3.1.2. Condenser Magnetic Lens
Electrons can be focused either by electrostatic forces or magnetic forces. Although electron lenses in principle behave the same as optical lenses, there are differences. Also, the quality of electron lenses is not nearly as good as optical lenses in terms of aberrations.
The electron beam is divergent after passing through the anode plate and must be collimated by condenser lenses and apertures into a relatively parallel stream. A magnetic lens is formed from two circularly symmetric iron pole-pieces with a copper winding in-between with a hole in the center through which the beam passes (Figure 3.1.2a). The pole pieces are separated by an "air-gap" in which the focusing actually takes place. The divergence of the magnetic flux along the optical axis imparts a force on electrons back towards the optical (Z) axis, resulting in focusing action. The magnetic field also causes a rotation of the electrons (and the image) about the Z axis in a corkscrew fashion.
|
|
Figure 3.1.2a. Cross section of an electromagnetic lens showing deflection of the electron beam by the magnetic field (lines of force shown as curved dashed lines) across the pole pieces; dimensions have been exaggerated for clarity (after Potts 1987). |
The condenser lens controls the amount of current that passes down the rest of the column by focusing the electron beam to variable degrees (Figure 3.1.2b). The sharper the focus, the less of the beam intercepted by an aperture (small hole) located below the lenses and the higher the current (Figure 3.1.2c). Some microprobes have a second condenser lens that is used to provide a better focus for SEM work. This lower condenser lens may be "decoupled" (not used) for microprobe work and "coupled" to the upper lens to make one long lens for SEM work.
 |
Figure 3.1.2b. Controlling the sample beam by changing the current through the windings of a condenser lens: (a) no current, electron beam is unfocused; (b) small current, lens has long focal length, (c) larger current, lens has a shorter focal length and a smaller cone of electrons is transmitted through the aperture. Dimensions have been greatly exaggerated for clarity (after Potts 1987). |
 |
Figure 3.1.2c.. Change in probe current resulting from varying the condenser lens current. The points a, b and c correspond to diagrams in Figure 3.1.2b (after Potts 1987). |
The beam characteristics are different for SEM and microprobe work. Probe beams are typically 1 to 5 µm in diameter with currents of 10-9 (1 nanoamp = 1 nA) to 10-7 amps (100 nA). There is no reason for the electron beam diameter to be smaller than the diameter of the excited region, which is about 2 µm. In contrast, because SE are produced only in the upper 10 A of the sample's surface, the beam size is not limited by the excitation area and much smaller beam diameters are possible. SEM uses much lower currents (about 10-12 amp = 1 picoamp = pA) and a smaller final aperture to limit the beam diameter.