Collection of Auger electron spectra with a microcomputer

Abstract

This paper describes the principles of the method of electron spectra collection based on the spectrum statistics, and its realization by a microcomputer system developed for electron spectrometer control and for data processing. Electrons passing through the bandpass energy filter are detected as pulses and counted by a 24-bit counter. Another counter collects the pulses of a reference signal; for fixed timing, the reference counter is clocked with an oscillator. Using the total secondary emission current as the reference signal, the emission fluctuations are eliminated and, finally, the signal from a second broader concentric window allows us even to eliminate the background. The measurement of the single spectrum run in any energy interval is governed by the ratio of signal to statistical noise. This value defines the binary order of the reference counter carry signal that stops the signal sample collection. In this way a constant value of signal-to-noise ratio is kept throughout the whole spectrum. The 24-bit sample is then fitted to a 16-bit window according to a preselected value of “sensitivity”. The 16-bit result is stored or accumulated in the “immediate” memory. A reduced 8-bit sample is stored and possibly displayed on a screen. To improve the signal-to-noise ratio, easily-observable fast spectrum runs are accumulated. In this case the microcomputer calculates the new measurement timing so that after the accumulation of 256 runs all bits in the selected dynamical range of measurement are statistically valid. Some results are presented to illustrate the advantages of the microcomputer controlled spectrum accumulation method with measurement time correction. They are compared with single sweep measurements obtainable with classical instruments.