ELECTRON MIKHOK MICROSCOPY 



One can, therefore, use as a fourth criterion details fall into tiie plane of the screen. Hence, 

 for the presence of magnetic fields in front "focusing" is accomplished by applying proper 

 of the specimen the displacements and shape bias to the mirror rather than by adjusting 

 deformations of the area corresponding to the voltages of the electron lenses. The lenses 

 the area of impinging electrons. In practice, mainly provide the desired magnification and 

 this check can be executed most conveniently are not used as focushig elements in the 

 by lateral movements of the specimen. In the meaning of the Gaussian dioptrics. This re- 

 presence of magnetic field components paral- quires, of course, that the electrical as well 

 lei to the plane of the specimen the spot rep- as geometrical roughness of the specimen be 

 resenting the impinging electrons will move rather small and uniform within the viewing 

 not only against the reference coordinate sys- area. This is a disadvantage in this kind of 

 tem of the specimen (as it does in the non- electron microscopy. The rather high field 

 magnetic case) but also with respect to the strength in front of the specimen is another 

 coordinate system of the viewing screen. Si- disadvantage. 



multaneously, the shape of the spot represent- There exists another mechanism for image 

 ing the area of impinging electrons will be contrast formation. Differences in surface 

 deformed, depending upon the shape of the potential across the specimen can result in 

 magnetic field. an area-dependent variation in the intensity 

 Because of the peculiar kind of image for- of the reflected beam. More electrons from 

 mation, care must be taken in the interpre- the velocity spectrum of the electron beam 

 tation of electron mirror micrographs. The will impinge on an area which is more posi- 

 potential reUef in front of the mirror is the five than its surroundings. This ''intensity 

 actual image-forming medium. Optically modulation" type of image contrast forma- 

 speaking, it represents a medium of changing tion is in most cases, however, masked to a 

 refractive index with inhomogeneities in considerable extent by the normal "deflec- 

 front of an irregular electron-reflecting sur- tion modulation" which is also always pres- 

 face. This irregular reflecting surface in con- ent and is in general more sensitive, 

 nection with the inhomogeneous refracting Electron mirror microscopy has a differ- 

 medium forms the image. The image is in ent goal from optical microscopy or con- 

 fact the electron density distribution of the ventional electron microscopy. ^Maximal re- 

 caustic of these two inhomogeneities in the solving power, a major goal in conventional 

 plane of the screen. The amount of the small categories of microscopy, is not the major 

 negative bias of the mirror against the cath- objective in electron mirror microscopy; 

 ode determines how far the electrons pene- rather, it makes accessible to direct visual 

 trate into the potential relief in front of the observation the distribution of those physical 

 mirror specimen and which potential surface properties which are not normally depicted 

 will become the reflecting one. This potential by other microscopic methods. Instead of 

 relief rapidly loses its detail and its refracting revealing the distribution of absorption, re- 

 influence on the electron beam with increas- flection or mass density scattering with high 

 ing distance from the mirror. The closer the resolution, electron mirror microscopy strives 

 electrons are permitted to penetrate toward for the obser^'ation of electrical and magnetic 

 the mirror-specimen the more detailed the patterns even if the resolving power is more 

 image formed by the potential relief will be. limited. Nevertheless, the resolution of the 

 One is, of course, inclined to bias the mirror few presently existing rather crude labora- 

 to obtain the most detailed image, i.e., in tory models of electron mirror microscopes is 

 such a way that the focal points of those about 1000 A, which is slightly better than 

 focusing irregularities which represent the fine the resolution of conventional optical mi- 



323 



