Quantitative Scanning Electron Microscopy of Surfaces

The emissive modes of secondary and backscattered electrons in a scanning electron microscope work more quantitavely for material and topographic contrasts and can better separate different types of contrast when using a multi-detector system. The detector system of conventional scanning electron microscopes often do not make the best use of the potentialities given by the electronspecimen interactions and a better detector strategy will be necessary also to get more quantitative results / l / . For example, the EverhartThornley detector designed for collecting as much secondary electrons (SE) as possible, only collects an undefined fraction of the emitted SE depending on the specimen position and the dimension of the specimen chamber, and a large fraction of the SE signal is formed by SE which are excited by the backscattered electrons (BSE) at the lower pole-piece and other parts of the specimen chamber. The micrographs contain an undefined mixture of topographic and material contrast. The BSE are not collected by electrostatic collection fields and it is necessary to use scintillator or semiconductor detectors with a large solid angle of collection. Therefore, experiments have been done to use BSE detectors at different positions /2-5/ and also two BSE detectors A and B for separating material and topographic contrasts with the signals A+B and A-B, respectivelyf6,7/. However, we show below that the A-B signal can contain image artifacts which are not caused by topography. I Backscattering and secondary electron emission For a better understanding of the concepts for improving the detector strategy, the most important laws of BSE and SE emission shall be summarized / 8 / . The backscattering coefficient increases monotonously with increasing atomic number Z for primary electron energies E > 5 keV (Figs. 1 and 5). Below 5 keV, Q decreases for large Z /9/ which can be explained quantitatively by Monte Carlo calculations /l@/ using Mott instead of Rutherford cross-sections for considering the large-angle scattering which is mainly responsible for the effect of backscattering. With increasing tilt angle 6 the total backscattering coefficient 0 increases (Fig.2). When looking on the angular characteristics d ~ / d R (Fig.3), this increase of Q with increasing @ is concentrated in a reflection-like maximum /11/. This increase will only be observed in the BSE signal when the detector is positioned at the corresponding take-off direction. With BSE detectors below the pole-piece, only the fraction of BSE emitted opposite to the electron beam will be collected. These BSE leave the specimen by complete diffusion. The corresponding BSE signal depends ogly weakly on and is approximately constant for tilt angles @ 5 0-50 and decreases for larger 6. The energy distribution Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984265 C2-292 JOURNAL DE PHYSIQUE