INORGANIC REPLICATION IN ELECTRON MICROSCOPY 803 



gold-manganin films are only slightly thicker, they are not particularly 

 difficult to process, especially if 500-mesh supporting screen is used. 



2.2 Granularity 



Resolution is also affected by the short-range migration which culminates 

 in recrystalhzation of many metallic films.^^- ^^' ^^' ^^ If the crystaUite size is 

 smaller than the resolution, i.e. less than J/e, this effect is not too important, 

 even though the granularity may be objectionable at high magnification 

 from an esthetic viewpoint. A fairly extreme example of recrystalhzation is 

 shown by the aluminimi repHca in Fig. 9. 



A second source of granularity is due to properties of plastics when 

 plastic molds are used as intermediate replicas. Because plastic molecules 

 are large, and because they associate into domains,^^ the plastic surface is 

 actually granular on a scale of the order of 100 A. Plastic granularity is not 

 observed in siKca replicas because of insufficient resolution, but it becomes 

 very evident in shadow-cast repHcas on account of the near-glancing inci- 

 dence of the shadowing material.^^- ^^ The occurrence of granularity due to 

 this cause in replica films of denser materials is an indication of their good 

 resolution. Since this granularity is real on the plastic surface, it shows 

 clearly the azimuth of the incident atom beam, whereas granularity due to 

 recrystalhzation shows no directional effect. 



3. Experimental Observations 



The foregoing material presents a rather idealized picture of the process 

 of replica film formation by condensation of inorganic substances evaporated 

 under good vacuum, i.e. at pressures preferably less than 10~^ mm, and 

 certainly not greater than lO"'' mm of mercury. Subsequent to film forma- 

 tion in the vacuum, it must be subjected to gross physical and chemical 

 processing to prepare it for electron microscopic examination. It must be 

 exposed to air, which may cause oxidation. Uranium films, for example, 

 appear to oxidize completely, and it is beheved that SiO films oxidize to 

 Si02. Most metal films yield good electron diffraction patterns characteristic 

 of the metal, although this does not preclude the possibihty of surface 

 oxidation, since the oxides are usually amorphous and diffuse rings due to 

 thin oxide layers would be difficult to detect. Then the films must be sepa- 

 ls R. G. Picard and O. S. Duffendack, //. App. Phys., 14, 291 (1943). 

 12 H. A. Stahl. //. App. Phvs., 20, 1, (1949). 

 13 H. Levinstein, Jl. App. Phys., 20, 306 (1949). 



14 R. S. Sennett and G. D. Scott, //. Opt. Soc. Am., 40, 203 (1950). 



15 C. C. Hsiao and J. A. Sauer, //. App. Phys., 21, 1070 (1950). 



i« R. C. Williams and R. C. Backus, Jl. App. Phys., 20, 98 (1949). 

 1^ Metallurg:ical Applications of the Electron Microscope, p. 11, Symp. of Inst, of Met., 

 November 1949. 



