Electron microprobe analysis: The upper limit of submicron spectroscopy

Abstract

In the electron microprobe, a beam of high energy electrons is focussed to a fine point on the surface of a fairly thick specimen and the x rays produced are analyzed to determine the chemistry of the "point". The spa- cial resolution of this instrument for chemical analysis, then, is defined by the volume of material from which the x ray signal originates. This, in turn, is related to factors such as the diameter of the electron beam, the spreading of the electron beam as it penetrates into the sample and interacts with the atoms of the sample to generate x rays, and the extent to which these primary x rays penetrate beyond the region of electron beam interaction and generate secondary x rays by the process of fluorescence. Beam voltage, current, and diameter can all be easily controlled by the analyst. Once the electrons enter the specimen, however, the analyst loses control of their density. Each individual electron follows a unique, erratic path as it passes near, passes through, or collides with parts of the atoms in the specimen. Monte Carlo calculations are a means by which many investigators have tried to model the paths of individual electrons and the interation volume that large numbers of such electrons define.3 “* It is well known that the size and shape of the region into which the electron beam penetrates and expends its energy is controlled primarily by the average atomic number, atomic weight, and density of the specimen in the region of interest and the beam voltage.