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

138 Chapter 5
Backscattered electrons can be detected by a scintillator/PMT detector if
the bias on the first grid is made negative, to repel secondary electrons. The
BSEs are recorded if they impinge directly on the scintillator (causing light
emission) or if they strike a surface beyond the grid and create secondaries
that are then accelerated to the scintillator (see Fig. 5-8). The BSEs travel in
almost a straight line, their high energy making them relatively unresponsive
to electrostatic fields. Consequently, more of them reach the side-mounted
detector if they are emitted from a surface inclined toward it, giving some
topographical contrast; as in Fig. 5-7b. However, backscattered electrons are
emitted over a broad angular range, and only those emitted within a small
solid angle (defined by the collector diameter) reach the scintillator. The
resulting signal is weak and provides a noisy image, so a more efficient form
of detector is desirable.
In the so-called Robinson detector, an annular (ring-shaped) scintillator
is mounted immediately below the objective lens and just above the
specimen; see Fig. 5-11a. The scintillator subtends a large solid angle and
collects a substantial fraction of the backscattered electrons. Light is
channeled (by internal reflection) through a light pipe and into a PMT, as in
the case of the Everhart-Thornley detector.
An alternative backscattered-electron detector is shown in Fig. 5-11b.
This solid-state detector consists of a large area (several cm 2 ) silicon diode,
(a)
scintillator
specimen
specimen
objective
primary beam
light pipe
(b) objective
primary beam
metal
HV
contacts signal
p-n transition region
light
to PMT
Figure 5-11. Backscattered-electron detectors installed below the objective lens of an SEM:
(a) annular-scintillator/PMT (Robinson) design and (b) solid-state (semiconductor) detector.