Hello everyone, welcome to this material characterization
course.
In the last two classes, we reviewed about
the electromagnetic lenses and it is 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 this electromagnetic lenses we have seen
will be used.
Before I just start this lecture on Fundamentals
of Scanning Electron Microscopy, I would like
you to carefully go through what is shown
on the slide.
So, before we get into any of this electron
optics or electron optics based instrumentation
which is used for imaging of materials to
reveal the micro structure details first we
should know about the interaction of electron
beam with the solid.
It is a very gentle information which one
should remember, I will tell you the importance
of this, the movement I finish this discussion
here.
Look at this schematic, what is shown in the
slide is you have a specimen and then this
is an incident high energy electron beam which
is falling on this sample.
Then you get to see quite a bit of signals
or which is coming out of this sample in all
the directions.
I would like you to carefully look at each
one of them.
So, what we are seeing is within this volume
of the sample what we have written is an absorbed
electrons, 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, then you have backscattered
electrons, then you have elastically scattered
electrons, then you have direct beam, and
you have inelastically scattered electron,
and then you have Bremsstrahlung x-rays.
By looking at this you just see that when
high energy electron beam interacts with the
specimen, it is always true that all this
signals are generated.
It is this the detecting system which you
employee 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 in optical microscope,
a characteristic x-rays can be used for n
number of spectroscopic techniques to analyze
the chemical details or chemistry of this
specimen in a very, very high resolution in
the materials which may contain very minute
or trace elements.
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 this 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 the
transmission electron microscopy.
You have all this, other signal also being
used in the transmitted electron microscopy
for different applications.
We will look at that in an appropriate time.
I just want you to look at all this signals
which is coming out of the specimen.
These can be broadly categorized into two
segments; one is a forward scattering signals,
all this are just direct beam, inelastically
scattered electron, elastically scattered
electrons, these are all forward scattering
signals.
Then you have backward scattering signals.
Out of these two categories, the scanning
electron microscopy uses only the backward
scattering signals.
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 high energy electron beam interacts
with the specimen all this 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 were
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 black board and then
write few things on an introductory remark.
In an introduction to the Scanning Electron
Microscopy, we should know what are its unique
capabilities?
Why do we opt for a scanning electron microscopy
investigation in comparison to a light optical
microscopy?
The primary objective is to obtain the magnification
with the high resolution.
So, in very simple terms, 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 10000x 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.
These unique characteristics be could not
do with the light optical microscope.
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 polish the metallic
specimen with the different kinds of emery
sheets.
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
looked very polished and so on, then what
we did it we also put that sample under the
optical microscope then we could observe very
closely spaced scratches.
I would say this closely spaced an 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 had looking at the very high magnification
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.
Any surface unevenness to the very micron
to nanometer scale can be analyzed, and also
the crystallography of the specimen and its
chemistry can be analyzed with the scanning
electron microscope primarily.
What are the parameters which enables this
microscopic to do that?
We will see that.
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 secondary electrons,
backscattered electrons, characteristic x-rays
and other photons of varying energies.
See each radiation will have very specific
energies which we will talk about.
The primary signals which is coming out of
this SEM as I said it is a backscattered signal
or backscattering signal back sorry I would
say backward scattering signals it will be
very clear, because there is another particular
signal is named as backscattered electrons
so you should not con confused with this because
this is only coming the backward you know
scattering.
This is what meant, all this signals are backward
scattering signals which are being primarily
used in SEM.
So, only these three signals are primarily
used in an SEM, of course they are characters
based on their energy that we will see in
an appropriate time.
Out of all this backward scattered signals,
only we talk about is, second electrons and
backscattered electrons why, why are we talking
only about this?
Because they vary primarily as a 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 core idea behind using this two
and what are the other important things.
The other important information you get from
SEM is, the x-rays that is characteristic
x-rays come from a 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 in the order of 1 micrometer diameter
and 1 micrometer depth from the surface.
These are the information you get from this
in general from SEM
We look at what are the Imaging capabilities.
So, if you look at the imaging capabilities
of this microscopy, the major reason for the
SEM's usefulness is the high resolution in
the order of 1 to 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 been exploited in electron
microscope.
In fact, 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 SEM's in the coming classes.
The SEM's are becoming very popular because
of the advances in the signal processing and
amplification, like the kind of signals you
receive second electrons, backscattered electrons
or x-rays and when you have advanced processing
signal processing and amplification detectors
and then gun design, etc.
So with that all this advances this SEM's
can 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.
These are all some of the highlights of the
imaging capabilities of the SEM.
You will see that next is Structural analysis.
If you look at what are the structural analyses
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 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 these lectures.
Then the diffraction of the backscattered
electrons emerging from the sample surface,
Electron Backscattered Diffraction, EBSD to
the low intensity also enables this capability.
Since, it is a low intensity we have very
high sensitive CCD camera recording, this
is Charge Coupled Device camera records the
backscattered so called Kikuchi pattern, which
is nothing but this signal, and it is analyzed
with the computer based indexing method.
Then you have today, SEM's with advanced indexing
and computer assisted crystal lattice orientation
mapping is called EBSD maps, which allow this
technique to identify the phase and the mis-orientation
across the boundaries.
So, this is also very powerful technique today
and it is been applied everywhere in this
itself separate research domain people can
extensively use this and very powerful technique
as for as SEM structural analysis concerned.
What else we can do with the SEM.?
So far we have seen Imaging capabilities,
Structural analysis and finally 
Elemental analysis.
If you look at the elemental analysis capability
of an SEM, you 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 complete chemical analysis.
Then you have this EPMA is specially outfitted
SEM with light optics and one or more WDS
units, Wavelength Dispersive Spectrometer.
We will see all this variants of the spectrometers
as I mentioned which uses the characteristic
x-rays which come out of the samples and then
do the chemical analysis we will look at them
in a separate lecture serious.
But then these are all the one of the primary
attachment to the SEM's.
The another one is Energy Dispersive Spectrometer,
which can detect the elements greater than
4 atomic number, correct 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 0.1 weight percent.
So, this WDS is much more powerful compare
to EDS.
We will see why and so all this details in
later, but these are all the basic details
one should have about the when you look at
the capabilities of scanning electron microscope.
I have few more points to add in this segment.
So, final point to the elemental analysis
with a modern EPMA, you can get quantitative
information from your specimen within a spatial
resolution of the order of 1 micrometer with
the accuracy of the order of 1 to 2 percent
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 diffracts,
a typically organic crystals which has got
large interplanar spacing which enables long
wavelength x-rays from the low atomic number
elements.
Since, this low atomic number elements has
the characteristic x-rays of large wavelength
or a long wavelength, so these crystals enables
the diffraction possible and then and they
can be measured with the WDS.
These are all the basic capabilities of a
Scanning Electron Microscope.
I have just put them into three categories;
one is imaging capabilities and structural
analysis and then composition analysis or
elemental analysis and SEM's are primarily
used for only this purpose.
Now, we will look at the some of the other
introductory remarks.
As the sophistication of the investigation
increased, the optical microscope often has
been replaced by instrumentation having superior
spatial resolution or depth of focus.
The resolution of the SEM can approach a few
nanometers, as I mentioned and it can operate
at magnifications that are easily adjusts
from about 10x to 300,000x.
Of course, this can be a subject of instrumentation
we will talk about it in appropriate time.
The depth from which all this information
comes varies from nanometers to micrometers.
This also a subjected to a specific instrumentation
details we will look at them in appropriate
time.
Likewise, the lateral resolution in these
analytical modes also varies and is always
poorer than the topological contrast mode.
The principle images produced in the SEM are
of three types namely; Secondary electron
images, Backscattered electron images, and
then you also have elemental x-ray maps.
These are three primary images one can obtain
in a normal SEM.
Secondary and backscattered electrons are
conventionally separated according to their
energies.
We will now see how this will have some schematics
to show how this SEM works.
You have the two separate entities; one is
Microscope Column on the top to bottom, the
other is a Control Console.
You have the Electron Gun, which comes from
the top of the microscopic column and then
you have a further all the Electron Lenses
and a Scan Coil and the beam reaches all the
way up to this Specimen chamber, which is
maintained at the with the vacuum of a 10
to the power minus 4 pascal, which is in the
order of one billionth of the atmospheric
pressure for your reference.
On the right hand side you have the CRT screens,
viewing screens, and then camera where all
this scanned images are being used.
This is primary classification of this equipment,
microscopic column and a control console.
Then you look at this next schematic, and
you have this complete schematic of the cross
section of the Scanning Electron Microscope.
What you see is then electron gun which generates
the electrons and accelerates to; typically
from 0.1 to 30 kilo electron volts, and then
it is being passed through electron lenses
and also it can scan coils.
So, these electron lenses what they do is,
the probe diameter which is being produced
by this tungsten hairpin, typically it is
not sharp enough to resolve the structure.
These electron lenses de-magnified to very
sharp spots or a sharp probe, and then they
are being rastered on to this coil, to this
specimen.
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 this Scanning Electron Microscopy
in next class as well.
Thank you.
