ELECTRON IVIICKOSCOPY 



The next section will describe the different 

 preparation techniques for obtaining thin 

 specimens. After that, a short introduction 

 to dislocations is given and an enumeration 

 of different metallurgical ai)plications. In 

 this article no work on moir^ patterns (the 

 interference patterns of difTerent superposed 

 crystal layers) will be discussed. We shall 

 refer as far as possible to review papers. 



The papers of a symposium on thin film 

 techniques are published in the August 1959 

 issue of the Journal of the Institute of Metals 

 (11). A review of Japanese work on electron 

 metallography including transmission work 

 is given in the booklet "The World through 

 Electron Microscopes (Metals)" (12). 



Note added in proof: Since this article was sent 

 in, many new papers have appeared, most of which 

 are collected in the Proceedings of the Delft Con- 

 ference 1960 (72). 



Preparation Techniques 



To be traversed by 100 keV-electrons a 

 crystalline specimen has to be thinner than 

 about 5000 A, and to get information about 

 the interior, the surface has to be as smooth 

 and clean as possible. When these conditions 

 are fulfilled, the information in the pictures 

 depends only on the internal state of the 

 specimen and its orientation. Specimens can 

 be prepared in two main ways: 



A. they can be built up directly as a thin 

 foil or 



B. they can be cut out of a block and 

 thinned. 



Class A covers specimens produced by vapor 

 condensation in vacuum, e.g., on cold or hot 

 rocksalt or by electrolytic deposition. A 

 method, where a metal ring is dipped into 

 liquid metal to produce a metal skin, anal- 

 ogous to a soap skin, has been developed by 

 Takahashi et al. (13). The evaporation 

 technique has been perfected to a high 

 degree by D. W. Pashley et al. (14). 



While the specimens of class A are used 

 mostly to study the behavior of the thin 

 films themselves, specimens produced by a 



method of class B are considered as repre- 

 sentative of the bulk material. Most of the 

 thinning techniques are based on the electro- 

 polishing method introduced by Heidenreich 

 (1). Heidenreich clectropolished a mechan- 

 ically thinned disk in a special holder, pro- 

 tecting the edges, branched as anode against 

 a pointed cathode, first from one side to take 

 away the mechanically damaged surface 

 layer and then from the other side until the 

 first tiny hole broke through. In the sur- 

 roundings of these holes, the specimens were 

 thin enough to be penetrated by the elec- 

 trons. Castaing (2) added to the electro- 

 polishing an ion bombardment treatment 

 with the ion gun built directly into the 

 electron microscope, and was able to control 

 this treatment to obtain the optimum condi- 

 tions. This technique is especially useful for 

 studying precipitation, e.g., CuAU in Al, 

 where the aluminum is preferentially dis- 

 solved by electropolishing, while the pre- 

 cipitates are thinned by the ion bombard- 

 ment. While Hirsch and his co-workers (6) 

 studied beaten Al foils annealed and chem- 

 ically etched in hydrofluoric acid, Bollmann 

 (7) continued on the line traced by Heiden- 

 reich. The modifications to Heidenreich's 

 arrangement were that 



(a) the electrolytic attack was symmetrical 

 from both sides of a specimen, 



(b) instead of stopping the attack when 

 the first small holes appeared, two large 

 holes were produced and the attack was 

 stopped when these holes joined. 



The symmetrical attack has the advantage 

 that the specimen is a symmetry plane of 

 the potential distribution, thus the current 

 is not especially concentrated at the edges 

 of a hole and does not round them off, so 

 that the edges remain sharp. The point (b) 

 has the advantage that the right moment to 

 stop the treatment can be foreseen. The 

 best specimens are found normally at the 

 point where the two holes join, and may be 

 cut out after cleaning in water and methyl 

 alcohol. It is even possible to continue the 



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