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

136 Chapter 5
Because each secondary produced at the SEM specimen generated about
100 photoelectrons, the overall amplification (gain) of the PMT/scintillator
combination can be as high as 10 8 , depending on how much accelerating
voltage is applied to the dynodes. Although this conversion of SEM
secondaries into photons, then into photoelectrons and finally back into
secondary electrons seems complicated, it is justified by the fact that the
detector provides high amplification with relatively little added noise.
In some high-resolution SEMs, the objective lens has a small focal length
(a few millimeters) and the specimen is placed very close to it, within the
magnetic field of the lens (immersion-lens configuration). Secondary
electrons emitted close to the optic axis follow helical trajectories, spiraling
around the magnetic-field lines and emerging above the objective lens,
where they are attracted toward a positively-biased detector. Because the
signal from such an in-lens detector (or through-the-lens detector)
corresponds to secondaries emitted almost perpendicular to the specimen
surface, positive and negative values of � (Fig. 5-6a) provide equal signal.
Consequently, the SE image shows no directional or shadowing effects, as
illustrated in Fig. 5-9b.
In-lens detection is often combined with energy filtering of the secondary
electrons that form the image. For example, a Wien-filter arrangement
(Section 7.3) can be used to select higher-energy secondaries, which consist
mainly of the higher-resolution SE1 component (Section 5.6).
Figure 5-9. Compound eye of an insect, coated with gold to make the specimen conducting.
(a) SE image recorded by a side-mounted detector (located toward the top of the page) and
showing a strong directional effect, including dark shadows visible below each dust particle.
(b) SE image recorded by an in-lens detector, showing topographical contrast but very little
directional or shadowing effect. Courtesy of Peng Li, University of Alberta.