Hello everyone.
Welcome to this material characterization
course.
In the last two classes we reviewed about
the electromagnetic lenses and its function,
fabrication and some of the parameters which
controls the electromagnetic lenses, how it
is being used in the electron microscope.
A kind of introduction with little more details
we have gone through.
Now from this class onwards, we will just
start the scanning electron microscopy where
all these electromagnetic lenses we have seen
will be used.
So, before I just start these lectures on
fundamentals of scanning electron microscopy.
I would like you to carefully go through what
is shown on this slide.
So, before we get into any of this electro
electron optics or electron optics-based instrumentation
which issued for imaging of materials to reveal
the microstructure details, first we should
know about the interaction of an electron
beam with a solid.
it is a very general information which one
should remember.
I will tell you the importance of this the
moment I finish this discussion here.
So, look at this schematic.
What is shown in this slide is, you have a
specimen and then this is an incident high-energy
electron beam which is falling on this sample
and then you get to see quite a bit of signals
are which is coming out of this sample in
all the directions.
So, I would like, like you to carefully look
at each one of them so what we are seeing
is within this volume of the sample what we
are is an absorbed electron.
Some that means some electrons are being absorbed
by the specimen and some of them actually
you get electron hole pairs generation and
then you see a secondary electrons, characteristic
x-rays, visible light and then you have backscattered
electrons and then you have elastically scattered
electrons and then you have a direct beam
and you have inelastically scattered electron
and then you have bremsstrahlung x-rays.
So, by looking at this you just see that when
a high-energy electron beam interacts for
the specimen, it is always true that all these
signals are generated.
It is this, the detecting system which you
employ to collect them and use them for imaging
or analysis that characterizes the particular
characterization equipment.
For example, you just see that the visible
light we used so far an optical microscope,
a characteristic x-rays can be used for n
number of spectroscopic techniques to analyze
the chemical details or chemistry of the specimen
in a very, very high resolution of in the
materials which are I mean which may contain
very minute or trace elements.
So, in order to characterize them we may use
this characteristic X-rays.
We will look at that all the spectroscopic
technique in a different lecture series but
you just see here this is also one of the
important signals which you get out of the
electron beam specimen interaction.
And then this backscattered electrons and
is secondary electrons are being used in SEM
and then you see that auger electrons are
used in auger electron spectroscopy and then
a direct beam which comes from the specimen
is used in transmission electron microscopy
and you have all this other signal also be
used in the transmitted electron microscopy
for different applications.
We will look at that in an appropriate time
So, I just want you to look at all these signals
which is coming out of the specimen.
These can be broadly categorized into two
segments.
One is a forward scattering signals.
All these are just direct being inelastically
scattered electron, elastically scattered
electrons.
These are all forward scattering signals and
then you have a backward scattering signals.
So, out of these two categories the, the scanning
electron microscopy uses only the, the backward
scattering signals.
So, this is primary important information
one should have before we get into the details.
So, all the other signals are not used in
the scanning electron microscopy.
We will see the details one by one but as
an introduction you should know, in general
when an electro I mean high, high energy electron
beam interacts with the specimen all these
signals are coming out and then the kind of
detecting system which we use actually defines
the characterization tool, whether it is a
scanning electron microscopy or a transmission
electron microscopy or any spectroscopy specific
spectroscopy which where we look at the chemistry
of the specimen.
So, this is primary important concept you
have to understand before we get into the
specific characterization tool.
So, with this introduction I would like to
start the scanning electron microscopy and
let me just go to the blackboard and then
right few things and an introductory remark.
So, in an introduction to the scanning electron
microscopy, we should know what are its unique
capabilities.
Why do we opt for in a scanning electron microscopy
investigation in comparison to a light optical
microscopy.
The primary objective is to obtain the magnification
with high resolution.
So, in a very simple terms, you can you can
obtain microscopic details 3D like images.
We will see how this effect comes and what
are the parameters which contribute to this
phenomenon or any effect I would say.
Magnification range of 10x to 10,000 x and
more in fact it could be more also.
Typically the signals are obtained from the
specific emission volumes within the samples
and can be used to examine the sample in terms
of surface topography, crystallography and
composition, etc.
So, these unique characteristics we could
not do with the light optical microscope and
what is surface topography, the surface unevenness.
So, you can just imagine what we have seen
in a light optical system.
If you recall we just polished the metallic
specimen with the different kinds of emery
sheets.
Right !
So, the final Emery sheets which had very
fine ceramic particles embedded in that sheet
and then we just rub the sample against them
and then that sample appeared almost like
shiny and so on.
With our naked eye the sample look very polished
and so on.
Then we what did it we also put that sample
under the optical microscope, then we could
observe the very, very closely spaced scratches.
I would say this closely spaced and impression
which was observed, like if you put the same
sample under the SEM, you will see that there
are hills and valleys because we are looking
at at the very high magnification and rather
a high resolution we are looking at it we
are able to observe the small hills and valleys
that is surface topology.
So, this is one classical example you can
just go and look back.
This can be so, any surface unevenness to
the very micron to nanometer scale can be
analyzed.
And also the crystallography of the specimen
and that under its chemistry can be analyzed
with the scanning electron microscope primarily.
So, what what, what are the parameters which
enables this microscopy to do that.
We will see that.
So, what are the typical signals we are going
to get from the SEM, secondary electrons,
backscattered electrons, x-rays and other
photons of varying energies.
Just you look at that slide again what I have
just shown here.
This is second electrons, backscattered electrons,
characteristic X-rays and other photons of
varying energies.
See, each radiation will have very specifi
energies which we will talk about.
So, the primary signals which is coming out
of this SEM as I said it is a backscattered
signal or back scattering signal sorry.
I would say back towards scattering signals.
It will be very clear because there is another
particular signal is named as backscattered
electrons.
So, you should not confuse with this because
this is only coming the backward you know
scattering.
This is what I meant all these signals are
backward scattering signals which are being
primarily used in may SEM.
So, only these three signals are primarily
used in an SEM of course they, they are characterized
based on their energy that we will see in
an appropriate time.
And out of all this backward scattering signals,
only we talk about second electrons and backscattered
electrons, why ? Why are we talking only about
this, because they vary primarily as the result
of difference in the surface topography.
The amount of secondary and backscattered
electrons which is coming out of the specimen
surface is primarily depending on the surface
topology.
This is the, the core idea behind using this
tool and what are the other important things
? So, the other important information you
get from the a SEM is, the x-rays that is
characteristic x-rays come from the sample
can yield both qualitative and quantitative
information from the region of 1 micrometer
diameter and 1 micrometer depth.
This is a rough indication.
You get what is the region size from which
you get this information which is of the order
of 1 micrometer diameter and 1 micrometer
depth from the surface.
So, these are the information you get from
this in general from SEM and we look at the
what are the imaging capabilities.
So, if you look at the imaging capabilities
of this microscope, the major reason for the
SEM usefulness is the high resolution in the
order of 1to 5 nanometers and another important
feature of SEM is the large depth of field.
We have already discussed in the fundamentals
of the optics.
We have seen what is depth of field, how it
is being exploited in electron microscope.
In fact, the what we have just stated in the
beginning 3D like images it is partly because
of this effect.
You have high or very large depth of field
in an SEM.
We will also see it using a ray diagram how
it enables this effect when we discuss the
other functions of SEMs in the coming classes
and the SEMs are becoming very popular because
of the advances in the signal processing and
amplification like the kind of signals you
receive secondary electrons, backscattered
electrons or x-rays and then you have advanced
processing signal processing and amplification
detectors and then gun design etc.
So, without all this advances this SEMs and
also do imaging something like using electron
channeling contrast by varying the crystal
orientation and also magnetic contrast from
the magnetic domains in the uniaxial and the
cubic materials.
So, these are all some of the, the highlights
of the imaging capabilities of the SEM.
You will see that next is structural analysis.
Okay.
If you look at the what are the structural
analysis one can do with this SEM, it has
got a capability to determine the crystal
structure and grain orientation of the crystals
on the surface.
Please understand you have to remember that
it is all whatever the information you obtain
is only from the surface with very limited
volume you will just understand that in much
more detail as we go into this lectures and
then the diffraction of the backscattered
electrons emerging from the sample surface.
Electron backscatter diffraction (EBSD) with
the low intensity also enables this capability
and since it is a low intensity we have very
high sensitive CCD camera recording.
This is charge coupled device camera records
the backscattered a so called Kikuchi pattern
which is nothing but this signal and it is
analysed with the computer-based indexing
method.
And then you have today SEMs with advanced
indexing and computer-assisted crystal lattice
orientation mapping, that is called EBSD maps,
which allow this technique to identify the
face and the Miss orientation across the boundaries.
So, this is also very powerful technique today
and, and it has been applied everywhere.
This itself separate is a research domain.
People can extensively use this and very powerful
technique as far as SEM structural analysis
concern.
And what else we can do with the SEM so, so
far we have seen imaging capabilities, structural
analysis and finally 
elemental analysis.
So, if you look at the elemental analysis
capability of an SEM, you can get the complete
compositional information using characteristic
x-rays.
The tool generally referred as Electron Probe
Micro Analysis (EPMA) which can get the chemical
composition from the very localized region
and then provide a complete chemical analysis
and then you have this EPMA is specially outfitted
SEM with light optics and one or more or WDS
units (Wavelength Dispersive Spectrometer).
We will see all this variance of the spectrometers
as I mentioned which uses the characteristic
x-rays which comes out of the sample and then
do the chemical analysis.
We will look at them in a separate lecture
series but then these are all the attachment
one of the primary attachment to the SEM.
This one, the another one is energy dispersive
spectrometer, which can detect the elements
greater than four atomic number, collect the
characteristic x-rays from the major elements
approximately you should have about 10 weight
percent whereas the WDS measures x-rays from
the minor or even a trace elements of point
one eight percent.
So, WDS is much more powerful compared to
EDS.
We will see why and so all these details say
later.
But these are all the basic details one should
have about the when you look at the capabilities
of a scanning electron microscope.
I have few more points to add in this segment.
So, the, the final point to the elemental
analysis with a modern EPMA, you can get a
quantitative information from your specimen
within a spatial resolution of the order of
1 micrometer with the accuracy of the order
of 1-2% the amount present and also this EPMA
has a capability of analyzing the very low
atomic number elements like boron, carbon,
oxygen.
Because the WDS spectrometer uses a large
interplanar spacing diffractors typically
organic crystals which has got large interplanar
spacing, which enables long wave length x-rays
from the low atomic number elements.
Since this low atomic number elements as the
characteristic x-rays of large wavelength
or a long wave length.
So, these crystals enable the diffraction
possible and then and they can be measured
with the WDS.
So, these are all the, the basic capabilities
of a scanning electron microscope.
So, I have just put them into three categories,
one is imaging capabilities and structural
analysis and then composition analysis or
elemental analysis and SEMs are primarily
used for only this purpose.
Now we will look at the some of the other
introductory remarks.
As the sophistication of the investigations
increased the optical microscope often has
been depressed by instrumentation having superior
spatial resolution or depth of focus so the
resolution of SEM can approach a few nanometer
as I mentioned and it can operate at magnification
that are easily adjust from about 10x to 300,000
X of course this can be a subject of instrumentation.
We will talk about it in an appropriate time.
The depth from which all this information
comes varies from nanometers to micrometers
is also a subject to specific instrumentation
details we will look at them in appropriate
time.
Like wise the lateral resolution in these
analytical modes also varies and is always
poorer than the topological contrast mode.
So, the principle image is produced in the
SEM are of three types, namely secondary electron
images, backscattered electron images and
then you also have elemental x-ray maps, and
these are the three primary images one can
obtain in a normal SEM secondary and backscattered
electron are conventionally separated according
to their energies.
So, we will now see how this will have some
schematics to show how this SEM works.
So, you have the two separate entities one
is a microscope column, from the top to bottom,
the other is a control console.
So, you have the electron gun which comes
from the top of the equipment column, microscopic
column and then you have further all the electron
lenses and as can coil and the beam reaches
all the way up to this chamber specimen chamber
which is maintained at the with the vacuum
of 10 to the power minus 4 pascal, which is
in the order of1 billionth of the atmospheric
pressure for the for your reference.
And you have on the right hand side you have
the CRT screens screens and then camera where
all this scanned images are being used.
So, this is a primary classification of this
equipment microscopic column and a control
console and then you look at this next schematic
and you have this a complete schematic of
the cross section of the scanning electron
microscope and what you see is an electron
gun which generates the electrons and accelerates
to typically from point 1 to 30 kilo electron
volts and then this being passed through electron
lenses also and also a scan coils.
So, these electron lenses what they do is
the, the probe diameter which is being produced
by this the tungsten hairpin typically, it
is not sharp enough to resolve the structures
and this electron lenses dig magnified two
very sharp spots or a sharp probe and then
they are being rested on to this coil to the
this specimen.
And then you have this detector the signals
which comes out of this specimen which is
kept under the vacuum and then it amplifies
and it goes to the display.
We will continue the discussion of this general
function of this scanning electron microscopy
in next class as well.
Thank you.
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