A Scanning Microscope with Electron or X-Ray Recording 



13 



DISPLAT 





r 



l— «nPLIFIER 



g 



- ELECTRON! GUN 



MftGMETIC LENSES 



CRYSTAL SPECTROMETER 

 'MAY Be INSERTED 



AMPLIFIER 



PULSE 

 ANALYSER 



Fig. 1. Block diagram of the scanning microscope. 



the lens aperture. The spot size is 0. 1 /< to 1 // and 

 the area of scan about \ mm square at largest. Part 

 of the emitted x-rays are collected through a window 

 in the polepiece gap by a scintillation counter, or 

 alternatively the phosphor of the counter may be 

 pushed into the vacuum to record high energy scat- 

 tered electrons. The amplified signal from the counter 

 modulates the brightness of a cathode ray tube 

 scanned in synchronism with the microscope beam, 

 so that an image is obtained of the variation of 

 scattered electron intensity or of x-ray emission over 

 the surface. 



The scan may then be stopped and the micro- 

 scope beam accurately positioned on any feature in 

 the specimen by observing the spot on the afterglow 

 of the picture on the display tube. Through another 

 window part of the emitted x-rays pass into a crystal 

 spectrometer for analysis of the emission spectrum 

 from that point, as in the method of microanalysis 

 developed by Castaing (I, 2). 



Alternatively, the crystal can be removed from 

 the spectrometer so that the x-rays pass straight into 

 a proportional counter, which gives for each quan- 

 tum a pulse of height approximately proportional 

 to its energy. A single channel pulse analyser can 

 be used to pass only pulses corresponding to a given 

 characteristic line and, with the beam scanning, 

 these can modulate the display tube so that the 

 distribution of the element emitting that line is 

 shown up. 



The second lens is shown in fig. 2. The electrons 

 are focussed on to the specimen which is level with 

 the lower pole face, and x-rays pass through a 

 beryllium window into the phosphor of the scintilla- 

 tion counter just outside the gap. The light generated 

 is transmitted by a perspex rod to the photocathode 

 of the multiplier outside the lens. The electrons 

 scattered from the specimen are deflected by the 

 lens field, but may be collected by removing the 

 window and pushing the phosphor into the polepiece 

 gap. The signal from the photomultiplier with elec- 

 tron recording is several hundred times that with 

 x-ray recording with other conditions unaltered. 



Through another window in the polepiece gap 

 x-rays pass into a hydrogen filled tube and into the 

 crystal spectrometer or proportional counter. The 



1 SPECIMEN 

 MOVEMENTS 



Fig. 2. Final lens. 



specimen can be moved laterally and vertically or 

 rotated and is insulated from earth in order to meas- 

 ure the incident electron current. The focal length 

 of the lens is about 4 mm; this is slightly short- 

 ened by a thick ferromagnetic specimen but the spot 

 is not distorted. 



Fig. 3 shows an area of 1500 mesh/inch silver grid 

 as photographed from the display tube with electron 

 collection in A and x-ray in B. The accelerating volt- 

 age was 25 kV giving an electron penetration in the 

 specimen of about 1 //. The topography of the bars 

 is shown up with electron collection (A) but not 

 with x-ray collection (B) unless the irregularities are 

 more than a few microns in size. With x-ray collec- 

 tion the resolution can be no better than the electron 

 penetration since x-rays are emitted throughout the 

 volume of electron diffusion, but with electron collec- 

 tion the resolution is well below this. Smith reports 

 a resolution of about 200 A with his scanning 

 microscope (7). 



The particular advantage in using x-rays to form 

 the picture lies in being able to show up the distribu- 

 tion of one element by using a proportional counter 

 and pulse analyser. For example, with a specimen 

 of silver and copper grids it is possible to make 

 either one appear the brighter by selecting the CuK 



A B 



Fig. 3. 1500 mesh inch silver grid, bars .^ /( and I \ /i wide. 

 A, electron collection; B, x-ray collection. 



