SPECIAL MK/niODS 



selected area corresponding completely to 

 the reflection image except in some special 

 cases, because in electron diffraction the in- 

 cident angle of the electron beam to the 

 specimen is less than 1°. 



Considerations on Resolution. In gen- 

 eral the resolution of the electron micro- 

 scope is determined mainly by spherical and 

 chromatic aberrations, astigmatism and diff- 

 raction. In the reflection electron microscope, 

 chromatic aberration is comparatively great 

 because of the larger amount of inelastic 

 scattering suffered by an electron beam. 

 Therefore, it is considered that chromatic 

 aberration is mainly responsible for limiting 

 the resolution obtainable in the reflection 

 microscope. 



Here, the order of magnitude of the reso- 

 lution is estimated which is expected in a 

 direction perpendicular to the plane of inci- 

 dence of the electron beam. Neglecting all 

 aberrations except chromatic, we may write: 



5x = «•/• 



AV 



(1) 



To achieve higher resolution, it is neces- 

 sary for the objective aperture to be smaller. 

 For example, to obtain a resolution better 

 than 100 A, it is estimated that the ol)jective 

 aperture about 10 n would be required. But 

 by using such a small aperture, it is doubtful 

 whether the intensity of the image would be 

 adequate for precise focusing at high magnifi- 

 cation. 



Menter has obtained the resolution better 

 than 400 A using a 30 /x aperture in electron 

 microscope with a focal length somewhat 

 longer than that in the above calculation. 



Application of Reflection Device. Fig. 

 4 is a reflection image of pearlite structure. 

 (a) is taken at 6 = 8° and (b) at = 30°. 

 It is found that the distortion of the image 

 is reduced by increasing the observation an- 

 gle. Fig. 5 shows the reflection image of mar- 

 tensite structure and Fig. 6 the reflection 

 image of pearlite structure of dra\^'n high 

 carbon steel. They were taken at 30°. 



Studies by a reflection method have not so 

 far given any further information to that 

 obtained by the replica method, but in future 



where 8±is the radius of the disc of confusion 

 in a plane perpendicular to the plane of inci- 

 dence due to chromatic aberration, a the 

 semi-angular aperture, / the focal length of 

 the objective, V the accelerating voltage and 

 AV the average energy loss of electrons scat- 

 tered from the specimen. The variations in 

 the high tension supply voltage and excita- 

 tion current in the objective lens being much 

 smaller compared with energy losses due to 

 scattering at the specimen, their effects are 

 neglected in equation (1). According to 

 Kushnir et al., AV is lOOF when the acceler- 

 ating voltage is 80 kV. Thus, AV/V = >^oo. 

 Putting a ^ 3 X 10-^ rad. and f = o mm, 

 we can find: 



8± ^ 190 A 



The resolution in a direction in the plane 

 of incidence will be worse by a factor 1/sin d, 

 i.e., for d = 6°, 5,, ^ 2000 A and for 9 = 30°, 

 5 1, ^ 400 A. 



Fig. 4. Reflection image of pearlite (above) 

 e = 8°, (below) e = 30°. 



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