Inorganic Replication in Electron Microscopy 



By C. J. CALBICK 



Contrast and resolution in electron micrographs from thin replica films are 

 determined by the geometrical relationships between the directions of incidence 

 of the condensing atom beam and the local surface normal, during film formation 

 by evaporation in vacuo, and the direction of incidence of the electron beam, 

 during subsequent exposure in the microscope. Rephca films may be formed of 

 any material suitable for vacuum evaporation. Metal atoms in general tend to 

 stick where they strike, moving only short distances, 100 A or less, to nucle- 

 ating centers where they form small crystallites. Oxides such as silica and 

 silicon monoxide, and also the semi-metal germanium, form amorphous films. A 

 portion of the incident material, about 50% in the case of silica, migrates large dis- 

 tances, 5000 A or more, before finally condensing; the remainder sticks where it 

 first strikes the surface. 



The existence of a minimum perceptible mass thickness difference, about 0.7 

 Mg/cm^ for 50 kv electrons, results in an optimum replica mass thickness of about 

 10 /iig/cm^. The resolution of the replica film is proportional to its linear thickness 

 and hence is inversely proportional to its density. Micrographs of silica, chro- 

 mium, gold-manganin, aluminum, aluminum-platinum-chromium and germanium 

 replicas are shown. The importance of stereoscopic methods in interpretation of 

 micrographs is discussed. 



THE basic purpose of micrography of surfaces is to exhibit structural 

 topography. Present day electron microscopes are transmission-type 

 instruments. Practical limitations of experimental technique establish a 

 voltage of the order of 50 kv as the most useful accelerating potential for 

 the electrons used for illumination. In bright field transmission microscopy, 

 the image consists of a field with local variations of intensity produced 

 because the object has partially absorbed, or scattered, the incident radia- 

 tion. In electron imaging scattering is the predominant factor, limiting 

 direct examination to objects whose mass thickness does not exceed about 

 50 ng/crri^* Thicker specimens can be examined only in profile. 



Optical microscopy of surfaces is concerned with their appearance as 

 seen by reflected light, the counterpart of which is not practicable^ with 

 electrons. The electron microscopist has therefore devised means of trans- 

 ferring surface structural details to thin films called replicas. ^ These films 

 must present to the electron beam locally varying thickness corresponding 

 to the surface details. A simple type is the plastic replica^ consisting of an 

 appropriately thin plastic film stripped from the surface. A second type 



* Some microscopes provide a range of accelerating potentials, up to 100 kv or more, 

 permitting direct examination of thicker objects. 



1 Zworykin et al. "Electron Optics and the Electron Microscope," pp. 98-106. 



2 /. Roy. Micro. Soc, 70, 1950, "The Practise of Electron Microscopy," ed, by D. G. 

 Drummond, see Chapters II and V. 



» V. J. Schaefer and D. Harker, //. App. PJiys., 13, 427 (1942). 



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