INSTRUMENTATION BY RALPH H. MÜLLER
"Where the telescope ends, the microscope begins. Which of the two has the grander view?"— rpTIH
ELECTRON
MICROSCOPE,
which
J- came into general use more than twenty years ago, revolutionized many branches of scientific research permitting one to see objects roughly three orders of magnitude smaller than is possible with the best optical microscope. Elaborate techniques for sample preparation, such as shadow easting and replicas, have increased the versatility of the instrument. For resolution, the electron microscope is still supreme, being of the order of 5-10 angstroms. For some seven years, scanning electron microscopy has been investigated (sec appended references) and this offers some considerable advantages not possessed by the transmission electron microscope. One of the most valuable of these is the great depth of focus— about 300 times as great as that of an optical microscope—and the realistic "three dimensional" images which it produces. On May 24th, a new Micro Topography Laboratory was dedicated by the Engis Equipment Co. at Morton Grove, 111. On this occasion, the Stereosean electron microscope was demonstrated. This instrument is manufactured by the Cambridge Instrument Co. and is based on development work done by Professor C. W. Oatley's group in the electrical engineering department at the University of Cambridge in England. Through the courtesy of Mr. T. Kolany of Engis Equipment Co., we are able to give some details about this new development. In the words of Professor Oatley, the schematic in Figure 1 explains the operating principles of the Stereosean. An extremely fine beam of electrons is focused onto the surface of the specimen and is caused to move over a small area of this surface in a zig-zag television raster or scan. The current, which brings about the deflection of the electron beam, passes also through the scanning coils of a cathode ray tube to produce an exactly similar, but greatly magnified, raster on the face of this tube. Electrons leaving the specimen are collected and the re• Circle No. 102 on Readers' Service Card
Victor Hugo
Figure 1. Schematic diagram of the scanning electron microscope suiting current is amplified and used to control the brightness of the cathode ray tube (Z axis modulation). Since there is a one-to-one correspondence between the position of the electron beam on the surface specimen and that of the spot on the cathode ray tube, the picture on the face of the tube will be, in some sense, a magnified image of the surface. I t turns out that the way in which the number of electrons leaving the surface of the specimen is affected by topographical features of the surface is very similar to the way in which these same features would affect the amount of light entering a microscope, if the surface were examined optically. Thus, a scanning electron microscope produces an excellent "three dimensional" image which can readily be interpreted. The electron beam current in the microscope is of the order of 10 micromicroamperes (lO^ 11 amp.) and, to allow time for a reasonable number of
electrons to fall on each picture point of the specimen, it is necessary to scan rather slowly. For visual observation, a cathode ray tube with afterglow screen is provided and a scanning rate of about one frame per second is used. When the appropriate area of specimen has been selected and focused, the image is recorded photographically with a second high resolution cathode ray tube and a much slower rate of scan to reduce background noise, perhaps one frame in three minutes. Incidentally, the electron beam current in the scanning electron microscope is very much less than in the conventional transmission instrument, so that the specimen suffers correspondingly less damage from heating, an important point with many specimens. An alternate technique of applying the Stereosean which is particularly useful in examining solid state devices has been described by Dr. Patrick Richard Thornton of the University College of VOL. 38, NO. 8, JULY 1966
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INSTRUMENTATION
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North Wales, Bangor [Science. Journal 1, No. 9, 66, (1965)]. I n this technique additional contrast can be produced b y applying a reverse bias, let us say in a diode, as a result of which the j> and η type regions will be at different voltages because t h e applied voltage will be dropped across the depletion layer. Thus, the ordinary surface topography revealed b y conventional operation of the Stereoscan, can be augmented by additional information obtained by elec trical manipulation of the specimen. Results obtained with the Stereoscan are most impressive particularly in t h e depth of focus which can be attained. A single example is shown in Figure 2. Additional information and new appli cations a r e rapidly becoming available from the Engis Co. (8035 Austin Ave., Morton Grove, 111.). Some other ref erences to t h e subject are to be found in: (1) Scanning Electron Microscopy, K. C. A. Smith, "Encyclopedia of Electron Microscopy," p . 241, Reinhold, New York, 1961. (2) T. E. Everhart, C. W. Oatley, O. C. Wells, J. Electronics and Control 7, 97 (1959). (3) I. M. Mackintosh, Proc. I.E.E.E. 53, 4,370(1965). (4) A. Boyde, A. D. G. Stewart, Nature 198, 1102 (1963).
INSTRUMENTATION I N SCIENTIFIC RESEARCH
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CONNECTICUT
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ANALYTICAL CHEMISTRY
On M a y 16th, we had the privilege of delivering t h e concluding lecture of the Chemical Instrumentation Institute which started Sept, 20, 1965, under the auspices of the Institute, for Continuing Education in Engineering and Applied Science of Washington University, St. Louis, M o . T h e planning of D r . Esterson, Director, a n d Chairman J . J . Marcus, resulted in one of t h e best u p to-date commentaries on t h e subject that we have ever encountered. Aside from our negligible role as initial a n d
INSTRUMENTATION
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Figure 2. Tensile fracture in soft iron wire Preparation: Stretched by hand to breaking point. Clamped onto specimen stub which was coated with "Aquadag." Specimen itself uncoated Print sizes: Micrographs A, B, and C are about 5 0 % of the size of the image shown by the instrument. Micrograph D is a print enlargement of C Magnifications: Approximately χ 73 (Α), χ 145 (Β), χ 445 (C), X 830 (D) Gun potential: 20 kv. for all results
final commentator on t h e subject, the bound notes and "proceedings" of the Institute of some 256 pages contain valuable information and suggestions from t h e lecturers who journeyed to St. Louis from all parts of the country. We would hope t h a t the notes might find wider circulation. For some time, we have been hoping t h a t Professor K u r t S. Lion of M.I.T. would find t h e time and inclination to extend the scope of his excellent mono
graph "Instrumentation in Scientific Research (Electrical I n p u t Transduc e r s ) " McGraw Hill Book Co., Inc., Ν . Υ., Ν . Υ., 1959. I t seems more important than ever to emphasize t h e general principles of instrumentation without reference to any particular or specific branch of science or technology. I t is one of the consequences of spe cialization t h a t still hinders the wider use of techniques being practiced by our fellow scientists in other fields.
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U. S. S T O N E W A R E Akron, Ohio 44309 Circle No. 158 on Readers' Service Card VOL. 3 8 , NO. 8, JULY 1966
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