ELECTRON MIHKOK MICKOSCOPY 



oratod to make Ihc suiiace conductive. De- 

 piction of the topography of a specimen is 

 certainly not a unique feature of electron 

 mirror microscopy and, in many cases, other 

 microscopic methods are available which are 

 either more convenient, such as optical mi- 

 croscop}^, or have considerably higher re- 

 solving power, such as conventional electron 

 transmission or reflection microscopy. In spe- 

 cial cases, however, electron mirror micros- 

 copy might be utilized quite advantageously 

 for the observation of the surface reUef 

 structure on a specimen because the image- 

 forming electrons do not penetrate, nor even 

 impinge on the specimen and thus no speci- 

 men heating problems exist. Furthermore, 

 no recourse to replica techniques is neces- 

 sary; the resolving power, although not ap- 

 proaching that of electron transmission mi- 

 croscopy, is somewhat better than that of 

 light microscopy, and, perhaps most im- 

 portant, the elevation differences or step 

 heights necessary to form sufficient image 

 contrast are extremely small. 



The step height of a single barium stearate 

 monolayer (24.4 A) can, for instance, be ob- 

 served with such pronounced contrast that 



Fig. 3. Noise-like and wave-like electric charge 

 movements on amorphous selenium film. Single 

 frame of a motion picture taken from the screen of 

 the electron mirror microscope. Magnification ap- 

 pro.x. 23 X 



considerably smaller step heights should cer- 

 tainly be observable. A genuine and some- 

 what imique feature of electron mirror mi- 

 croscopy is, however, its capability of depict- 

 ing purely electrical and magnetic patterns. 

 Besides depicting contact potential distribu- 

 tions or surface charge distributions, it can, 

 for example, also depict the electrical con- 

 ductivity distribution of layers of very low 

 conductivity above conductive substrates. 

 This can be achieved by letting a few elec- 

 trons from the tail end of the Maxwellian dis- 

 tribution impinge on the mirror-specimen, 

 using the majority of the electrons, however, 

 for image-forming purposes. It is a particu- 

 larly valuable property of electron mirror 

 microscopy that it can, in such cases, depict 

 without any time delay electrical charge 

 movements which occur on such layers if 

 their conductivity is extremely low. 



Figure 3 shows as an example a single 

 frame of a 16-mm motion picture taken di- 

 rectly from the screen of an electron mirror 

 microscope. (The dark shadow extending 

 from the center of the picture to the right is 

 caused by the electron gun protruding from 

 the center of the \'iewing screen.) The speci- 

 men was an amorphous selenium film de- 

 posited on a germanium film substrate. The 

 observations which can be made if one ap- 

 plies a small positive bias potential to the 

 substrate film of such a specimen demon- 

 strate quite well that electron mirror micros- 

 copy has some unique capabilities not to be 

 found in any other type of microscopy. 



One of the many phenomena which can be 

 observed on such an amorphous selenium 

 film is briefly described here as an example. 

 The small positive bias potential on the sub- 

 strate of the mirror specimen lets some elec- 

 trons impinge on the selenium, thus creating 

 a voltage drop across the thickness of the 

 selenium film. This voltage drop in turn 

 causes a slightly negative surface potential 

 equilibrium so that most of the electrons are 

 still reflected by the equipotentials in front 

 of the selenium film. A genuine electron mir- 



318 



