818 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



Many artifacts are also detectable by stereoscopy. Since two pictures of 

 the same field are available, photographic emulsion flaws are readily de- 

 tected. The three-dimensional view also helps to identify many other arti- 

 facts, such as foreign material present on the replica or local wrinkling of 

 the replica film. 



Unfortunately, half-tone reproductions are not suitable for stereograms, 

 because half-tone detail is objectionably enlarged by the viewing stereo- 

 scope. Despite this, some idea of the value of stereograms may be obtained 

 from the figures. 



Conclusions 



A unified picture of replication by evaporated films has been presented. 

 Thin-film replicas may be made by any material which can be evaporated 

 in vactw and whose physical and chemical properties are suitable. For good 

 contrast, a considerable angle should separate the directions of incidence 

 of atom- and electron-beams. The intrinsic resolution of the replica is about 

 half the film thickness and is therefore inversely proportional to the density 

 of the repHcating material. Multiple point sources and sources of extended 

 area are equivalent from a shading standpoint to a single point source 

 properly placed. The oxides SiO and Si02, and presumably many others, 

 form amorphous films, whereas the metals tend to recrystaUize although 

 the crystallite size may be less than 50 A for some metals. Germanium, a 

 semi-metal, forms an amorphous film. Although not dense compared to the 

 heavy metals, it is more than twice as dense as Si02, and should be valuable 

 as a replicating material because it combines electrical conductivity and 

 high resolving power in an amorphous fihn and is chemically rather inert. 

 Among the metals, chromium appears to be the most generally useful repU- 

 cating material. Gold-manganin has sufficiently small crystallite size for 

 many purposes, and is very easy to evaporate. The platinum group suffers 

 from the disadvantage of being very difficult to evaporate. 



Finally, all inorganic repUca films studied retain the third dimension. 

 Electron stereo-micrograms may be used to reveal three-dimensional topog- 

 raphy, largely eliminating the need for correlation of photographic density 

 variations with the surface structure. 



APPENDIX I 

 CONTRAST IN REPLICA FILMS 



(a) Local Thickness When Every Atom Sticks Where It Strikes. 

 Referring to Fig. 1, it is evident that: 



/f == /a r-= = U cos 6a (1 -\- tan da cos <Pa tao 6 cos fp). (1) 



fn 



