The authors describe a modification to the alignment algorithm typically used for electron beam lithography. Extra markers are used, which increases accuracy, but more importantly enables misplaced markers to be detected and discounted. This has the advantage of eliminating the effect of occasional poor markers and also gives a measure of the expected alignment accuracy after marker search and prior to writing. Two methods for identifying outliers are discussed, the first being a conventional least squares approach in which outliers are rejected one by one. The second approach uses the method of trimmed least squares, which is a more robust regression method, and was found to give better results. With 16 or more evenly distributed markers around the edge of the pattern both methods were found to give good results. Both modeled and experimental data were used to evaluate the use of additional markers. The experimental results showed that with 24 markers, 10 of which had position errors between 100 and 300 nm, a final alignment accuracy of 20 nm could be obtained. Finally this technique can be used to identify rogue markers, which is an important first step toward rectifying the process issues which led to the formation of poor markers in the first place. This is particularly important during process development, which is a major part of the typical throughput of an electron beam lithography tool.
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