Hello everyone.
Welcome to this material characterization
course.
In last class we have just discussed about
some of the special contrast mechanisms that
is operating in SEM namely the electric field
contrast, in terms of voltage contrast and
then magnetic field contrast.
And then we also had gone through couple of
examples where these contracts mechanisms
can be realized under the SEM.
And today we will discuss two more some of
the special contrast mechanisms are which
will come under the special topics of scanning
electron microscopy namely electron channeling
contrast as well as electron backscatter diffraction.
This electron backscatter diffraction itself
is a very popular technique these days for
the characterization of microstructure as
well as the quantification.
The subject is quite vast and is becoming
very specialized these days but, for the sake
of completion I will briefly discuss about
the principles behind it and also we will
show some of the lab demonstration how this
things are done in a much more brief manner
so, that you will have some kind of idea about
what is this EBSD is all about.
So first we will briefly discuss about what
is this electron channeling contrast for that
I use the some of the schematic on the blackboard
it is also called a crystallographic contrast.
So what I have drawn here is a representation
of 3D lattice in 2D.
And suppose if you have this is the electron
beam which is coming and falling on this.
Where the electron beam travels all the way
deeper inside the crystal.
And here the electron beam is being stopped
by this kind of a random randomness.
Suppose if you assume that this is a kind
of ordered alloy or ordered system like this
in relative to the amorphous, amorphous is
always kept as a reference.
Suppose if the electron beam is able to pass
through this path where you have a very least
resistance that means the density of the atomic
path is less here as compared to here.
So then the electron beam can travel all the
way inside the crystal and then the chance
of coming back, that is that BSE as, a BSE
electron after scattering is less.
Whereas if, if it is stopped in the surface
itself then the of probability of getting
the BSE that is in terms of yield is more
here.
So you have the difference in the contrast.
We will write few remarks about it then we
will move on to this explanation.
So what I have written here is along certain
directions the path of low atomic density
are found.
Something like this.
These so called channels which permit the
fraction of beam electrons to penetrate more
deeply into the crystal before beginning to
scatter.
That means after reaching this point only
the scattering even starts then the BSE signal
or SE signal will come out of this.
That kind of a signal will have very low yield.
That is the η value will be lower and on
the other hand if some other orientation where
the denser atomic packing is found and the
beam of electrons begin to scatter immediately
something like this, where you do not have
a clear channel here the electron beam start
scattering from the surface itself which promotes
the BSE yield.
So these two produces the, the difference
in the signal produces a contrast and if you
see that the modulation of η, that is a backscatter
electron yield between the maximum and minimum
is very small it is not very big number here,
it is within the contrast differences only
five percent which produces the actual the
image.
So though it is not very powerful in terms
of producing the contrast but still it is
being used sometimes and produces a electron
channeling pattern, something similar to back
scattered pattern EBSD which we are going
to discuss now.
So this is also one of the imaging contrast
so called a crystallographic contrast under
the SEM.
Now what we will do is we will, will drive
our attention to another important imaging
technique called EBSD.
So this EBSD pattern will look like this.
It is also called a Kikuchi pattern.
You will see the, the bands of bright and
dark line pairs.
And we will now see the some of the basics
about this image formation and one of the
primary use of this EBSD pattern is to analyze
the microstructure in terms of crystallography
and grain orientation and so many other parameters
are measured through this technique and it
is very powerful and is becoming popular and
popular these days and we will just go through
the basics of this technique very briefly.
So this electron backscatter diffraction EBSD
is also called Kikuchi diffraction, the in
elastically scattered electrons can subsequently
be elastically scattered that is back diffracted
by the lattice planes to produce a phenomenon
known as Kikuchi lines.
So you see all these signals whatever we get
from this SEM is because of inelastic scattering
and when the inelastic scattering electrons
subsequently subjected to elastically elastic
scattering or you say Bragg diffraction by
the lattice planes which produce the Kikuchi
lines.
And Kikuchi lines will be best seen in the
diffraction patterns from the areas of specimen
that have a low density of defects and are
about half the thickness that the beam can
penetrate or thicker.
You need a thicker sample and if the specimen
is thinner, only spots will be seen if it
is very thick only Kikuchi lines will be seen.
Of course this is with respect to some of
the transmission mode we will also discuss
this when we go to the appropriate section.
And this is how it has been interpreted how
Kikuchi lines are forming.
So this is a intensity of the inelastic scattering
as a function of scattering angle.
So what I have just shown here is two lines
this is one reference 1 and this is reference
2.
Let us consider these two rays.
So compared to 1 and 2 the, the ray 1 has
as a forward scattering in fact you can see
that the intensity of the ray 1 is much higher
compared to the intensity of the ray 2.
So you keep this in mind then we will look
at the next animation to understand this better
What you are now seeing in the schematic is
the specimen this is an electron beam which
is falling and this is the transmission axis
and I will just play this animation just closely
observe this.
It is a thick specimen and this is the screen
and inside the specimen we consider the lattice
planes and the ray which I mentioned as 1
and 2 are here and as the ray 1 is closer
to the forward direction than the ray 2, it
is more intense and an excess number of electrons
over the background will arrive in the back
focal plane at B. So this is please understand
all this diffraction takes place in the back
focal plane which you all know.
So here the ray 1 which I am talking about
is this ray so obviously compared to ray 2
this is this is more intense because you can
see that compared to this point this point
will have higher intensity.
So the excess number of electrons over the
background will arrive at the back focal plane
and B here and there will be a deficiency
of electrons at D. So you are talking about
an electron diffraction which is forming a
kind of a cone.
We will just see what this cone which I am
talking about.
And what you have to understand is one ray
with excess electron or higher intensity falls
in the back focal point B and the deficient
line will fall here.
And there is a bright line at B and the dark
line at D in the diffraction pattern.
And these are all Kikuchi lines.
So you can see that go back and look at this
pattern again, a bright and a dark line which
is coming a parallel line is because of this
diffraction effect.
We will understand this little more now.
So once the crystal is rotated little bit
then everything falls in the, the ray 2 falls
within the optic axis the ray 1 falls through
the diffraction spot.
The diffracted rays actually form cones of
semi angle (90-θ) called Kossel cones.
The cone which I am talking about this.
In 3D it will appear as a cone where I will
show you one more schematic you will appreciate
that.
What we see in a diffraction pattern is a
pair of parabolas where the cones intersect
the Ewald sphere.
The parabola appears as straight lines in
the diffraction pattern because the angles
involved are very small.
You see in an electron microscopy we just
discussed in the fundamentals that you can
with the increase in the acceleration voltage
your α can be reduced or controlled to very
small value.
And because of that you can see this and one
of the primary differences between an x-ray
diffraction and electron diffraction if you
recall, the if you probably if we go and go
back and discuss about these fundamental principles
on a Ewald sphere you will appreciate this
and if you are not able to pick up this at
this.
So the ±g pair of lines and the region between
them is known as Kikuchi band.
The angular separation of the pair of lines
is 2θ, their spatial separation in the diffraction
pattern in the back focal plane is g and the
lines are perpendicular to the g-vector.
Each reflection has an associated pair of
Kikuchi lines attached to it.
So this is a schematic you can look at it
and you can appreciate what we are now talked
about.
So you have the specimen here the incident
electron comes and interacts and they are
subjected to diffraction.
Suppose if you consider this sample is so
thin and then if you look at the three dimensionally
the electron beam which falls, it produces
a cone like this.
It is a projection here it is actually a three
dimensional cone on.
For each plane if the cone is produced on
both sides.
So one these are all called Kossel cones and
when these cones are intersects the Ewald
sphere or what actually we are looking at
is only this parabola because it is the only
the intersection of this cone on a two dimension
is appears with which appear like this.
And you can see that this is a kossel cone
intersects a Ewald sphere here.
And this side is also the other cone will
intersect.
All this pattern is appearing in the diffraction
pattern that is widely described as DP.
If you look at if you assume this and then
come back to this diagram what we have just
discussed, for the convenience we can imagine
it like this in 2D.
This is the specimen you have this HKL planes,
where the electron beam comes and then it
produces the cone here.
The one we talked about nexus line another
is deficient line intensity and the angular
measure between these two lines is 2θB, 2θB
because of the Bragg diffraction.
And then you can see that the deficient line
will appear dark and the excess line will
appear bright and again you may wonder that
since it is a, a very flat cone and the theta
is so small here for the same reason, actually
the parabola in all practical purpose it appears
a straight line in the in the electron diffraction
pattern that is EBSD pattern.
That is because of the very, very small α
which, which you experience in the electron
microscope.
So this is the typical schematic of Kikuchi
map for a diamond cubic crystal.
So we will just see that some of the applications
of this.
You can as I mentioned that you can map the
grain orientations and orientation mapping
and then you can identify the faces and you
can quantify all the micro structural parameters.
We will just show you some glimpses of all
this if not in detail.
And the Kikuchi lines and the Kikuchi maps
are one of the most important aids we have
when orienting and or determining the orientation
of the crystalline materials.
Identification of orientation of the specimen
is essential for any form of quantitative
microscopy.
So this is one major application here quantification.
And if you can summarize this, the Kikuchi
lines consist of an excess line and deficient
line in a diffraction pattern.
In the DP the excess line is further from
the direct beam than the deficient line.
The Kikuchi lines are fixed to the crystal
so we can use them to determine orientations
accurately.
The trace of the diffracting planes is midway
between the excess and the deficient lines.
So for time being you just try to understand
this with a simple diffraction phenomenon
by looking at this schematic and now we will
just go to the laboratory demonstrations,
where we will actually look at some of these
samples which is being loaded in the SEM.
So this is a sample which is loaded in the
specimen stage and then you can see that the
specimen stage is tilted to about 70°.
So then only you can produce that very flat
cone and then α can be very small and you
can see that that camera just came that EBSD
camera just came.
And this is your pole piece what you are just
seeing is a pole piece and this is the sample
which is kept at angle of 70°.
And the camera has come very close now.
And now we will see how the Kikuchi map is
generated with this sample.
What you have to do is the one of the primary
requirement of producing EBSD sample is the
very fine polish, which is very difficult
which is done by this electrolytic, electrolytic
polishing and you first generate a secondary
electron image of the sample.
So now the second electron image is getting
focused.
So you can see that some of the features start
appearing.
This sample is being investigated by one of
our scholar for his PhD thesis Mr. Devendar.
And now we will demonstrate that 
EBSD pattern which is obtained from this sample.
Normally what happens is once you obtain in
a secondary electron you just grab it on the
another screen where the orientation microscopy
software called TSL, which handles this EBSD
analysis.
So there now what happens is the, the beam
is connected to directly connected to I mean
synchronized with your mouse.
So wherever you put the cursor on the sample
and then click then the corresponding Kikuchi
lines are generated here at each point.
And this information is coming from the sample
about 20 nanometer thicknesses.
So you have to be very careful about this
aspect when you talk about representation
of the bulk texture or bulk orientation and
so on.
And normally what happens is I will just briefly
tell you how this is the analysis is done
by this software.
So the electron beam just goes and then you
can just click the mouse and then it produces
the Kikuchi light and if you know the crystal
system of the specimen in this case it is
nickel so a database belongs to this nickel
is selected.
And then the software will generate a orientation
which similar to what, what is being generated
in your sample and these two patterns are
overlapped because this is a for example this
is the orientation, now the software will
superimpose this pattern which is very close
to this because this is already a well-known
pattern, which is already index so this will
get superimposed and then your actual specimen
EBSD data also will be indexed.
So like that the each yeah now you can see
that it is a superimposed with the specimen
data.
So now you can identify some of this zone
axis like this.
And on each point your probe will generate
an EBSD pattern like this and it will record
the orientation data and then you can you
have to select the area under which you want
to do this mapping.
So the area is being selected and also the
spot I mean the step size, there is something
called a step size.
That means under what are the minimum distance
an electron beam has to travel after it scans
one spot or one location.
That is just step size here it is one micron
is selected.
That means the electron beam will move one
micrometer after it collects one signal that
is one data crystallographic data to another
region.
That means you have to be very careful about
this step size.
If the step size is on comparison with your
grain size, then you will not be able to get
the meaningful a crystallographic data because
at least you are supposed to scan a grain
within, within the grain 2, 3, 4 orientation
information should be attained in order to
get a meaningful data.
So your step size is very crucial here.
So in this particular example this region
is being selected.
And now the, the beam will scan this sample
like this line by line and as I said it will
index automatically and then record it and
then you go back again it will record.
So a typical scan of this range in a normal
EBSD a conventional camera takes about six
to seven hours.
So it is a very time-consuming process but
today you have a modern recording media where
very high speed camera is employed if you
have that kind of facility you can reduce
this time by one third.
So this is how the, the indexing is done.
What now you are looking at is this is the
beam scanning and it is getting automatically
indexed and finally it will get recorded.
So what, what, what I will do now is since
it is going to take long time I will go to
the final result for example typically you
get the this is a inverse poll figure map.
So you see a very nice color colorful picture
like this.
So you have to be very careful in understanding
this.
Each color indicates it is a orientation mapping.
So be very careful about it this is not a
micro structure this is a orientation map.
What is the orientation map?
You look at this key here so this particular
color blue color belongs to (111) orientation,
this green color belong to (101) orientation
and red color belong to (001) orientation.
So the each color indicates the whole grain
orientation belong to this particular number
that is what it means.
And another important thing we can do is see
what this color is trying to do is to look
at the miss orientation between these two
grains.
See he has what he has done is, is just taken
the cursor and then drawn this line here between
these two lines these two boundaries.
You can see that the miss orientation angle
between these two is about 60.
So that he confirmed this as a twin so you
can really readily understand the misorientation
between the two boundaries.
So these boundaries are characterized as twin
boundaries on the other hand if you do a scan
here and then this is only about 30°.
So definitely it is not a twin boundary.
So these are the very, very powerful tool
to determine the, the grain orientation instantaneously.
And you can do a lot more calculations like
you have the orientation spread and then you
have the misorientation distribution you can
be show your distribution also we can see
from this sample.
So like that you have all this very useful
quantitative information can be obtained from
this technique and another very important
aspect is like you can also look at the surface
texture information.
Yeah so this is a poll figure which also shows
the texture within this top and 20 nanometer
layer of the sample.
And it shows kind of a random texture here
it is not showing any particular texture and
we will show you some of the sample where
it exclusively shows a very nice texture.
And you can also look at the quality maps
like this and some of this IQ maps that is
quality maps also widely used in some of the
re-crystallized grains and deformed grains
and so on.
I am just giving you every glimpses of it
I am not getting into the details.
So just basically I am just highlighting the,
the usefulness of this EBSD technique.
And finally I would like to show some of the
a sample which is exhibiting a very strong
texture, which I want to give you an example.
So let me go back and take one more shot okay.
So look at this map where it shows mostly
(001) orientation that means most of the grains
are oriented towards (001) orientation.
So if you take a poll figure then it will
clearly show you the cube texture (001) texture.
This is one classical example you can see
how the cube texture is shown.
Yes so 
this is very nice a poll figure shows a cube
texture (001) any material which exhibits
cube texture will show the poll figure of
this kind of three different orientations
here.
So in this case this is a nickel sample wherever
a student processed it to obtain this cube
texture.
And again I am telling you this is this information
is coming from the top 20 nanometer the surface
layer.
So if you really want to do it or prove it
as a material property you may have to do
it in an x-ray texture, EBSD is not a characterizing
the bulk behavior of the sample.
So that point you have to be very careful.
Other than that it is very useful to characterize
this again this you can see that miss orientation
angle, you can see that a low angle and high
angle boundary distribution which is readily
available used using this software interface.
So I think we will stop here, what I would
like to say is so as a whole we have now gone
through a number of concepts involving may
SEM apart from the conventional imaging technique
like scanning I mean secondary electron imaging
or back scattered electron imaging.
And we have also very briefly introduced the
special contrast mechanisms and, and this
particular technique just we have just shown
I have not gone into the details for the lack
of time constraint but then the EBSD itself
a separate course one can go through to get
into all the details but as a part of this
SEM course I think whatever I have just shown
is I hopefully it is I hope it is useful to
realize that is one of the powerful tool which
gives about a crystallographic information
and so on.
And with that I will finish all this discussion
on the scanning electron microscopy and in
the next class I would like to do some more
tutorial problems.
And you can just go through those tutorial
problems and you get back to me whether you
have any doubts.
Thank you.
IIT Madras Production
Funded by
Department of Higher Education
Ministry of Human Resource Development
Government of India
HYPERLINK "http://www.nptel.ac.in"www.nptel.ac.in
Copyrights Reserved
