, specified overlay end-point (ground-rule). We describe the principal techniques using the MANN 4800* system as an illustrative vehicle, while realizing that such principles are general and not limited to this particular machine. We point out that the scheme depends upon the final results that the machine produces on a wafer, as distinguished from some internal references or adjustments within the machine. What we seek to do is to use the apparatus described below to modify the vendor's lithographic camera to permit measurement of overlay errors in situ by the interferometer system inherent in the camera, to digitize such data, pass it via an appropriate I/O interface to the internal computer of the camera or an auxiliary processor to perform the calculations, and to use the results in order to correct the machine. If in place of a product wafer, a fixed standard reference wafer is used as a basis of measurement, and if certain controls in the machine are motorized, a tune-up of the machine becomes possible automatically by closing the loop. On a similar basis, specifications of precision may be entered into the system suitable for the job to be performed and the system may be made to operate automatically to conform to the specifications or to reject the...