Imaging and Analysis of Surfaces with a Scanning Electron Microscope and Electron Spectrometer

Abstract

The topography and the composition of a surface are in many cases of equal importance (catalysis, electroplating, pretreatment of foils and sheet metal, corrosion, passivation, adsorption, coating offibers, etc.) , and this explains the great interest in methods of investigation that reveal both. I f the demands on the resolving power, the analytical possibilities, and the thickness of the surface layer are not too exacting, combined devices like the scanning electron microscope and its analytical accessories can be used. When it is necessary to avoid the compromises involved in simultaneous imaging and analysis, the investigations must be carried out with separate equipment. As an example o f a method for the analysis of surfaces we consider briefly photo-and Auger electron spectroscopy (ESCA). Principle of the Scanning Electron MicroscopeIt has been an old dream of scientists to possess a device enabling them to image a surface with the highest possible spatial resolution, i.e. to study its topography, and in addition to obtain the most complete possible information on the composition of this surface. There is, at present, no such universal apparatus, although numerous attempts have been made using electrons, ions, and X-rays"'. The scanning electron microscope may be considered as a good approximation as regards the demands of microscopy. As a supplement to this we shall discuss a special surface analysis technique which utilizes photo-and Auger electron spectroscopy with X-ray excitation (ESCA). The two methods have, in comparison with others, the advantage that preparation is fairly easy, that they can be used universally, and that they are therefore suitable for routine industrial investigations.[ ' I Dr. R. Holm Farbenfabriken Bayer A. G. 1ng.-Abt. Angewandte Physik SO9 Leverkusen (Germany) [**I Since it is impossible to list the large number of publications dealing with the scanning electron microscope; the reader is referred to the following bibliographies: The scanning electron microscope works on the pantograph principle : The essential elements are two electron beams operated synchronously by a scan generator (Figs. 1 and 2). The primary beam is produced in an evacuated column by thermionic emission, and is focused to a dia- meter of about 1008, by electromagnetic lenses. This beam scans the object point by point and line by line. The other beam records the picture point by point and line by line on the phosphor screen of a cathode ray tube. The magnification is given by the ratio of the picture area to the scanned area of the object.When the primary beam impinges upon the object, a variety of interactions occurs between the electrons and the solid material (Fig. 3). Both secondary electrons and backscattered electrons are used for imaging the surface. The secondary electrons are generated by impact ionization or the Auger effect. They possess a significantly lower energy than the primary electrons and those that are backscattered elastically or with only small energy losses. The secondary elec...