INORGANIC REPLICATION IN ELECTRON MICROSCOPY 



801 



2. Intrinsic Resolution of Replica Films 



In the vicinity of a sharp change in surface gradient, the local thickness 

 te does not change abruptly, but the change occurs over a short distance d 

 as illustrated in Fig. 2. A mathematical formulation for this intrinsic resolu- 

 tion d is given in Appendix II, with some detailed discussion. Intrinsic 

 resolution varies locally over the surface, dependent on the geometrical 

 relationship between atom source, electron beam, and local surface to- 

 pography. Observable resolution includes also instrumental resolution and 

 contrast factors. Except at shadow boundaries it is probably never less 

 than J/e, where ie is the average value of /g, and perhaps may be as poor as 



ELECTRON 

 BEAM 



Fig. 2 — Diagram showing resolution of replica film at a sharp corner, (a) Every atom 

 sticks where it strikes, (b) All atoms diffuse, finally condensing into film of uniform local 

 thickness. 



several times \lc- Shadow profiles and edges often show short lengths of 

 extreme sharpness. The resolution across these portions of shadows may 

 often beassumed to bethe instrumental resolution. The reasons for this are 

 developed in the Appendix. 



2.1 Efect of Film Thickness 



The average linear thickness h is a factor in the expressions for resolution. 

 To reduce d and thereby improve the resolution, the most effective method 

 is to reduce h. With a given material, as the repUca film is made thinner, 

 the contrast is reduced also, so that, just as in photography a feature that 

 is visible in a properly exposed negative may be lost in a thin negative, 

 some features may not be repHcated with sufficient contrast to be detectable. 

 For 50 kv electrons, the optimum average thickness of a replica lies between 

 S and 10 /xg/cm^. This is 7-15 times the minimum perceptible thickness 



