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The historical development of data systems for Auger electron spectroscopy (AES) is reviewed with the purpose of putting the role of potential modulation differentiation in perspective. Typical current AES systems employ completely analog controls, signal trains, and data output devices. They use potential modulation differentiation with a lock-in amplifier measuring the first harmonic signal component as an approximation to the derivative of the secondary electron energy distribution. This approach to the generation of Auger spectra makes a considerable sacrifice in the signal-to-noise ratio and the quantitative accuracy in an attempt to improve signal “contrast” in the data. This sacrifice is not necessary when the E∙N(E) output, which is available from the cylindrical mirror analyzer (CMA), is used.
An AES system is described which uses a mini-computer, a microprocessor, a disk drive, and a graphics terminal to provide digital control functions, a digital signal train, digital data storage and processing, and reconversion to analog only in the final data display devices. This permits the acquisition of the E∙N(E) signal as measured by the CMA with no arbitrarily imposed modulation or system time-constant effects. This approach provides a high dynamic range in the data storage so that all data may be immediately stored in unprocessed form, which precludes the type of information loss that commonly results from incorrect sensitivity settings in analog systems. After the data are stored, a copy of the data may be digitally processed to provide the smoothing or averaging effect of a time constant, to differentiate the data, to integrate over specific peaks, etc. By converting the signal to digital form immediately after it leaves the CMA, the sensitivity to the pickup of stray noise is drastically reduced. The benefits of the digital approach are illustrated through examples from actual applications and through comparison with data from typical analog systems.
surface analysis, Auger electron spectroscopy (AES), computer-controlled instrumentation, digital data processing
Technical staff member, Solid State Laboratory, Hewlett-Packard Laboratories, Palo Alto, Calif.