Search

The spatial resolution of X-ray microanalysis in thin foils is defined in terms of the incident electron beam diameter and the average beam broadening. The beam diameter is defined as the full width tenth maximum of a Gaussian intensity distribution. The spatial resolution is calculated by a convolution of the beam diameter and the average beam broadening. This definition of the spatial resolution can be related simply to experimental measurements of composition profiles across interphase interfaces. Monte Carlo calculations using a high-speed parallel supercomputer show good agreement with this definition of the spatial resolution and calculations based on this definition. The agreement is good over a range of specimen thicknesses and atomic number, but is poor when excessive beam tailing distorts the assumed Gaussian electron intensity distributions. Beam tailing occurs in low-Z materials because of fast secondary electrons and in high-Z materials because of plural scattering.

Nonconducting samples invariably need some sort of treatment before they can be examined and analyzed under optimal conditions in electron-beam instruments that rely on an emitted signal to provide information. The treatment is necessary to eliminate or reduce the electric charge that builds up rapidly in a nonconducting specimen when it is scanned by a beam of high-energy electrons. Figs. 4.63a–c and 4.64a–c show examples of pronounced and minor charging as observed in the SEM. In addition to charging phenomena, which result in image distortion, the primary beam also causes thermal and radiation damage, which can lead to a significant loss of material from the specimen. In many situations the specimen may acquire a sufficiently high charge to decelerate the primary beam, and the specimen may even ultimately act as an electron mirror (see Fig. 4.62).

The versatility of scanning electron microscopy and of x-ray micro-analysis is derived in large measure from the rich variety of interactions that the beam electrons undergo in a specimen. These interactions can reveal information on the specimen’s composition, topog­raphy, crystallography, electrical potential, local magnetic field, and other properties. The electron—specimen interactions can be divided into two classes.

This laboratory explores some of the principal phenomena observed at low beam energies. Images prepared at “conventional” beam energies, e.g., 15 keV and above, are compared with low-beam-energy images, e.g., 5 keV and below. The possibility of examining uncoated insulators by taking advantage of enhanced emission of secondary electrons at low energy is also examined.

The light element regime traditionally defined as the elements Be through F (Z = 4 to 9) has posed many difficulties for the analyst. Poor detectability due to low count rates caused by insufficient beam current at low voltage and small spot sizes has made microanalysis very difficult Adequate matrix correction procedures are just being developed and quantification is difficult without very good standards. Contamination of the sample during analysis often makes the results questionable. These problems are slowly but surely being corrected in today’s modern electron microprobes. New WDS crystals and recent ultra-thin-window (UTW) have contributed to improved detectability; modern electron columns have much better low-voltage performance; and finally, today’s SEMs and microprobes are designed with anticontamination devices. However, with all these improvements, the measurement and subsequent quantification of light elements is far from routine. This laboratory will illustrate operational aspects of light element analysis background measurements, peak overlap problems, and quantitation techniques. More details may be found in SEMXM, Chapter 8.

Experiment 8.1: E-T Detector Collection Efficiency. The solid angle and efficiency of a specific E-T detector for direct collection of BSEs is: (a) Area, A, of scintillator (cm2) = 1.5 cm2. (b) Distance, r, from specimen to scintillator (cm) = 4 cm. (c) Solid angle, Ω = A/r2 = 1.5/16 = 0.094 steradians. (e) For a specimen set normal to the beam (0° tilt), approximate take-off angle = 45°.