ELECTRON MIHKOK AIICKOSCOPY 



croscopes. One can reasonably expect that 

 the resolution will be improved to about 100 

 A. The factor finally limiting the resolving 

 power is the rather long de Broglie wave- 

 length of the rather slow electrons at the 

 location where the information is picked up 

 in the form of the image-formhig deflections. 

 Theoretically (13), the smallest resolvable 

 distance 8 should be 



d[A] = 



3.3 • W 



volts"! 

 cm J 



in which E stands for the electrical field 

 strength on the surface of the mirror-speci- 

 men. Such a reasonable field strength as 

 40,000 \'olts/cm at the viewed area of the 

 specimen leads to the abo\'e mentioned figure 

 for the obtainable resolving power. 



Electron mirror microscopy has a number 

 of advantageous properties and it also has 

 some inherent disadvantages. The require- 

 ment of a very smooth and uniform speci- 

 men surface and the fact that there will al- 

 ways be inherently a comparatively high 

 field strength in front of the specimen have 

 already been mentioned. The peculiar kind 

 of image formation can sometimes result in a 

 rather complicated correlation between the 

 object and the object's electron mirror mi- 

 croscopical image, a correlation which at first 

 sight appears in some cases as not easily in- 

 terpretable. Fortunately, however, there is 

 an additional parameter available, namely, 

 the bias potential on the mirror-specimen. 

 By varying this bias potential and observ- 

 ing the corresponding changes in the electron 

 mirror microscopical images a correct inter- 

 pretation of the images becomes compara- 

 tively easy after some experience has been 

 gained in this respect. 



Electron mirror microscopy may appear 

 at first thought as a unique research tool for 

 the broad field of surface physics which is 

 becoming increasingly important. In many 

 respects, of course, it can be utilized for that 

 purpose; in many cases, however, its applica- 



bility is severel}^ restricLed by the vacuum 

 obtainable in present day electron mirror 

 microscopes. The design of a normal elec- 

 tron mirror microscope neither permits out- 

 baking nor sealing off; the obtainable vac- 

 lumi will therefore not be considerably better 

 than 10~^ mm Hg. This in turn does not 

 permit a clean surface in the meaning of 

 present-day surface physics. Difficult as it 

 may be, it should not be impossible, howe\'er, 

 to design simplified electron mirror micro- 

 scopes for special applications in which the 

 extremely high vacua required in modern 

 surface physics could be maintained. 



Despite its shortcomings electron mirror 

 microscopy promises to become a valuable 

 research tool extending microscopic obser\^a- 

 tions into areas which are not easily accessi- 

 ble by other means. What makes it even 

 more interesting and versatile is the fact that 

 rather often one can observe with an electron 

 mirror microscope not only static electric 

 and magnetic patterns but actually obsen-e 

 some dynamic behavior, such as strangely 

 moving and changing electric charge pat- 

 terns or magnetic domains set in motion by 

 mechanical strain or applied magnetic fields 

 or magnetic stray fields growing out of grain 

 boundaries. Although the basic features of 

 electron mirror optics have been known for 

 many years (14), its utiUzation for a new 

 type of electron microscopy is rather recent. 

 But even with only a few electron mirror 

 microscopes now in operation, the flow of in- 

 formation from them is considerable and 

 often exceeds the observers' capabilities to 

 digest it. 



REFERENCES 



1. Mayer, Ludwig, /. Appl. Phys., 26, 1228 



(1955). 



2. Bartz, G., Weissenberg, G., and Wiskott, 



D., "Proc. Third Interntl. Conf. Electr. 

 Microscopy," London, 1954, p. 345; Lud- 

 wig Mayer, /. Appl. Phys., 24, 105 (1953). 

 See also ref. 14. 



3. Mayer, Ludwig, J. Appl. Phys. ,28, 259 (1957). 



4. Mayer, Ludwig, /. Appl. Phys., 28, 975 



(1957); G. V. Spivak, et al., Dokl. Akad. 

 Nauk SSSR, 105, 965 (1955). 



324 



