ELECTRON OPTICS 



ditions the optimum bias potential is in- 

 creased but the brightness and hence the 

 image intensity is unchanged. 



Haine, Einstein and Borcherds (1958) 

 have discussed the use of automatic bias, i.e., 

 the generation of bias potential by the po- 

 tential drop across a resistance connected in 

 series with the high voltage supply. Apart 

 from the simplicity of this arrangement, the 

 negative feedback action of the gun mutual 

 conductance and series resistor gives a high 

 degree of stabilizing action to the beam cur- 

 rent. Further restrictions are imposed on the 

 choice of geometrical configuration to en- 

 sure that the bias potential is maintained 

 at the optimum value. 



To obtain adequate image intensity for 

 very high resolution working (3 to 10 A), the 

 electron gun cathode must be operated at an 

 emission density of 1 to 3 amp/cm-. This 

 emission current density requires an operat- 

 ing temperature at which the tungsten cath- 

 ode life, as limited by evaporation, is only a 

 few tens of hours (Bloomer 1957). 



Electron Lenses 



The requirements of the electron micro- 

 scope are met with magnetic or electrostatic 

 lenses which provide a "bell-shaped" magnetic 

 field or electrostatic potential distributions 

 along the axis. The magnetic lens comprises 

 a solenoidal excitation coil wound on an axi- 

 ally symmetric kon circuit with a gap in the 

 core and an axial hole bored to allow the 

 passage of the electrons. Basically the lenses 

 comprise parallel pole pieces spaced S cm 

 apart and an axial hole diameter D cm with 

 an excitation NI ampere-turns applied be- 

 tween the pole pieces (Figure 2). Little or no 

 advantage is gained by changing the shape 

 of the pole pieces from this simple geometry. 



The electrostatic lens comprises basically 

 three parallel plate electrodes with axial 

 holes. The outer plates are at ground poten- 

 tial and the central one at the negative po- 

 tential of the electron gun cathode. The elec- 

 trodes are usually shaped to minimize the 

 surface electric field strength to avoid flash- 

 overs, but the lens theory applied to the 



O CWDI 002 0-03 0-04 005 0-0 6 



Vr/CNI)' 



Fig. 2. Curves showing the focal properties of magnetic lenses as functions of the excitation param- 

 eter. Vr/{NI)\ 



149 



