Characterization and performance improvement of secondary electron analyzers

Abstract

The detection of voltage contrast in scanning electron microscopes requires the use of secondary (SE) analyzers that allow the SE spectrum shift to be determined accurately and with high signal-to-noise ratio. In order to improve the performance of a given analyzer, a characterization scheme has been developed that allows one to pinpoint critical detector parts and provides guidelines for design changes. Performance determining factors, e.g., the operation point of the analyzer, the contribution of backscattered electrons (BSE) and SEs generated by BSEs (tertiary electrons) to the background signal, and changes of the S-curve shape, are determined separately by using specially developed characterization techniques. The optimum analyzer operation point, where the signal difference between two sample voltages is a maximum, is determined by simultaneously varying the retarding voltage and the voltage at the sample. Reflecting the primary electron beam with a negatively biased electrode simulates BSEs and reveals areas of high tertiary electron generation. Replacing the sample by a low energy electron gun with a small energy spread simulates SEs. This allows a detailed energy resolution determination of the analyzer to be performed.