ELECTRON ^IIRROK MICROSCOPY 



the electron beam is reflected on the eqiii- 

 potentials ui front of the specimen M which 

 is generally biased a few tenths of a volt 

 negative with respect to the cathode C and 

 acts therefore as an electron optical mirror. 

 Every electron-deflecting irregularity on the 

 mirror-specimen whether caused by the 

 topographical relief structure or by differ- 

 ences in contact potentials, in surface 

 charges, in electrical conductivities or in 

 magnetization, influences the low velocity 

 electron beam. The electrons returning 

 through the electron optics EO, which may 

 consist of electric or magnetic lenses, project 

 a magnified pictorial representation, a kind 

 of shadow-schlieren image, of the irregulari- 

 ties on the specimen M onto the viewing 

 screen S. 



The other design version (2) of an elec- 

 tron mirror microscope is shown in Figure 

 lb. Here the incoming illuminating electron 

 beam Bi is separated for part of its path 

 from the returning image-carrying beam B2 

 by a magnetic field H normal to the plane 

 of drawing. (Separation of the two beams 

 by simply deviating from normal "incidence" 

 of the electron beam on the mirror-specimen 

 would not be feasible because the mirror- 

 specimen necessarily constitutes a part of 

 the electron optical system, in the vicinity of 

 which axial symmetry must be retained in 

 order to avoid intolerable aberrations.) The 

 auxiliary magnetic field H and the resulting 

 knee-shaped design necessarily complicate 

 the design and construction but result in 

 two basic advantages. First, it leaves the 

 viewing screen unobstructed and, second, 

 it permits the introduction of separated 

 lenses to act on the electron beam before en- 

 tering and after leaving the auxiliary mag- 

 netic field H. This is advantageous because 

 these separated lenses can be optimized in- 

 dependently for their different tasks. The 

 one lens can be designed for optimal per- 

 formance of the incoming, illuminating beam 

 whereas the other lens can be operated for 

 optimal image formation and magnification. 



In the case of the less complicated straight 

 design, the entire electron optics acts on both 

 the incoming electron beam as well as on the 

 returning, image-carrying beam so that the 

 electron optics cannot be of optimal design 

 for either piu'pose but must be a compromise. 



The capabilities of electron mirror micros- 

 copy are numerous and varied. In general, 

 every electron-deflecting field caused by an 

 irregularity on the mirror-specimen can be 

 depicted because it is the electron deflection 

 which forms the image contrast. Electron 

 mirror microscopy permits, therefore, visual 

 observation of the distribution of such purely 

 electrical properties as contact potentials (1), 

 surface charges, space charges (2) and elec- 

 trical conductivities (1, 3). It also proved to 

 be a feasible method for the visual obsen^a- 

 tion of magnetic patterns (4), particularly of 

 magnetic domain patterns. Electron mirror 

 microscopy is also capable of depicting the 

 surface relief structure (2, 5) on a specimen 

 because this structure is retained in the 

 structure of the equipotentials immediately 

 in front of the mirror-specimen. 



Figure 2 illustrates this latter possibility. 

 It shows an electron mirror micrograph of a 

 cleaved surface of a LiF single crystal (5) 

 onto which a thin Pt film had been evap- 



FiG. 2. I'^lcctron mirror micrograph of a cleaved 

 surface of a LiF single crystal. 



317 



