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Technique of Characterizing Calibration Grids in E-Beam Lithography Tools

IP.com Disclosure Number: IPCOM000050464D
Original Publication Date: 1982-Nov-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 4 page(s) / 144K

Publishing Venue

IBM

Related People

Loughran, JF: AUTHOR [+2]

Abstract

The non-ideal calibration grids employed to calibrate E-beam lithography tools are characterized by a filtering technique which improves the repeatability and accuracy of the grid parameters.

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Technique of Characterizing Calibration Grids in E-Beam Lithography Tools

The non-ideal calibration grids employed to calibrate E-beam lithography tools are characterized by a filtering technique which improves the repeatability and accuracy of the grid parameters.

Electron-beam lithography system tools are typically employed to overlay patterns written over previous levels, exposed by optical means or other electron beam tools. This requires the calibration of the system deflection apparatus to a nominal reference. One technique employed is to utilize a calibration grid comprising an array of square holes in a gold film over a silicon wafer attached to the system's X-Y table. The E-beam scan errors are calibrated to this reference grid by scanning the grid with the writing deflection.

However, since the reference grid itself is not ideal, it is necessary to devise techniques for characterizing the grid in terms of the ideal.

Taking advantage of the inherently smooth character of deflection scans, the scan errors may accurately be defined from a mathematical model. Errors in the deflection scans are measured by scanning, in a raster pattern, the calibration grid which is an array of square holes in a gold film over a silicon wafer, as shown in Fig. 1. The grid is one of several calibration fixtures attached to the X-Y table. Fig. 1 shows only a small portion of the grid. For a given deflection, a discrete subset of grid holes at points throughout the writing field are selected based on the error characteristics of the writing deflection. Typically, a thousand holes (27 x 32) are enough, for example, for 5 mm deflection.

For measurement, the E-beam is unblanked only over the grid holes selected to be measured. When the grid is scanned, the electron backscatter from the gold is greater than that from the silicon. The modulated backscattered electrons are then intercepted by solid-state diodes. The output current signals of the diodes are current to voltage converted, then summed. The "hole-crossing" positive and negative transmissions are used to identify the times at which the beam goes into and out of the grid hole. A digital circuit utilizes each transition to sample the system clock. The two clock samples are averaged to produce a clock value which represents the position of the center of the grid hole with respect to the deflection scans.

Two sets of scan errors are measured, one in the X-direction and the other in the Y-direction. For purposes of simplicity, the discussion here refers to scan (or grid) errors in the X-direction. However, it should be understood that the scan (or grid) errors in the Y-direction are treated independently in an identical manner.

The scan errors are defined as the difference between the measured deflection coordinates and the ideal coordinates of the scans. It is very useful to think of the scan errors in terms of a surface in a three-dimensional space. The error surface may be defined mathematica...