ELECTRON MK.KOSCOPV 



. . . . I. i.V . / MEAN PLANE 

 . f < < <V /^^ ^ OF SPECIMEN 



-t -< 



APERTURE IN 

 " OBJECTIVE LENS 



\ DEFLECTIN" 

 J SYSTEM 



SCREEN 



Fig. 3. Geometry of beam scanning system. 



ducing the deflecting system into the lower 

 pole-piece of the scanning objective lens (26). 

 While the electromagnetic system is slightly 

 more complex, the change of magnification 

 of the instrument with change of beam-volt- 

 age is considerably less than in the case of 

 the electrostatic system because in the elec- 

 tromagnetic system the deflection sensitivity 

 is inversely proportional to the root of the 

 beam voltage. 



One of the simplest electromagnetic sys- 

 tems utilizes the square counterwound yoke 

 type of configuration (19) which may be 

 conveniently fabricated from ferrite rods 

 of rectangular cross section (7). 



The geometry of the scanning system is 

 shown in Figure 3. The deflecting system is 

 placed immediately before the objective since 

 there is not sufficient space between objec- 

 tive and specimen. With this arrangement, 

 the portion of the beam which is used to form 

 the spot changes during the scan; since the 

 magnification is reduced and the scanning 

 amplitude thus increased, the spot is formed 

 by rays further from the axis of the beam. 

 It should be noted that at the higher magni- 

 fications the amplitudes involved are so 

 small that the use of slightly off-axis rays 

 has no significant effect on resolving power. 



The portion of the beam represented by 



ray OP is very nearlj^ parallel to the axis 

 of the beam since h is small; therefore, the 

 deflection angle 62 is approximately equal to 

 di = p/f, where 2p is the amplitude of scan 

 on the specimen and f is the focal length of 

 the objective. The magnification of the in- 

 strument will change with focal length; al- 

 lowance must be made for this when travers- 

 ing an obliquely scanned specimen in the 

 vertical direction. The deflection system is 

 placed as close to the lens as possible because 

 this will, at a given minimum magnification, 

 reduce h; the deflecting system may be made 

 smaller and the efficiency increased. 



The lowest operating magnification in the 

 scanning instrument is about X200. If the 

 picture on the display is 12 cm square, the 

 amplitude of the raster on the specimen, 2p, 

 will be 0.6 mm and di and h would have the 

 typical values of 0.03 radian and 3 mm, re- 

 spectively. 



When operating at high values of spot 

 current, the beam diameter, D, may be re- 

 duced sufficiently to cause the beam to be 

 completely swept off the objective aperture 

 at low magnifications. This will result in 

 vignetting of the picture. A balanced double- 

 deflection sj^stem in which the axis of the 

 beam is arranged to always pass through 

 the objective aperture prevents vignetting 

 under these conditions (7). 



The magnification of the scanning instru- 

 ment is altered simply by altering the scan- 

 ning amplitude on the specimen while main- 

 taining that of the display constant. As 

 shown in Figure 1, this is accomplished in 

 the electromagnetic system by a simple com- 

 bination of shunt and series resistors, the 

 scanning generator providing a constant- 

 current output. 



Formation of the Image. The image 

 observed in the scanning electron microscope 

 is foreshortened in the vertical direction ac- 

 cording to the angle, 6, which the incident 

 beam makes with the mean plane of the 

 surface. The effect is exactly as if the sur- 

 face were viewed at this angle; accordingly. 



246 



