CHAPTER 11 

 THE SCANNING MICROSCOPE 



IT has been mentioned in chapter 5 that attempts to produce 

 an electronic counterpart of the metallurgical microscope were 

 so far not very successful, nor are they likely ever to achieve 

 the perfection of the transmission type of instrument. Trans- 

 parent replicas will give information on the shape of the surface, 

 but none on its constitution. It is therefore fortunate that we 

 possess in the scanning microscope an instrument which gives 

 information based on secondary emission, which is a highly 

 sensitive function of the chemical constitution and physical 

 state of the surface constituents. 



The scanning microscope is entirely a by-product of tele- 

 vision. Max Knoll, who by that time had left the Technische 

 Hochschule, Berlin, for the Television Department of Tele- 

 funken, was the first to conceive the idea of producing an image 

 of a surface by scanning it with an electron beam, collecting the 

 secondary electrons, and applying the impulses to the modulat- 

 ing electrode of a television tube which is scanned in synchronism 

 with the first beam.^^ A» similar tube, called Monoscope, was 

 later designed by C. E. Burnett,^^ of the R.C.A., for the purpose 

 of testing television tubes. The same principle was utilized by 

 M. von Ardenne for the first scanning microscope. ^^ Finally, 

 a highly perfected instrument with many novel features was 

 developed by V. K. Zworykin, J. Hillier and R. L. Snyder,^"^ 

 in the R.C.A. Laboratory in Camden, a short description of 

 which follows. 



Figure 35 is a diagram of the electron optical arrangement. 

 The problem here is to produce an extremely fine scanning spot 

 of less than 500 A diameter, which yet carries sufficient current 

 to produce a signal at least ten to twenty times above the noise 



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