Physical Principles of Electron Microscopy: An Introduction to TEM ...

The Scanning Electron Microscope 141
Figure 5-13. SEM voltage-contrast image of an MOS field-effect transistor with (a) the gate
electrode G at –6 V, source S and drain D electrodes grounded; (b) gate at –6 V, source and
drain at –15 V. From Reimer (1998), courtesy of Springer-Verlag.
Voltage contrast arises when voltages are applied to surface regions of a
specimen, usually a semiconductor IC chip. The secondary-electron yield is
reduced in regions that are biased positive, as lower-energy secondaries are
attracted back to the specimen. Conversely, negative regions exhibit a higher
SE yield (see Fig. 5-13) because secondaries are repelled and have a higher
probability of reaching the detector. The voltage-contrast image is useful for
checking whether supply voltages applied to an integrated circuit are
reaching the appropriate locations. It can also be used to test whether a
circuit is operating correctly, with signal voltages appearing in the right
sequence. Although most ICs (such as microprocessors) operate at far too
high a frequency for their voltage cycles to be observed directly, this
sequence can be slowed down and viewed in a TV-rate SEM image by use
of a stroboscopic technique. By applying a square-wave current to
deflection coils installed in the SEM column, the electron beam can be
periodically deflected and intercepted by a suitably-placed aperture. If this
chopping of the beam is performed at a frequency that is slightly different
from the operational frequency of the IC, the voltage cycle appears in the SE
image at the beat frequency (the difference between the chopping and IC
frequencies), which could be as low as one cycle per second.
As an alternative to collecting electrons to form an SEM image, it is
sometimes possible to detect photons emitted from the specimen. As
discussed in connection with a scintillator detector, some materials emit
visible light in response to bombardment by electrons, the process known as
cathodoluminescence (CL). In addition to phosphors (see Fig. 5-14), certain
semiconductors fall into this category and may emit light uniformly except in
regions containing crystal defects. In such specimens, CL images have been
used to reveal the presence of dislocations, which appear as dark lines.