Examples of the Microscopic World through the Scanning Electron Microscope

Leonardo, Vol. 9, pp. 133-136. Pergamon Press 1976. Printed in Great Britain EXAMPLES OF THE MICROSCOPIC WORLD THROUGH THE SCANNING ELECTRON MICROSCOPE B. A. Meylan* and N. E. Flower* Over the last few years a considerable number of pictures produced by scanning electron microscopes have been published, most of them as illustrations in scientific articles and books. Many people find that these photographic images are very exciting, quite apart from their direct scientific interest, both because of their dramatic 3-dimensional appearance and because of the aesthetic appeal of the microscopic detail of natural objects. Optical and transmission electron microscopy, both of which require very thinly sliced or very flat specimens for magnifications over about 200 times, cannot provide the same kind of images. Scanning electron microscope images are not produced by light but by means of low energy secondary electrons emitted from the surface of a specimen when a very narrow beam of high energy primary electrons strikes it. The microscope itself consists of: (a) A source of primary electrons that is somewhat like a light bulb filament. The electrons are accelerated towards the specimen by a high voltage. (b) A series of magnetic lenses that gathers the electrons into a very narrow pencil-like beam that is about 1/10,000 mm in diameter. (c) A means of causing this beam to raster across (scan) the specimen. (d) An electron detecting and display system. The image from the microscope is displayed on a television screen, which can be photographed with a conventional camera. When the high energy primary electrons penetrate the specimen surface and pass deep into the specimen, they knock out large numbers of low energy ionization electrons, losing their own energy in the process. The low energy electrons cannot travel far within the specimen without being recaptured, so that only those produced very close to the specimen’s surface can escape. As the image is a result of these low energy electrons, only the surface of the specimen is seen. Furthermore, as the primary electron beam is extremely narrow and as the secondary electrons do not have to be focussed, but merely collected for display, a very large depth of field is achieved (some 500 times greater than that of an optical system at the same magnification ). Thus, it is possible to examine specimens with a high relief. Samples that are electrically conducting can be readily examined directly, but non-conducting ones, such as biological samples, have first to be given an electrically conducting surface by coating them with a * Physicist, Dept. of Scientific & Industrial Research, Physics & Engineering Lab., Private Bag, Lower Hutt, New Zealand. (Received 16 Jan. 1975). very thin layer of metal, usually gold or a mixture of gold and palladium. This preparation, together with the fact that the whole microscope works under vacuum, means that in general only non-living specimens can be studied. The range of useful magnification is from about 15 to 20,000 times on biological specimens and somewhat more on other materials, such as metals, which do not need coating. In scanning electron microscopy the very narrow electron beam is made to scan the specimen in a raster, rather like that of a television screen, though with a larger number of finer lines. The time taken to scan the specimen when recording an image is generally quite long, of the order of 40-150 sec. This relatively slow process of building up a 6nal picture is necessary because the amount of information or detail present in a final picture is approximately proportional to the length of time the primary beam stays in any one place on the specimen. The final picture of the object is 3-dimensional in appearance because of the effect of perspective and of the presentation in light and shade (Figs. 1-4). The light and shade arises because for surfaces sloping towards the electron detector a large number of electrons are collected and the image on the television screen appears bright; for surfaces that slope away from the detector there are fewer collected and so these surfaces appear darker. This can be compared to photographing an object under strong side lighting, traditionallyused in photography...