The scanning electron microscope (SEM) is a type of electron microscope that
images the sample surface by scanning it with a high-energy beam of electrons in a
raster scan pattern.1-2 With irradiating the sample with electron beam in a vaccum
chamber, secondary electrons (SE), backscattered electrons (BSE), characteristic x-rays
and other signals are generated as indicated in Fig 3.1. 3 The SEM mainly utilizes SE or
BSE signals to form an image. SE are produced near the sample surface, and SE image
obtained upon detecting these electrons reflects the fine topographical structure of the
sample. BSE are beam electrons that are reflected from the sample by elastic scattering.
BSE are often used in analytical SEM along with the spectra made from the
characteristic X-rays, and BSM image therefore reflects the compositional distribution
on the sample surface. Because the intensity of the BSE signal is strongly related to the
atomic number (Z) of the specimen, BSE images can provide information about the
distribution of different elements in the sample. Moreover, x-ray detector can be
equipped to the SEM, so the SEM is also applicable as an x-ray analyzer for
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determining what elements are included in the sample.
Fig. 3.1 Signals that result from electron beam-specimen interaction.
The typical SEM instrument is made up of electron column, scanning system,
detector(s), display, vacuum system and electronics controls as shown in Fig. 3.2. 4 The
electron column of the SEM consists of an electron gun and two or more
electromagnetic lenses operating in vacuum. An electron beam is thermionically emitted
from an electron gun fitted with a tungsten filament cathode. Tungsten is normally used
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in thermionic electron guns because it has the highest melting point and lowest vapour
pressure of all metals, thereby allowing it to be heated for electron emission, and
because of its low cost. The electron gun generates free electrons and accelerates these
electrons to energiesin the range 1-40 keV in the SEM. With the the electromagnetic
lenses (condenser and objective lenses), the electron beam generated by the electron gun
is coveraged into a fine beam in a high-vacuum column. Typically the electron beam is
defined by probe diameter, probe current and probe convergence. And by applying a
scan signal to the deflection coils, the electron beam is scanned along the sample
surface in X and Y direction. Objective lens is used to converge the electron beam into a
fine beam and focus it onto the sample surface. When the primary electron beam
interacts with the sample, the electrons lose energy by repeated random scattering and
absorption within a teardrop-shaped volume of the specimen known as the interaction
volume, which extends from less than 100 nm to around 5 μm into the surface. The
energy exchange between the electron beam and the sample results in the reflection of
high-energy electrons by elastic scattering, emission of secondary electrons by inelastic
scattering and the emission of electromagnetic radiation, each of which can be detected
by specialized detectors. A secondary electron detector for detecting signals produced
from the sample could converts signal to electric one. Finally the photomultiplier are
used to amplify the signals, the amplified electrical signal output is displayed as a
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two-dimensional intensity distribution that can be viewed and photographed on a video
display, which are displayed as variations in brightness.
Fig. 3.2 The structural scheme of a typical scanning electron microscope.
When imaging in the SEM, samples must be electrically conductive at the surface,
and electrically grounded to prevent the accumulation of electrostatic charge at the
surface. Metal objects require cleaning and mounting on a specimen stub for SEM. It
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tend to charge when scanned by the electron beam for nonconductive specimens, and
this causes scanning faults and other image artifacts especially in secondary electron
imaging mode. Therefore, they are usually coated with an ultrathin coating of
electrically conducting material by sputter coating or evaporation. Additionally, coating
could increase signal/noise ratio for samples which is composed of low atomic number
(Z) atoms. This improvement arises because secondary electron emission for high-Z
materials is enhanced. Figure 3.3 show a photo of the SEM (JEOL, JSM-6500F) image
used in this work.
Fig. 3.3 Photo of scanning electron microscopy (JSM-6500F, JEOL)
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The image details and resolution in the SEM are determined not by the size of the
electron probe by itself but rather by the size and characteristics of the interaction
volume, which is the area of the sample excited by the electron beam to produce a
signal. When the accelerated beam electrons strike a specimen they penetrate inside it to
depths of about 1 μm and interact both elastically and inelastically with the solid,
forming a limiting interaction volume from which various types of radiation emerge,
including BSE, SE, characteristic and brehmsstrahlung x-rays, and
cathodoluminescence in some materials. The combined effect of elastic and inelastic
scattering controls the penetration of the electron beam into the solid. The resulting
region over which the incident electrons interact with the sample is known as interaction
volume. The interaction volume has several important characteristics, which determine
the nature of imaging in the SEM. The energy deposition rate varies rapidly throughout
the interaction volume, being greatest near the beam impact point. The interaction
volume has a distinct shape as shown in Fig 3.4. For low-Z target it has distinct pear
shape. For intermediate and high-Z number materials the shape is in the form of
hemi-sphere. The interaction volume increases with increasing incident beam energy
and decreases with increasing average atomic number of the specimen. For secondary
electrons the sampling depth is from 5 to 50 nm and diameter equals the diameter of the
area emitting backscattered electrons. BSE are emitted from much larger depths
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compared to SE. Finally the resolution in the SEM is controlled by the size of the
interaction volume.
The effective interaction volume can be calculated using the electron range (R): 5
ൌͲǤͲʹܣܧଵǤ
ߩܼǤ଼ଽ ሺߤ݉ሻ
Where A is the atomic weight (in g/mole), Z is the atomic number,Ȱis the density
(in g/cm3), and E0 is the energy of electron beam (in KeV)
Fig. 3.4 The range and spatial resolution of backscattered electrons, secondary electrons, X-rays, and Auger electrons for electrons incident on a solid.
24 3.2 Cathodoluminescence (CL)
When an electron is promoted into the conduction band, the electron and hole
become free and they can move independently in corresponding bands.6 The major
electron-hole recombination pathway between the conduction and valence bands
involve donor and/or acceptor levels, recombination via deep level traps, and
recombination at the surface. Electron beam excitation in general leads to emission by
all the luminescence mechanisms present in the material. Photoluminescence emission
may strongly depend on the excitation hv, which can be used for selective excitation of
particular emission processes. Cathodoluminescence analysis of materials, on the other
hand, can provide depth-resolved information by varying the electron beam energy as
shown in Fig 3.4. In general, electron beam energy of upon to 30 keV can be used.
Cathodolumiscence analysis enables one to assess various properties of the material
with a spatial resolution down to 1 ƶm or less. Spectroscopic CL and monochromatic
imaging can be used in identification and measurement of luminescence center
concentration and distributions, as well as in the determination of the composition of
compound materials.
Here, the CL spectrometer (Gatan, MonoCL3) as shown in Figs. 3.5 is adapted to
the SEM with the energy of electron beam from 1kVto 30 kV. PMT and detectors are
used to gathered the visible and IR emission spectra.
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Fig. 3.5 Photo of the CL spectrometer.