Use of fast Fourier transform methods in maintaining stability of production CD-SEMs

Benjamin D. Bunday; Mark P. Davidson

doi:10.1117/12.386459

2 June 2000 Use of fast Fourier transform methods in maintaining stability of production CD-SEMs

Proceedings Volume 3998, Metrology, Inspection, and Process Control for Microlithography XIV; (2000) https://doi.org/10.1117/12.386459
Event: Microlithography 2000, 2000, Santa Clara, CA, United States

Abstract

CD-SEM dynamic accuracy and stability is fundamentally linked to the quality of beam tuning. One source of beam instability is stigmation drift. Uncorrected stigmation drift can account for, in extreme cases, up to 20 nm of CD error on a 0.18 micrometer linewidth. As most current CD-SEMs do not feature robust auto-stigmation systems, periodic beam tuning is usually performed manually. This solution, however, does not yield the desired results, as a 0.18 micrometer linewidth can still err by unacceptable amounts due to human variability. One method of monitoring beam quality is the use of fast- Fourier transform (FFT) techniques, as first described by Postek and Vladar of NIST, where an FFT of an image of a very fine, random, isotropic, high-contrast defect surface is used to calculate beam sharpness and eccentricity. Advantages of this are: (1) beam-tuning is controlled to the point that measurements of 0.18 micrometer linewidths after the beam tuning vary by only 2 nm, (2) this result is rendered operator-independent, and (3) the resulting sharpness and eccentricity values quantify the beam quality and do not bias with time due to charging, as do repeated linewidth measurements (i.e. as in a 'hammer test'), allowing trend- chart to be used for process control. This FFT technique has been implemented in SEM Monitor, a software package from Spectel Company, under contract by Sematech AMAG. This software is now used for periodic qualification of our production CD-SEMs. This article demonstrates our evaluation data of the Spectel SEM Monitor workstation, resulting beam- stability trend data of our OPAL 7830Si CD-SEMs, and details about the sample used with this technique.

Citation Download Citation

Benjamin D. Bunday and Mark P. Davidson "Use of fast Fourier transform methods in maintaining stability of production CD-SEMs", Proc. SPIE 3998, Metrology, Inspection, and Process Control for Microlithography XIV, (2 June 2000); https://doi.org/10.1117/12.386459

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