Electron-Specimen Interactions

1
Electron-Specimen Interactions
2
Primary Signals Secondary Electrons Backscatter
ed Electrons X-rays
3
The size and shape of the region of primary
excitation can be estimated by carrying out
simulations that use Monte Carlo calculations and
take into account the composition of the specimen
4
An interaction volume can also be used to predict
the types of signals that will be produced and
the depth from which they escape

• Monte Carlo simulations of electron trajectories
  are based on
• the energy of the primary beam electron,
• the likelihood of an interaction
• the change in direction and energy of the
  electron,
• the mean free path of the electron and
• a random factor for any given interaction.

5
Actual image of beam penetration into PMMA
showing size and dimension of region of primary
excitation
6
The angle at which the beam strikes the specimen
and the distance from the surface are important
factors in how much of signal escapes from the
specimen.
7
Sometimes one can take advantage of the this
effect and increase useable signal by tilting the
specimen towards the detector and at an angle
relative to the primary beam
8
Different electron signals depend on the
interaction of the source electrons with the
atoms within the speciman. Backscatter,
Secondary, x-ray generation and auger
9
The probability of an elastic vs. an inelastic
collision is based primarily on the atomic weight
of the specimen.
10

• Secondary electron
• lt 50 eV
• Backscatter electron
• gt80 of primary electron energy
• X-ray 0.5 20 KeV

11
Secondary electrons are usually the result of an
inelastic collision in which the transferred
energy of the primary beam is transferred to an
electron that is then emitted from the atom.
Secondary electrons typically have and energy of
50 eV or less
12
Although secondary electrons are produced
throughout the interaction region they can only
escape from the uppermost portion due their low
energy
13
Backscattered electrons are the result of elastic
collisions with atoms of the specimen. They
result in emitted electrons that have an energy
of 80 or more of the original energy of the
primary beam electron
14
Backscattered electrons are also produced
throughout the interaction region but because of
their greater energy can escape from deeper in
the specimen.
15
X-rays are indirectly produced when an electron
is displaced through a collision with a primary
beam electron and is replaced by another
electron. The resultant loss of energy is given
off in the form of an X-ray. The energy will
always be less than the energy of the primary
beam electron.
16
Because of their high energy X-rays can escape
from very deep in the specimen.
17
Resolution in an SEM is ultimately determined by
the size of the region from which signal is
produced. Thus for the same region of excitation
the resolution from the three signals differs and
decreases from secondary to backscatter to
X-rays.
18
Factors affecting size of the interaction
region Diameter of the primary beam Energy of
the primary beam Atomic weight of the
specimen Coating of specimen
19
Final primary beam probe size from a field
emitter is 10-100X smaller than that of a
conventional tungsten filament or LaB6 emitter.
This is one reason why FESEMs have the best image
resolution.
20
FESEMs also tend to remain stable at very low
accelerating voltages (0.5 5 KeV) resulting in
shallow regions of excitation and thus higher
image resolution.
21
Effects of Accelerating Voltage
Z Atomic Weight E Energy of primary beam
22
3.0 KeV 20.0 KeVEffects of
Accelerating Voltage
More signal (brighter)
23
3.0 KeV 20.0 KeV
But reduced resolution
24
Effects of Coating

• Sputtered Gold Chromium
• Mycoplasma pneumonia

25
The relationship of accelerating voltage (E0) to
atomic weight (Z) of the specimen and its affect
on the depth of penetration can be summarized as
above.
26
If the region of excitation remains small then
signal will be produced from a small region and
there will be no overlapping from adjacent
regions. In this case each individual spot is
resolved from its neighbors.
27
If the beam is scanned in exactly the same
positions but the region of excitation is larger
then the regions of signal production will also
be larger and overlap with adjacent ones. Such
an image would therefore not be resolved.
28
Even a slight increase in size of the region of
signal production can result in decreased
resolution.
29
Overlapping of signal production is also the
primary reason why it is so critical to have the
beam of an SEM properly stigmated. Even if the
size of the region is kept small, it is only
those regions which are perfectly circular that
will produce the best resolution
30
Astigmatic regions may not reduce image
resolution in one dimension.
31
But can still reduce resolution by overlapping
with adjacent regions.
32
The position of the secondary electron detector
also affects signal collection and shadow. An
in-lens detector within the column is more
efficient at collecting secondary electrons that
are generated close to the final lens (i.e. short
working distance).
33
Secondary Electron Detector
Side Mounted In-Lens
34
A conventional secondary electron detector is
positioned off to the side of the specimen. A
faraday cage (kept at a positive bias) draws in
the low energy secondary electrons. The electrons
are then accelerated towards a scintillator which
is kept at a very high bias in order to
accelerate them into the phosphor.
35
The Everhart-Thornley detector has an aluminum
coating (10-12 KeV) that also serves to reflect
the photons back down the light pipe.
36
The scintillator is a phosphor crystal that
absorbs an electron and generates a photon
37
The photons produced in the scintillator are
carried down a fiber optic light pipe out of the
microscope.
38
Most of the secondary electron detector lies
outside of the SEM chamber and is based on a
photomultiplier tube (PMT)
39
A PMT works by converting the incoming photons
into electrons which are then drawn to dynodes
kept at a positive bias. The dynodes are made of
material with a low work function and thus give
up excess electrons for every electron that
strikes them. The result multiplies the signal
contained in each photon produced by the
scintillator.
40
The electronic signal from the PMT is further
increased by a signal amplifier. Thus an
increase in gain is accomplished by voltage
applied to the dynodes of the PMT and
alters the contrast of the image. An increase in
the black level is made by increasing the
current in the amplifier and alters the
brightness of the image. Signal is thus
increased at the scintillator, PMT, and amplifier.
41
An in-lens detector does not use a faraday
collector as this would affect the primary beam
electrons but instead depends on the natural
trajectory of the secondary electrons to strike
it. It takes advantage of the focusing action of

the lens to bring these electrons to cross over
and then spread out to strike the annular
detector.
42
Backscatter electrons have a greater energy and
can escape from deeper within the specimen than
can secondary electrons but because they are more
readily produced by high atomic weight elements
they can be used to visualize differences in
elemental composition
43
2o
BS
Blood cells with nuclei stained with a silver
compound are visible in backscatter mode even
though they are beneath the surface of the cell
membrane
44
Since backscattered electrons have a high energy
they cannot be collected by way of a faraday cage
or other device
45
The most common design is a four quadrant solid
state detector that is positioned directly above
the specimen
46
Gold particles on E. coli appear as bright white
dots due to the higher percentage of
backscattered electrons compared to the low
atomic weight elements in the specimen
47
Backscatter image of Nickel in a leaf
48
The topography of the specimen will also affect
the amount of backscatter signal and so
backscatter imaging is often carried out on flat
polished samples
49
Backscatter image of a composite (polished cement
fragment) in which low atomic weight particles
appear dark and high atomic weight particles are
white.