Electron interaction with the surface
Suppose
emitted electrons beam, from the electron source, as shown in Figure 1,
collide to the sample surface. In this collision, according to the
energy of the beam and type of collided particle, it is possible that
observe various particles or beam, which each one is characteristic of
the different surface properties, and as a result, with use of different
detectors can be used each of them to interpret the different
characteristics of surface atoms.
According to figure 1, the
secondary backscattering, Auger electron, and X-Ray, constitute the most
common particles or radiation emission resulting from the electron beam
collision to the sample surface, which we will review them in
following.
Figure 1, the interaction between high-energy electrons with solid sample.
1.
The electron beam. 2. The scape range [of Auger electrons 3. The escape
range of secondary electrons 4. X-ray range. 5. The exit range of
backscattering electrons
The secondary electrons:
These
electrons are obtained based on the inelastic collision between initial
incident electrons with conductive band electrons, and sometimes, with
the electrons in the valence band. The emitted electrons, which are
called secondary electrons, have the energy between 5 and 10 eV.
Considering to their low energy, these electrons are collected easily
with a positive voltage 300-100 V, at the front of the detector of the
type PMT. Because it usually is collected more than 50 percent of these
electrons, thus is created a three-dimensional image of the sample with
high the field depth.
Backscattered Electrons
Backscattered Electrons
These
electrons are emitted based on elastic collision of incident electrons
with the atoms nucleus of the sample surface and any material that its
atoms have more protons, causes to rise these backing electrons.
Because,
the collisions are elastic, there will be no change in the energy of
the scattered electrons. The scattering angle also can be from zero to
180 degrees. Because these electrons have high energy, the secondary
electrons are not easily collectable.
The most common detector, for
this purpose is the Surface barrier detector, which is placed on the
sample and below objective lens, and electrons colliding with it are
identified. these detectors are made of semiconductor materials. A
semiconductor material has a filled valence band and an empty conductive
band, when backscattering electrons collide to this detector, electrons
move from the valence band to the conductive band, and electrons in
this band, travel freely or return to the the valence band.
X-Ray
We
know that, when high-energy electrons collide to a metal sample, cause
the production of x-ray radiation. The energy of X-Ray is associated
with difference electron energy levels of the collided atoms and since
these levels and their variations are known for each substance with
detecting output beam can be identified some features of atoms on the
surface. Among these characteristics, it can be noted to determination
of the elements amount, in the sample or doing quantitative analysis.
In
electron microscopes, a technique that has been formed based on the use
of output beams, like X-ray, in order to identify substances called
Energy dispersive spectrometry (EDS) technique, and is placed usually on
the scanning electron microscope as one of the lateral equipment.
Auger electrons:
When
the electrons collide to atoms sample, it is possible that an electron
be removed, from one of the atomic level, and _ leave a hole. In this
case, for which atom returns _ to stable state. According to figure 2,
an electron moves from a higher level, _ toward the hole, and therefore,
is released, a lot of energy, which may occur, three modes, depending
on the difference in energy levels:
Figure 2: the production of Auger electron
If the emitted electron be from the external circuit, the emitted
energy is low, and is emitted as a photon. If the level holes be
insider, the released energy become greater, and in this case X-Ray
emit, which is discussed above. Sometimes, additional energy is
transferred to one of the inner electrons, and makes it emits.
Said
to these emitted electrons Auger electrons. Their detecting is called
the detector AES, and offers important information of material including
its type and concentration. Analysis of Auger electrons is applicable
according to their very low energy only in certain circumstances, such
as high vacuum.
For better understanding, we consider the energy of
two state, X-Ray emitting and Auger electrons for titanium. The energy
of emitted x-ray due to electrons transition from the level 4M two level
2 L is equal to:
EX=EL2-EM4=457.8 ev
Its corresponding for the Auger electrons, their energy is equal to:
EAuger=EL2-EM4=423 ev
In Figure 3, the X-ray production process and Auger electrons, is shown, as comparative.
Auger electron production process X-Ray production process
Figure 3: X-ray and electron Auger formation process
Sometimes, as shown in Figure 4, the electrons energy and the type
of sample materials as well as layer thickness is in such a way that the
electron beam passes through the thin sample and in this case more
particles being emitted, such as elastic passing of scattering electron
and inelastic passing of scattering electron.
Figure
4: The interaction between high-energy electrons, with solid sample, in
analysis of STEM or TEM, 1- incident electrons. 2. The secondary
electrons. 3. Backscattering electrons. 4. Heat. 5. The elastic
scattering electrons. 6. Passing electrons. 7. Inelastic scattering
electrons. 8. The thin sample. 9.. 10 X-Ray.