INORGANIC REPLICATION IN ELECTRON MICROSCOPY 799 



is the oxide film,- produced by controlled oxidation of the surface when it 

 is aluminum or another suitable metal. For other materials, a pressure mold 

 of the surface in pure aluminum may be utilized as an intermediate repHca 

 in a two-step process.'* A third type is the silica replica,^ ^ produced by 

 the condensation of silica vaporized by a hot source in vacuo, either on the 

 surface in a one-step, or on a plastic mold of the surface in a two-step, 

 process. A fourth type is the shadow-cast plastic rephca,^- ^ produced by 

 similar deposition of a suitable metal at near-glancing incidence upon a 

 plastic replica. 



The purpose of this paper is to discuss the process of evaporated film 

 formation as it is related to properties important in microscopy. The resolu- 

 tion and range of contrast available in the finished micrograph determine 

 the faithfulness with which the original surface is depicted, and depend 

 on the relative orientation of the surface, vapor source, and electron beam, 

 on the density and average thickness of the repHca, and on the mechanism of 

 condensation. In principle, it is pointed out that any material of suitable 

 physical and chemical properties may be used for evaporated repHca films, 

 and a number of examples are shown in micrographs. Inorganic repHca 

 films retain the third or vertical dimension, a fundamental advantage which 

 permits stereoscopic study. The material presented perhaps provides a uni- 

 fied view of repHca tion techniques and a method for the evaluation of 

 micrographs relative to the faithfulness of portrayal of the original surface. 



1. Local Thickness of Condensed Material 



Figure 1 illustrates how the thickness te in the direction of the electron 

 beam is dependent on the local surface normal n. The thickness /« in the 

 direction of the atom source is constant, depending only on the amount of 

 material reaching the surface. The thicknesses due to two or more sources 

 obviously may be vectorially added, and hence an arbitrary assembly of 

 sources may be replaced by a single source properly located, since each yields 

 the same thickness distribution on the surface S. A simple analogy is the 

 shading produced when ordinary objects are illuminated by direct Hght. 

 It is clear that atom source and electron beam must differ in direction for 

 shading to occur. The atoms or molecules of some materials, notably silica 

 and silicon monoxide, do not all stick where they strike, but some wander 

 over the surface^ as a ''two-dimensional gas" before condensing. This 



* J. Hunger and R. Seeliger, Metallfarschung, 2, 65 (1947). 



6R. D. Heidenreich and V. G. Peck, //. App. Phys., 13, 427 (1943). 



6 C. H. Gerould, //. App. Phys., 17, 23 (1947). 



7 H. Mahl, Konosion u. Metallsclmtz, 20, 225 (1945). 



8R. C. Williams and R. W. G. Wyckoff, //. App. Phys., 17, 23 (1946). 

 9 R. D. Heidenreich, //. App. Phys., 14, 312 (1943). 



