ELECTRON MICROSCOPY 



solvent. The silica or carbon replicas are 

 picked up on specimen screens and studied 

 as usual. The use of carbon and silica will 

 be covered in greater detail in the section 

 on positive techniques. Preshadowed repli- 

 cas afford the obvious advantage of provid- 

 ing better detail by eliminating the inter- 

 ference of the replica structure itself. 



Shadow Casting 



Shadowmg of paint replicas with metal is 

 needed in order to increase the contrast since 

 the replica alone scatters electrons more or 

 less uniformly over the entire area and con- 

 trast in electron microscopy is dependent 

 mainly on the difference in electron scatter- 

 ing among area increments of the specimen. 

 After shadowing, areas in which the depos- 

 ited metal is the densest are the most opaque 

 to the electron beam. 



This process of shadowcasting requires the 

 use of a vacuum evaporator which consists 

 essentially of a bell jar evacuated by a dif- 

 fusion pump backed with a forepump. The 

 mechanics of evaporation can be best illus- 

 trated with the aid of Figure 4. Here, the 

 replica preparation at B is placed below and 

 to one side of the tungsten filament C, which 

 can be charged with a given weight of metal 

 and heated electrically to the temperature 

 required for ^-aporization of the metal. 



If a sufficiently good vacuum exists in the 

 bell jar at the time of evaporation (10~^ mm. 

 of Hg) the metal atoms will travel in straight 

 hues in all directions from the filament and 

 some of them will be deposited on the sam- 



S4MPLE 



Fig. 4. High vacuum evaporation. 



pie. If a metal is used that does not ingrate 

 after deposit, its thickness will be greatest on 

 those aspects of the preparation that face 

 the oncoming atoms, and there will be re- 

 gions of the substrate lying behind high de- 

 tail that will be so shielded as to receive no 

 metal. These clear regions will appear as if 

 they were shadows when viewed under the 

 electron microscope. The varying opacity 

 that results from the varying thickness of the 

 metal creates the impression that one is see- 

 ing the preparation in three dimensions il- 

 luminated by light having the direction of 

 the oncoming shadowing atoms. 



Although in principle the process of vac- 

 uum evaporation is simple there are certain 

 conditions which must be fulfilled if satis- 

 factory preparations of paint films are to re- 

 sult. A suitable shadowing material must be 

 used and the evaporation must take place 

 under vacuum. It would be expected that 

 sharp shadows could not be obtained unless 

 the mean free atomic path in the vacuum 

 evaporator bell jar exceeds the distance from 

 the heater to the sample, that is, unless the 

 evaporated atoms can follow a collision-free 

 path. Experience has shown that the vacuum 

 should be considerably better than this mini- 

 mum. Seemingly, this good vacuum is needed 

 partly for adec^uate degassing of the surface 

 of the preparation before evaporation and 

 partly to take care of some of the gas evolved 

 during evaporation. 



The best material to use for shadowing will 

 depend both on the type of preparation and 

 the kind of observation to be made. A 

 shadowing material must not have any per- 

 ceptible structure of its own and must not 

 migrate over the face of the paint replica af- 

 ter deposition. In addition, it must absorb or 

 scatter so strongly that the needed thickness 

 does not appreciably distort the shape or de- 

 tail of the sample. 



Chromium was the first metal to be suc- 

 cessfully used for metal shadowcasting and it 

 continues to be used for much work. How- 

 ever, when the thickness of the chromium 



202 



