Scanning Electron Microscope

In contrast to the TEM, the SEM can easily observe nearly
any reasonably sized sample and furnish the chemical mi-
croscopist with much information with little effort. Pri-
mary signals generated in an SEM are secondary elec-
trons (SE), backscattered electrons (BSE), characteristic
X rays, X-ray continuum, Auger electrons (AE), low-loss
electrons (LLE), electron energy loss (EEL), and trans-
mitted electrons. Of value to the chemical microscopist
are primarily the SE, BSE, and X-ray spectrum.
The SEM is related to the TEM only by virtue of the fact
that it also employs a beam of focused electrons to bom-
bard a sample and generate an image; however, once the
beam of electrons passes through the condenser lenses, the
similarity ends. After the beam has been passed through
a final aperture, a set of scanning coils deflects the beam
at various rates across the sample.
The striking three-dimensional images which have pop-
ularized the SEM are the result of secondary electrons
ejected from the surface of the sample by the bombarding
primary electron beam. The SE image is collected by a
scintillator or phosphor-coated light pipe which transmits
the signal to a photomultiplier and finally to a viewing
CRT, which scans simultaneously with the primary elec-
tron beam. Since the area scanned is quite small in rela-
tion to the area of the viewing CRT, magnification is thus
achieved and can be varied by changing the size of the area
scanned across the sample. Resolution and magnification
in the SEM are not as good as with the TEM: commercial
SEMs now routinely work at 60-A resolution and magni ˚ fi-
cations of 500,000× are achievable although the theoreti-
cal MUM is ∼20,000×. Most work on an SEM, however,
is below 5000×, and very high resolution is rarely im-
portant. The lower resolution of the SEM is due partly to
lower accelerating voltages in the SEM (0.5–40 kV), but
mostly to scattering effects of various types of radiation
occurring below the surface of the sample with electrons
and X-rays emerging from an area larger in diameter than
that of the primary beam. Varying the accelerating volt-
age also changes the depth to which the primary beam
penetrates the sample, usually from 1 to 15µm.
The SEM can reveal much chemical information about a
sample; the three-dimensional images can aid in the study
of crystalline material and sample morphology and BSE
images can often be used to determine areas of varying
composition. The signals of most import to the chemical
microscopist, however, are X-rays since their wavelengths
or energies can be measured, thus identifying the individ-
ual chemical elements in the sample.