ELECTRON MICROSCOPY 



to a facsimile recorder scanning in syn- gether with direct viewing of the picture 



chronism wilh the electron beam. Contrast realized the possibility of an instrument 



in the image was produced partly as a result suitable for practical laboratory application. 



of variations hi the composition of the sur- The techniques and applications described 



face and partly because of the surface topog- in the following sections relate largely to 



raphy. The energy of the electron beam in- this instrument (6) and developments there- 



cident on the specimen was made low, only from (7, 8, 9). 



800 volts, in order to achieve a high sec- Formation of the Electron Spot. The 



ondary-emission coefficient. Focushig of this function of the electron optical system is to 



instrument was achieved by observation of focus a fuie electron spot of high intensity 



the video signal with an oscilloscope; the onto the specimen. Two principal factors 



high-frequency content of the signal was limit the performance in this respect — aber- 



used as a criterion for adjustment. rations of the lenses, and limited brightness 



A disadvantage of this instrument was output from the electron gun. Expressions 



that no means was provided for observing are derived below which indicate how these 



the field prior to recording. The use of a factors affect the performance, 



low-energy beam reduced the intensity of The electron spot which is a reduced im- 



the spot, thus necessitating rather long re- age of the virtual cross-over in the electron 



cording times of about 10 minutes. Also, at gun is formed by a two-stage lens system 



low primary energies, contamination of the (Figure 1). By varying the strength of the 



specimen surface with organic vapors, al- first lens, the demagnifying power of the 



ways present in a demountable vacuum sys- system, and thus the size of the spot, may 



tem, was likely to affect significantl}^ the be controlled over a wide range without 



secondary-emission coefficient. affecting significantly the position of the 



A scanning electron microscope, similar spot. Because the effective aperture at which 

 to the above instrument but incorporating the first lens works is very minute, the aber- 

 several new features, was proposed by Oat- rations of this lens do not contribute signifi- 

 ley and McAIuUan in 1948 and constructed cantly to the limitation of the performance, 

 by McjMullan at the Engineering Depart- The spot is focused on the specimen by 

 ment, University of Cambridge. This instru- means of the final or objective lens. The 

 ment used high-energy electrons (10-40 kV) specimen, which may be as large as 1 cm in 

 to reduce the effects of surface contamina- diameter, is placed as close to this lens as 

 tion, oblique scanning of the specimen, direct possible since the focal length and conse- 

 amplification of the emitted electrons with quently the spherical aberration is reduced 

 an electron multiplier suitable for use in a thereby. If ferrous specimens are to be exam- 

 demountable vacuum system, and direct ob- ined, the working space must also be field- 

 seni-ation of the field on a long persistence free. The requirements of a magnetic scan- 

 cathode-ray tube display prior to recording ning objective are best satisfied by an 

 the picture. While oblique scanning resulted asymmetrical pole-piece design (10); typical 

 in a foreshortening and reduction of resolv- values of focal length and spherical aberra- 

 ing power in the vertical direction of the tion coefficient for this type of lens with a 

 image, it allowed a much simpler and more working distance of 0.5 cm are 1 cm and 

 efficient collection arrangement. The fea- 3 cm, respectively. Astigmatism of the ob- 

 tures incorporated in this instrument re- jective caused by pole-piece ellipticity may 

 moved most of the disadvantages of earlier also be a significant fimitation. 

 instruments. In particular, the reduction of Spherical aberration and astigmatism of 

 recording times to 5 minutes (or less) to- the objective and, at the smaller spot sizes, 



242 



