ELECTRON IVI I C KOSCOP Y 



Fig. 3. Reflection electron micrograph of the 

 surface of a polished sapphire ball (diameter 1.27 

 cm). Though curved, the surface is well focused. 



Q J -|-©„ = 11°,© i~l to2° (varying across field), 

 nu = 2500X,m|| + 400 X. 



Applications 



There are two main types of specimen for 

 which reflection electron microscopy is a 

 particularly suitable method of surface 

 examination. First, there are very smooth 

 surfaces such as polished metals, glass, 

 cleavage surfaces of some crystals, etc. Fig- 

 ure 3 shows a micrograph of the surface of a 

 polished sapphire ball and is representative 

 of the results which can be obtained from 

 this type of surface. If the angles di and do 

 are known, then the heights of asperities can 

 be calculated from the shadow lengths using 

 simple geometrJ^ The method can give useful 

 quantitative information but there are a 

 number of possible sources of error to be 

 kept in mind. There may be transmission of 

 the electron beam through the edges of asper- 

 ities; the local surface on which the shadow 

 falls may be at an angle to the plane of the 

 specimen surface as a whole; the incident 

 electron beam may not be strictly parallel 

 and may therefore give penumbra; electron- 

 induced contamination can build up on the 

 edges of scratches, etc. and give misleading 

 results. However, comparison of results ob- 

 tained by this and other methods (see e.g., 

 Bailey and Seal 1956) has shown that for 



o 



surface roughnesses of 100 A or more the 

 method gives accurate results provided that 

 reasonable care is taken in the operation of 



the instrument and interpretation of results. 

 Features down to about 30 A in height can 

 be resolved by the shadows they cast, but 

 for such small features the quantitative re- 

 sults are less reliable. 



The second type of application depends 

 on another effect. This is the high depth of 

 field inherent in the electron microscope be- 

 cause of the very small angular aperture of 

 the objective lens. The depth of field of a 

 microscope objective is ± 5/tan a where 5 is 

 the resolution and a. the semianglar aperture 

 of the objective. For an optical microscope 

 at high magnification 8 would be about 2500 

 A and a about J-^ radian giving a depth of 

 field of ztVo u. A reflection electron micro- 



^ o 



scope would have 5 about 300 A and a about 

 3.10"^ radian giving a depth of field of ±10 n. 

 Thus the depth of field of a reflection electron 

 microscope is considerably greater than that 

 of an optical microscope at similar magnifi- 

 cation. Consequently, focused pictures of 

 highly curved objects such as balls, wires, 

 fibers, etc., and of features such as scratches 

 or grooves can be obtained. Examples are 

 shown in Figures 3 and 4: the polished 

 sapphire surface shown in Figure 3 had a 

 radius of curvature of 0.63 cm; Figure 4 

 shows one groove of a gramophone recording. 



Although it is possible to examine gross 

 features of this kind there is a restriction: 

 they must either be single isolated features 

 on an otherwise smooth surface or have 

 extent in one direction only. It is not possible 

 to examine rough surfaces of large extent 

 since the shadows cast by asperities would 

 obscure neighboring detail: in such cases 

 the mountain tops onlj^ are illuminated and 

 the valleys left in shadow. With a feature 

 such as a scratch the illumination should be 

 parallel to the scratch since otherwise the 

 edge would probably cast too long a shadow. 

 Figure 5 shows a surface (of lathe-turned 

 brass) which is about as rough as can usefully 

 be examined by this technique. 



In reflection electron microscopy one 

 examines the specmien directly and there is 



226 



