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THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



difference, a rule applicable for electrons of any energy. The local thickness 

 of the replica varies from a fraction of to several times its average thickness, 

 and a repHca of near optimum thickness provides a range of 20-40 detectably 

 different shades of contrast. Although for special purposes a replica film 

 may be made thinner or thicker, usually the average thickness should be 

 selected in the optimum range. The resolution is then determined by its 

 density, or more precisely, by its electron scattering power. Because the 

 denser materials are composed of elements of higher atomic number which 

 are more efficient scatters, scattering powerf tends to increase rather faster 

 than density, but the difference is not sufficiently great to invahdate for 



Table I 



Resolutions of Replica Films 



10 ij.s:/cm^ 



* Included for comparison only, 



** By volume. 



*** On exposure to air, U oxidizes. 



the present purposes the assumption that the two are proportional. The 

 existence of an optimum average mass thickness then implies that intrinsic 

 resolution of replica films is inversely proportional to the density of the material 

 of which they are composed. 



Table I presents a comparison of the resolutions associated with various 

 materials, based on the assumption that the resolution is Yte. As discussed 

 above, this is about the best observable resolution, for favorable topo- 

 graphic features. The resolution of plastic films is not susceptible to calcula- 

 tion, and is probably greater than indicated. The resolutions of evaporated 

 films decrease from about 200 A for silicon and silica to about 25 A for the 

 very heavy metals. However, it is difficult to process the exceedingly thin 

 films of these metals particularly if they recrystallize as does gold. Although 



t Ref. 1, Chap. 19 and p. 158. See also C. E. Hall, Jl App. Phys. 22, 658, 1951. 



