3.3. MICROSCOPY
53
Actual diverging
electron beam
Figure 3.16. Double-deflection system of a scanning electron microscope. When the upper
scan coils /,-A deflect the beam through an angle 0, the lower scan coils h-l2 deflect it back
through the angle 20, and the electrons sequentially strike the sample along the indicated line.
The upper inset on the left shows the sawtooth voltage that controls the current through the /,
scanning coils. The lower inset on the left shows the sequence 01 points on the sample lor
various electron trajectories 1.2.3.4.5 downward along the microscope axis. Scan coils I,-/,
and I,-/, provide the beam deflection for the sequence 01 points 1.1’. 1” shown in detail A. [From
J. D. Verhoeven, chapter in Whan (1986). p, 493.1
occurs in many television sets. The magnetic field produced by a coil is proportional
to the voltage Vapplied to the coil. We see from the upper insct on the let? side o f
Fig. 3.16 that a sawtooth voltage is applied to the pairs o f coils / , , I , and /2,/2. The
magnetic field produced by the coils exerts a force that deflects the electron beam
from left to right along the direction of the line drawn at the bottom on the sample.