In this lecture we learn about Auger electron
spectroscopy. Here, we say, but o u g g e
r
it is not pronounced as oogar, but it is pronounced
as o j a u z e y. so, pronunciation is
more like Auger electron spectroscopy and
that we utilize Auger electrons for the for
achieving a particular spectrum, so that is
the reason we use Auger electron
spectroscopy.
Auger electron spectroscopy, the Auger effect
was initially observed by Pierre Auger a
French physicist in mid 1920 s and this process
have developed only in the late 1960 s.
This particular process utilizes emission
of low energy electrons which are generated
by
the Auger process. What is this Auger process,
we will see we will understand that later
on, but it is a surface analytical technique
for determining the composition of the surface
layer. So, it is highly surface sensitive
technique and that is what we will see in
this
particular lecture.
So, this Auger electron it was discovered
by Pierre Auger on which this particular
spectroscopic technique is named on. It was
later on developed only in 1960 s, which
utilizes a low energy electron which is a
Auger electron to achieve a chemical
composition mainly of the surface, it is highly
surface sensitive technique.
Once we initiate a electron beam how does
this particular process emerges, we will see
initially the how the electron beam interact
with the material. So, we have this electron
beam which comes and interacts with the material,
so there is there are certain
interactions which are which keep occurring.
It forms overall interaction is contribute
certain region for the depth of a material,
so we certain regimes which are more like
this
that we see only a very surface depth of 0.5
to 5 nanometer, which generate something
called Auger electrons. This is nothing but
the incident beam which is tracking on to
the
surface of a material and we need the surface
we will have some interactions from which
comes out of the secondary electron.
Then, we have some elastically scattered electron
some regime of back scattered electron
then we have something, which is corresponding
to the regime of interaction with
electron beam to produce x rays. Then, this
is regime of primary electrons and this
regime is coming out from the core level ionization,
so we see that the Auger how the
electron beam is interacting with the material
we see that the incident beam. The incident
beam actually which is out here like this
is the incident beam, it is interacting with
the
material and the surface layer generates Auger
electrons and Auger electrons are
characteristic within a certain regime of
around 0.5 to 5 nanometers.
Beneath that, we have some electrons which
are emerging which are called secondary
electron these are again elastically scattered
electrons. Then, we have some elastic
scattering with the material, which result
back scattered electrons and in this particular
lecture we are more interested in what is
happening at the surface layer. So, we are
more
interested in the Auger process for this particular
case and the Auger analytical volume it
means how much volume is being interacted
when the electron beam is interacting with
the material to generate some Auger electrons.
So, we see that once the electron beam is
being indenting on to the material it is interacting
only in very small amount which is
corresponding to around 0.5 to 5 nanometer.
This particular analytical volume this is
the overall interaction volume and then if
we
consider this is an electron beam which is
indenting onto the material only a very low
level of volume. It may be limited to may
be couple of nanometers 5 to 10 nanometersis
really interacting with the material to result
Auger electrons and as we say that this is
highly surface sensitive because all the interactions,
they are they are being coming to
the detector only within a surface regime
of 5 to 10 nanometers. It does not mean that
the
Auger electrons are not being generated in
any other location, it just mean that only
the
surface electrons surface Auger electrons
which are generated they are able to escape
the
surface of the material or the sample to reach
the detector.
So, Auger electrons are being generated even
inside the material once the beam is
interacting with the material. So, we see
in this particular process though electrons
are
Auger electrons are being generated they do
not acquire sufficient energy to come back
to the to leave the sample surface and that
is what we see that the that the interaction
volume is limited to around 5 to 10 of nanometers.
So, basically that is what the
characteristic of Auger electron is that Auger
electron fail to emerge with their
characteristic energies, if they are deeper
than about point five to five nanometer from
the surface.
So, that is what is all about the Auger analytical
volume, so we see how the beam is
interacting with the material and the Auger
electrons are being generated only from the
surface. Though, they generated throughout
the material like till where the electron
beam
is interacting with the material, all the
Auger electrons cannot escape out, so that
is what
leads to the Auger analytical volume which
is which makes it highly surface sensitive.
Since it is highly surface sensitive and it
is coming out from a very narrow regime, it
basically has very high resolution and it
can spatially resolve chemical images, which
are
approximately to the order of 100 angstroms
or 10 nanometers. Additionally, Auger
electronic spectroscopy can also be utilized
for performing depth profiling and it
basically we remove the surface layers, we
see what is on a surface and we startutilizing
some iron etching. We can start removing the
surface layer and see what the composition
on the top surface is beneath the material,
so we can have and it might be an oxide of
for
a particular material.
Then, we start etching out using certain iron
guns and then once we etch out the material
we see what is the composition beneath that
particular layer and that is called depth
profiling. Auger electron spectroscopy can
be utilized to determine the underlying
compositions, so that is what so nice about
it and it is a one of the very essential
evaluation tool in the microelectronic industry.
We want to see what are the basic
interconnects or what is there any oxide forming
on the surface or there are very certain
connectors or the goal coatings, which are
basically plated on to microelectronic devices.
So, AES becomes a very essential tool in terms
of analyzing or in terms of confirming
whether the proper connection is achieved
or what is the overall, the surface is very
clean
enough to have the conduction at particular
level. So, that is what it has become an
essential tool and it is highly versatile
and sensitive as it can detect up to 0.1 atom
present for a particular composition.
Therefore, it has become a standard analytical
tool also in the research lab, so that tells
its applicability of how Auger electron spectroscopy
can be utilized because it has very
high resolution in terms of 10 nanometers.
Then, it can it is highly sensitive in terms
of
being able to detect up to 0.1 atom present
concentration for a particular sample and
that
makes it very useful in research as well as
proper analysis of certain materials.
Though it is highly sensitive, it can detect
up to one monolayer which is lying on the
surface so that tells its capability in terms
of detecting a particular composition, which
is
even a single or a monolayer. It can detect
all the elements except hydrogen and helium
why these things cannot be detected, we will
come back as we learn about the Auger
process. It needs a minimum of three electrons
in the process the Auger process and
since hydrogen and helium they only have one
two electrons in the outer shell.
It cannot yield an Auger electron and since
it is highly sensitive it can also be used
for
monitoring the surface cleanliness of samples.
Since the process is highly surface
dependent like what is our surface composition
it is highly surface sensitive that is the
reason we can also detect or monitor what
is the cleanliness of a particular sample.
Again, it can also do some quantitative compositional
analysis of surface regimes by
comparing it with some standard sample. So,
basically you come to that it can detect
even single monolayer, it can also monitor
the overall surface cleanliness. Also, it
becomes a essential tool in terms of quantitative
composition analysis only once when
we have some standard sample also available
for its comparison.
As we said earlier, the limitations extend
to that it cannot detect hydrogen and helium
just because Auger process itself requires
a minimum of three electrons in its outermost
shell. That is the reason hydrogen and helium
are eliminated from promotes detection
and at the same time it will not provide a
nondestructive depth profile. So, if you want
to
see what is beneath the surface, we obviously
need to cut it we need to remove, so that
is
the reason Auger electron, Auger electron
spectroscopy.
It can provide depth analysis only once the
surface layers are removed it is unlike the
x
ray diffraction, but x ray can penetrate much
more depth in many microns to be able to
detect the information from the bulk. But,
AES Auger electron spectroscopy cannot do
that because it is highly surface sensitive
it can penetrate only a depth of around 5
to 10
nanometers not more and it is also required
that samples be small and compatible with
high vacuum. This is because that Auger electron
they were low energy electron and
since if the electron has to come out of the
surface, it has to come out without any
interaction.
It should be able to come out without any
collisions with the atmospheric atoms if it
is
colliding with the atmospheric atoms it means
it is losing the information. That is the
reason that the samples they have to be compatible
with the high vacuum and most of the
time sometimes that non conducting samples
also become a problem because we are
always bombarding the sample with certain
electrons. Once you are bombarding the
sample with the electrons that charge also
has to be removed, it has to be grounded if
sample itself is non conducting in nature.
Basically, the charge is not going away it
is not getting earthed, so for this particular
Auger electron spectroscopy need the sample
to be conductive and because to avoid any
charge development on the surface. So, in
this particular case the non conducting
samples, they develop charge and it becomes
impossible to really analyze them because
of the electron beam bombardment. So, that
is the particular one more limitation of it
that
hydrogen and helium cannot be detected, it
cannot provide nondestructive depth profile
and it requires the sample be compatible to
that of for high vacuum.
It becomes little bit problem in certain biological
sample or polymer samples, where the
samples themselves cannot take much of vacuum
as they start as the mic start
decomposing. So, this is one of the major
limitation that it cannot be used for detecting
certain biological samples, so and again it
also cannot have it cannot take the non
conducting samples as well, so that makes
it certain limitations of the Auger electron
spectroscopy.
Once the electrons are interacting with the
matter, so the basically the excited electrons
it
can return to its lower energy state because
if sample is interacting with the with an
electron beam, it can excite the electron
and the excited electron can come back. It
can
relax in certain different processes, so there
are two competing processes, one is that the
electron is simply return to the core level
state. So, once we had excited electron it
has
gone out one electron is knocked out from
the core shell and then one electron will
come
back to its core level state, which was earlier
knocked off and the differential energy will
yield to some x ray fluorescence.
So, we have certain knocked off electron that
knocked off electron core shell is being
occupied by electron from higher energy shell
and that basically the differential energy
is
left as a x ray fluorescence. Secondly, it
can also happen that the when the knocked
off
shell is it is gone when electron is knocked
off, one electron can jump from higher
energy to the lower energy straight or from
a higher shell to a lower shell. Once it has
happened, that particular energy can be acquired
by a second electron, which basically
leaves the particular sample surface as Auger
electron.
So, instead of going out as a photon with
certain with certain x ray, x ray photon that
energy can be absorbed by a secondary electron
which is in a little higher shell. Then, it
can have acquire certain energy as well and
that is called a Auger electron to provide
a
picturesque explanation of this one.
How the particular things really work out,
but before that it just makes that Auger and
the x ray they are more complementary in nature.
So, whatever we have like we have
some atomic number out here, so we write atomic
number atomic number in this
particular axis x axis and say we have yield
of either x ray or Auger. So, we will realize
that the lower atomic number the lower atomic
number elements will have very high
Auger and it will drop down with atomic number.
So, Auger is typically very higher for say
up to 15 to 20 atomic number, whereas x ray
yield is complementary to it. So, it will
start building up from here and then the total
will
always be 1, so x ray plus Auger will yield
a total yield of 1. So, if say this was 1.0,
this
one will be 0.0 and generally the Auger yield
is very high for lower atomic number
elements. So, it is very high for lower atomic
number that elements, whereas x ray yield
is typically very higher for higher atomic
number and the Auger yield is typically very
low for higher atomic number.
So, this Auger electron spectroscopy is generally
utilized for low atomic number
elements since the yield is very higher so
the detection becomes very easier. So, the
overall explanation of overall yield which
is which combines Auger and x ray, so this
is
what the overall chart tells you about that
the Auger yield will drop down with increasing
atomic number, whereas, x ray yield will keep
increasing for higher atomic number.
We keep Auger limited to like up to 15 then
I will come to the transitions, but it can
be
detected up to like 40 or 45 atomic number,
we can have a considerable Auger yield.
Until there, we can utilize the Auger electron
spectroscopy because after that x ray yield
is so high that it becomes much easier to
go with x ray and the nit might give a very
high
background for higher atomic number elements.
So, Auger process how it is to be defined
it is defined by three basic steps, so as
we
saythat we bombard the surface with very high
energy high energy beam and that
basically leaves leads to atomic ionization.
Atomic ionization means that we are
removing an electron of from a core shell
or K shell, so the first step is that a electron
is
removed from the K shell of the material.
Now, the material or the sample is in higher
energy state now that higher energy state
has to go out either via x ray fluorescence
or
via electron emission, which leads to the
Auger process.
Third step is once the Auger electron has
released from the surface it is the characteristic
of a particular material and then we analyze
that emitted Auger electron and that
basically completes the process of Auger spectroscopy.
So, initially we have atomic
ionization relaxation by the emission of Auger
electrons and then detecting that
particular Auger electron to get the overall
composition of a material. So, that basically
completes the Auger process and how exactly
what is happening at the atomic level let
us go to that particular part.
First of all, the electronic structure which
is being defined, it will have certain non
zero
value of orbital angular momentum like we
have principal quantum number, we have
angular quantum number. Then, we have shells
like p d f level shells and all these show
spin orbit splitting, because we have more
than one electron in the p shell. So, like
in s
we have plus spin and minus spin, whereas
in p we have certain shells, which lead to
the
splitting of the shell as we have s and p
orbitals.
So, like in one s we have one level, but once
it curves to 2 s and 2 p so 2 s and 2 p will
show different energy levels because the overall
structure of this s shell and p shell itself
is different. So, it can keep going on for
once we have 3 s 3 p 3 and similarly, for
other
levels we can see certain splitting between
these particular bands.
So, these all will lead to certain splitting
in the electronic structure let us not come
to that
right now, but again once crossing that we
have a vacuum level vacuum level and then
we also can have band if required out here
for some conducting materials. So, the
electron has to cross certain binding energy
that is that is what being defined, where
the
electron is 1 s 2 s 2 p. So, that is overall
the electronic structure, which is defined
by the
overall shells which the material has valence
vacuum levels, where it has to overcome
the barrier in terms of getting release from
the surface.
So, the Auger state can be defined like this,
so we have initial state, we have then
intermediate state and then the final state.
So, intermediate state is more like that,
initially we take the material we take the
particular electron, so we have particular
electron and then we are supplying certain
energy with h mu. So, we have electrons in
the s shell K shell or L shell and then higher
shells, so we have certain particular
materials so because of this high energy photon,
it basically will lock off a electron. So,
that electron in the K shell is basically
knocked off, now what will happen that the
intermediate state we have we have one electron
jumping from higher energy shell to a
low energy shell.
So, basically what is happening is we have
this K shell L shell m shell, so say an electron
is jumping from m shell to K shell to basically
take the position of a electron, which was
knocked off or the vacancy which was created
out here. So, we can lower its energy, so
we have some sort of a relaxation, which is
occurred by the jumping of electron from
higher shell to a lower shell this is something
called intermediate state, but the final state
will be more like this. The additional energy
which is being released by the material, it
is
acquired by electron in a higher shell it
acquires the overall energy and then it basically
leaves off as a Auger electron.
So, in the first state we had knock off electron
from K shell and in the second in the
intermediate state, we had jumping of electron
from higher shell jumping of electron
from higher shell to K shell or L shell. It
can also be L shell, but coming to a core
level,
core level shell and then the energy is being
absorbed by electron, which is in the higher
shell. That basically is the Auger electron
which comes out with certain kinetic energy
and this kinetic energy is the typical characteristic
of a particular material.
So, that is what we see that in the initial
stage, we have knocking off electron from
core
shell and then in the intermediate state,
we see a electron is jumping to the core shell
and
in the final state, we see the emission of
a Auger electron.
So, that is what is telling the Auger process
and these things are classified as ionization
relaxation and Auger emission, so we see that
Auger process is again divided into three
parts.
So, coming back to the electronic structure
of it, we can see that we have a vacuum level
and then we have shells we can call it L 2
3 for the p orbital L 1 for the s orbital
and then
we have K shell which is nothing but the or
1 s orbital. So, we will have 2 electrons
out
here, 2 electrons out here and basically six
electrons out here and then we will have a
vacuum level. So, first process ionization
is nothing but the removal of electron from
K
shell, so we have our vacuum level it remains
as such and then L 2 3. It will still have
six
electrons L 1, it will have 2 electrons, but
in K shell we have one core or the one of
the
electrons has been knocked off.
So, we say that this electron was knocked
off, because of the high incident energy,
which
is being incident on the particular material
and the primary energies are in the range
of 2
to 10 K eV, which are nothing but the ionization
energy of any material. So, we see that
once we are indenting certain energy or electron
beam on a certain material, it is
knocking off an electron from core level shell
which is the K shell. It creates a vacancy
or a core in the K shell, so the first so
the first step in this particular Auger processes.
We
see that a core is created and then we the
other shell remains as such so L 1 and L 2
three
remains as such and the only thing is we are
creating a particular core hole in this first
step.
Going to the second step of relaxation Auger
emission, we see that that one electron
from a higher level jumps to fill an initial
core, so the same thing we see here that we
had
created a hole earlier. So, this was our vacuum
level and then we had this L 2 shell, then
we had L 1 shell L 2 3 and L 1 and then we
had a K shell. Since we had a core an
electron can jump from higher shell to a lower
shell or it means from L 1 or L 2 3, any
one of the electrons will jump to the lower
energy shell. So, we see that one of the
electrons is jumping out here from L 1 to
K and then this released energy because once
the electron is jumping from a higher energy
shell to a lower energy shell.
Basically, there will be some additional energy
which will be available with the material
and that particular extra energy is now being
released. So, first step was ionization in
the
second step we have jumping of electron from
high energy shell to a low energy shell.
Therefore, there is some gap or some differential
energy which is being released so that
is what we are seeing, so we will have certain
energy release and that will again lead to.
So, that can again go back that we know our
K shell is already filled and L 1 and L 2
3 if
the electron had jump from L 1. So, we had
only two electrons out here, now we will
have only one electron out here and then we
will have certain say couple of 6 electrons,
but now that additional energy can be occupied
by this particular electron. It can go out
as Auger electron while overcoming any binding
energy, so this Auger electron has to
cross certain energy barrier to be able to
get release from the surface.
So, we see the overall thing firstly a material
is getting ionized K shell electron is getting
released then there is jumping of electrons
from higher energy shell like L shell, say
in
this case we had jumping of L 1 electron to
the K shell. So, now that will release certain
energy and that particular energy is being
absorbed by the electron in the L 2 3 shell
and
that goes off as Auger electron while overcoming
the binding energy barrier. So, this
energy is basically being utilized for overcoming
the binding energy of this second
electron, so this particular second electron
is this particular case and this thing.
Basically, it is coming out as Auger electron,
it will have certain kinetic energy as well
because the overall energy is overcoming binding
energy plus acquiring certain energy
which is nothing but the kinetic energy of
the released electron. So, we have this energy
which was attained by the jumping of electron
from L shell to K shell that energy is
being absorbed by an electron which overcomes
which that energy it is utilized in
overcoming certain binding energy.
Rest of it becomes its kinetic energy, so
that is nothing but Auger electron with certain
kinetic energy. So, we have this particular
energy as the sum of the binding energy plus
some kinetic energy, so this tells us the
overall process what is the Auger electron
spectroscopy.
That we can come back to it that we can make
a rough estimate of the Kinetic energy of
the Auger electron from the binding energy
of the various levels in which the electrons
are involved. In this particular case, we
can see that we had the energy, which is being
released is the difference between the energy
levels of e K minus e L 1, because we had
e
L 1 electron L 1 electron had jumped to the
K level electron. So, this is the overall
energy which was being released and this energy
is again absorbed by the L 2 3 in
overcoming its binding energy.
So, we see that kinetic energy is equal to
e K minus e L 1 minus e L 2 3 again if we
rearrange this particular equation as below.
We can make it kinetic energy of the electron
the Auger electron is e K minus e L 1 and
minus e L 2 3 or in the bracket L 1 plus L
2 3.
If this become similar to e K minus e L 2
3 minus e L 1, it means had the electron jump
from e L 2 3 and the e L 1 would have gone
as Auger electron. Still, the kinetic energy
of
the Auger electron would not have changed,
it means that all the three electrons which
are participating the kinetic energy depends
only on those three electron. It does not
matter whether the electron had jump from
L 1 or L 2 3, so these two electrons become
in differentiable.
Since the latter two terms can of the energy
could be interchanged without any effect
because the kinetic energy is remaining the
same as we saw earlier that kinetic energy
is
remain same whether it is e L 2 3, which is
subtracted first or e L 1. This is being
subtracted first, so the overall kinetic energy
is just written as e K minus e L 1 plus e
L 2.
So, this tells that these two energy terms
could be interchanged, so which makes that
it is
actually impossible to say which electron
fills the initial core level and which electron
is
ejected as a Auger electron. So, that is the
beauty of this particular this particular
process
that one electron is jumping from higher energy
shell to fill the core level shell and
second electron is getting emitted as Auger
electron. These two electrons are
indistinguishable because the energy will
matter only where the initial core level electron
was there and where the two electrons which
have participated from a higher shell.
So, overall these two electrons are indistinguishable,
the two electrons which are
participating which are one is jumping which
one is coming out, they are basically
indistinguishable. Therefore, an Auger transition
is characterized primarily by the
location of the initial hole in this case
it was at K level K shell and the location
of final
two holes one is via jumping one core is created
by the jumping of electron from L shell
L 1 to K shell.
Second one was emission of Auger electron
from the L 2 3 shell, now we had core in L
1
and L 2 3, so this is the location of final
two holes and this is the location of our
initial
hole. So, overall Auger process will depend
what was the location of the initial hole
the
K shell and what is the location of the final
two holes which was either jumping L 1 and
L 2 3, this one will come out here. The location
of initial hole was k and the location of
final two holes is L 1 and L 2 3, so that
is what tells that is all what governs the
overall
Auger process.
So, basically we can interchange whether the
transition was occurring from L 2 3 2 K
and then L 2 had released as Auger electron
or the or vice versa. So, these two electrons
are nothing but indistinguishable and energy
remains same the kinetic energy remains
same whether the electron is jumped from L
1 or L 2 3, so that is what the overall
significance of this particular Auger process.
As we said earlier that Auger electron depends
on the atomic number and we also saw
that the yield is much higher in case of lower
atomic just because the probability is very
high that the K electron can go away or the
electron can go away. They can be jumping
from higher shells higher energy shells to
yield this particular process, but for higher
atomic number the x ray takes the predominance
and the overall Auger level basically
keeps going down.
Basically, we see that the there are certain
Auger peaks which are which can come out,
when a incident beam is applied on a material
and then stronger K L l signals. It means
that the location of three electrons is from
K shell L shell and L shell and m n n signals
are much stronger for higher atomic number.
It means that the transition is occurring
or
creating a hole in the m shell and then n
shell and n shell are participating in the
jumping
of electrons to m shell and then release of
Auger electron from the n shell.
Generally, typically we see that the if atomic
number is on this on this particular on
particular abscissa your y axis, then we see
that this is atomic number and in this case
we
have say electron energy which is being acquired
electron energy. This is being supplied
to the material for creating the ionization,
we will see something like that this is nothing
but the atomic number this case and we see
that the K L l transitions are occurring
something like this. Then, L l m transitions
will occur more like this and m n n n
transitions will occur more like this something
like this.
So, we will see that K L l transitions are
very predominant for certain atomic number
and
then we will have L m m transitions for little
higher atomic numbers which are more
predominant and then m n n. This is so true
because if this is up to maybe say 15 to 20
atomic number, we are more K L l transitions.
Till then we do not have m shell and as soon
as the m shell starts coming in, it governs
that basically the L m m transitions become
much more probable because it is very
difficult for a particular atom. If it has
the m shell n shell higher order shells, it
is very
difficult for electron beam to penetrate through
the material and knock off a K electron
because this is again surrounded by L shell
m shell n shell. So, this becomes very un
probable, so that is the reason this Auger
process is very dominant for K L l transitions
in
lower atomic numbers and L n n transitions
for little higher atomic number higher atomic
number elements.
Then, again m n n transitions become very
predominant for even higher atomic numbers
so approximately up to around 50 element transitions
are much more probable and again
80, 90 atomic number element transitions.
They are much more probable and so we can
see that the detection limit or the K L l
transitions, they are very stronger for the
low
atomic numbers up to 50 it is medium atomic
numbers. We have this element transitions
which are more probable and for high atomic
numbers, we have basically m n n
transitions which are much more probable and
we request certain level of energy to
basically ionize a particular material.
Those basically run upto may be up to from
maybe say couple of K e v, so we have the
incident beam from running from 2 to 10 K
e v to ionize the core shells. So, that is
what
is required in terms of ionizing a material
and leading to a generation of a core hole
and
so that the Auger process can really occur.
In this particular process, we require at
least
three electrons one transition to cause ionization
and two more electrons to cause a jump
and release of Auger electron. That is the
reason we need minimum 3 electrons and
hydrogen and helium cannot be detected, so
minimum element which can be detected is
lithium, so that is what the overall dependence
of this K L l transitions and above.
So, the Auger spectrum looks more like this
that we have we have to supply certain
electron beam which around 2 to 10 K e v to
ionize a particular material and then, but
these peaks are basically they generate they
are on a very high background. It also
undergoes an inelastic scattering processes
during the Auger while achieving the Auger
spectrum. So, we have certain signal and the
K eV which is being detected from the
particular from a particular detector, so
we have signal and again it has very high
background and certain peaks for certain material.
Then, it goes off, so the overall background
is very higher for particular signal in the
Auger process. So, basically that is not really
good for detecting, it is not good for
achieving a particular repeatability, so that
is the reason this particular signal is
differentiated with respect to the kinetic
energy of a material.
Then, basically we can achieve one more spectrum
which is much more repeatable, so
we have something d n with respect to d e
and then we have this particular in terms
of K
e and electron volts. Then, we see that we
achieve a very nice background with peaks
very sharp peaks and very repeatable peaks,
so let me draw it again. So, we have this
particular peak and then we achieve certain
peaks and then it goes on, so this is nothing
but a kind of a zero background and we achieve
certain peaks which are highly
repeatable. So, it is actually possible to
measure spectra directly in this particular
form
and, it gives very good sensitivity, because
you know this peaks are more repeatable.
They are also very sensitive to particular
materials, so it is again reproducible for
a
particular referencing. If you want to reference
it, say this was peak for say silver and
then the differentiated form will also give
out this particular peak. Again, at the same
location it would not be much with the background,
it would not be more hazy like we
saw in the earlier case the peaks were more
hazy and they were not really very sharp.
So,
that is the advantage of utilizing a differentiated
form which is d n by d e the number of
with respect to the kinetic energy because
it also consider a way of taking the
probability, which was not so being defined.
In earlier case in this case we know particular,
will occur at a certain energy level, so that
differentiation with respect to energy makes
it very sensitive and reproducible so that
is
the advantage of using a differentiated p.
Also, the sensitivity is gone high because
this
particular ratio with respect to background
it also increases, so that is the advantage
of
utilizing a differentiated form of AES that
we have much more higher peak or better
peak.
Again, it can be utilized for detecting very
small concentrations like we had certain
spectra something like this, It can be certain
spectra like this in the differentiated form.
We can see that there will be certain peaks
which will correspond to similar same
elements because we have transitions from
K L l, L m m and m n n. So, we can see that
the higher energy, the higher atomic number
materials they can have multitude of peaks.
So, we can see certain peaks which can say
belong to say chromium, then again
chromium or they can also have more peaks
say for something nickel or nickel.
It can also be that like in case of certain
materials like in steel we have very small,
very
fine contributions from sulphur, phosphorous
elements like that which are very which are
very low concentration less than say 0.03weight
percent. Still, we can see that this Auger
spectra can detect, so even such a lower concentration
can be defined.
Let us say sulphur or say some peaks in phosphorous,
so those things can also be
detected very easily in in the Auger spectra.
So, we had this n in terms of energy and it
can be again differentiated with respect to
energy and this again the energy. So, we can
detect always some peaks which are which were
really not probable with concentrations
of less than 0.03 weight percent. We have
seen that Auger spectra can detect the major
peaks which can come out either say from major
peaks can be either from chromium or
say nickel or even iron because that for a
stainless steel the major composition will
be
say iron.
Then, chromium will be say around may be say
15 to 25 percent then nickel can also be
from 12 to say 15, 20 percent so that those
are the major peaks what you might expect
to
see in the Auger spectra. Then, again sulphur
and phosphorous very low in quantity, but
those will also be detected if utilize the
Auger spectra. So, that is the beauty of this
particular process that even elements with
very small concentrations less than 0.03
weight percent can also be detected in the
Auger process.
In this particular processing, we require
very ultra high vacuum this is the order of
10 to
power minus 9e torr, why because the Auger
electrons they are very low energy
electrons and once they have to leave the
surface and get detected at the detector.
So, they need to undergo detection process
without collision with the atmosphere if it
is
colliding with the atmosphere atoms then it
is basically losing the energy and that will
increase the background. So, this particular
vacuum level will create a mean free path
which is approximately 40 kilometer and as
we said earlier that the Auger effects are
much dominant for atomic number less than
15 and for L and m shell transitions, it can
be measured up to atomic number of up to 50.
So, that is the overall thing which we saw
earlier because of the presence of the different
shells and this is the very much
requirement that it requires ultrahigh vacuum
of order of 10 to the power minus 9 torr.
Coming to the components of Auger electron
spectroscopy, basically we require an
electron source which will have a variable
energy because we want to see what the
overall kinetic energy is. How it is dependent
on the ionization because we are applying
certain energy and it is basically knocking
off K shell electron. So, we want to see how
the transitions are occurring and again this
particular variable source of energy is also
useful in terms of describing a very fine
spot of electrons. So, we should have a electron
source we should have a variable energy source
and then this electron has to go through
a electron energy analyzer.
There are different types of analyzer which
can be a spherical sector or hemispherical
sector and then it has to basically pass through
a electron detector. So, electron energy
analyzer will separate out of the energies
and those energies once it is filtered, it
will
basically get at electron detector, where
it can basically be detected. This is an electron
and then that the intensity of that particular
detection will be counted at the same time
the measurements have to be done in ultra
high vacuum just because that the Auger
electrons which can travel from surface to
detector without any collision.
Secondly, the surface cleaning is also one
of the very critical features out here that
once
as soon as we take the sample from the particular
location to the Auger chamber, then it
can get contaminated depending on its reactivity.
So, we want to keep the sample clean
and so to avoid that particular contamination,
we want to use a ultra high vacuum, it is
to
the order 10 to the power of minus 9 torr.
There are certain other options, which can
be available with Auger electron spectroscopy
is something called ion source ion source
can be utilized for cleaning the surface prior
to
the analysis. So, if we want to measure by
clean surface, say if we take a sample it
get
oxidized you want to clean the surface to
see what is the actual surface. So, we can
measure its concentration depth profile and
then we can measure it, so that is an
accessory to the Auger electron spectroscopy.
Coming to the analyzer part there are certain
analyzers one is one of them is cylindrical
mirror analyzer, it has basically some concentric
electrodes and then those electrodes are
basically all it is it is it is we apply certain
bias to it. Then, we can allow only certain
energy levels to pass through it if it is
higher energy, it will be absorbed by one
of the
electrodes lower energy. Then, it will be
absorbed by another electrodes and that those
two electrodes are again, so basically certain
energy gap can only travels through a
particular concentric electrode set up.
So, cylindrical mirror set up, so that basically
determines the pass energy and then it
allows only a certain or a narrow range of
kinetic energy to pass through it. Basically,
the
similar system also can be utilized by using
a hemispherical analyzer, so how the
particular analyzer looks like. Let us go
back to it and let us see how the cylindrical
mirror analyzer looks like, so we have this
particular electron energy source, which goes
and bombards a particular sample surface.
Then, we have the electrodes like this and
then only a certain level of energy the Auger
electrons will pass through this particular
electrodes set of electrodes and it will reach
the detector.
If it basically coincides with one of the
electrodes this electrode or this electrode,
the
energy level all the electron will get absorbed
on to the electrodes. So, the bias on this
two electrodes will basically decide how much
how much the particular energy is
allowed through the particular analyzer. So,
we have electron energy source which
interacts with the sample and then we have
the Auger electrons which are emitted and
they pass through the analyzer. Only certain
energy is basically allowed to pass through
and to reach the detector and that is all
this particular cylindrical mirror analyzer
really
works.
This was a one pass filter we can also have
two pass filter where we can see more like
this that electron gun, it is interacting
with the material and then basically it is
coming
out at a certain energy is coming out and
to verify that. This is again the same energy
level we can have one more set of electrodes
out here again these electrodes will make
a
second pass. So, this is first pass and then
again you have certain kind of opening and
then you again have these two electrodes,
which will again decide the overall energy
and
then it’ll have a detector. So, this is
nothing but the second pass, so we had this
particular
electron gun it is interacting with the sample
out here and then we have Auger electrons
which pass through the first.
The first becomes the second pass and then
we have again electrons going out from
through the analyzer like this, so we have
again a detector out here and which detects
the
overall intensity of electrons through this
particular process.
Now, comes the important part once it is pass
through the detection pass through the
analyzer. Now, we have to detect what is the
overall kind of the Auger electrons, so
basically we have we have to focus the electrons
which were basically coming out. So,
basically we have focused electrons are incident
on a sample and then they are pass
through the cylindrical mirror analyzer for
the analysis part. Then, we have its detection
where they are multiplied and signal is sent
to the data processing, so we have
photomultiplier tubes or the dynodes which
basically take the detect the electron and
multiply it.
So, again we can have single channel detector
or multichannel detector depending on it
can be a continuous dynode surface or a photomultiplier
tube and then the collected
Auger electron are basically captured. Then,
they are plotted as a function of energy
against the broad secondary electron background
spectrum, so since it has a very huge
background.
Then, again it can be again later on differentiated
to achieve a very nice repeatable
sensitive Auger spectrum, which is d n by
d e with respect to the energy and that is
what
we have. We have first its analysis and then
the detection part comes to the multichannel
or the single channel detector or the photomultiplier
tube or through continuous dynode
surfaces. One more thing to mention is that
the energy of Auger electrons is in between
that of a secondary electron and a backscattered
electron.
Again, once we have detected it we can have
the quantitative measurement of a Auger
composition that the atomic concentration
of an element can be given by this particular
equation where x is the intensity of the unknown
specimen. So, x is the intensity coming
out from an unknown specimen and i is the
pure element, so i is the pure element and
x
is the unknown specimen. Then, we have i is
the intensity of the Auger signal and s is
coming out from the is the relative intensity.
So, by comparing in this particular manner
we can see what the overall signal, which
is
coming out from the sample and what is overall
signal which is coming out from the
standard sample. So, by comparing those two
we can say what is the overall
concentration of x in particular sample? So,
we need to have pure sample available to be
able to say what the overall concentration
of that particular material in the unknown
sample.
Again, it can be utilized for Auger depth
profiling and it can detect what is beneath
the
surface or the buried layer, what are the
overall compositions which are basically can
be
attained out here. So, basically we have a
surface and it is being bombarded by an ion
source, so it basically starts eating the
surface, now we have some surface which is
beneath the original layer, the Auger process
it can detect its composition from step one.
It can also detect composition after it has
the sample has been etched, so this can
particularly provide what is the composition
at level one and what is the composition at
level two.
While this particular ion beam is etching,
the surface it is necessary to know what the
etching time is because depending on the material
depending on the energy of the ion
beam. The way it will eat out the material
will be very different one material will be
eaten away quicker second material can be
eaten away little slower also depending on
the current or ion beam intensity again the
etching level be little different.
So, then the Auger signals which are coming
out from the surface they can be correlated
with the overall depth of a particular material
and it can provide us information of what
is the particular material concentration with
respect to depth. So, this depth or the etching
time can be correlated here with respect to
percent composition out here so coming to
just one example.
So, say if we had a particular material and
say we saw some things out here and then it
is
dying out like this, it means this particular
material was sensitive only on the surface
for
say certain nanometer regime, say it was 2
or 3 nanometer in terms of death and in terms
of percent composition. So, this particular
material and then we can also see something
more is something more is forming on the surface,
so this was oxygen and say this one
was silicon.
So, we can say there is some formation of
silicon oxide on the surface, so we can say
that
the oxygen level will go down and then we
can say that there was some higher
concentration of a material say silicon. Then,
again oxygen we can say silicon oxide was
basically predominant at this particular on
the surface, whereas say the bulk was say
something like this the silicon and then oxygen.
Then, we had something like indium, so we
can say the bulk of it is formed of indium
whereas, it was probably coated with some
silicon and which is formed oxide. So, we
can say that Si O 2 is forming in an indium
bulk, so that is how it will tell that what
is the
overall material what is the overall material,
which is basically being what is really
happening on the surface. How this depth profiling
can help us identify what is
happening beneath the surface, so by ion etching
we can by this particular process we
can see with the increase in depth.
We are seeing increasing concentration of
indium and decreasing concentration of silicon
and oxygen, so it means that there was some
silicon oxide which might have formed on
the surface. One more dimension can be given
to it is that it can also happen that we had
silicon and then it was going like this and
then we had some oxygen. Then, it drops much
earlier to that, so it means that that silicon
oxide is forming only to a certain thickness
and there is again some more regime where
silicon is still intact. So, we have formation
of some silicon oxide on the surface beneath
that we have some silicon, and then we
have some indium on which the particular silicon
might have been coated, so that tells
overall depth profile of a particular Auger
process.
So, now coming back to the overall features
of the Auger electron spectroscopy to
summarize the overall lecture that we have
characteristic energy losses. They can occur
because of plasmon losses or which can again
be channeled through the electron
photoelectron spectroscopy. So, they basically
create the background and then there can
be a charging of insulating samples, so by
choosing proper incidence angle and by
choosing proper beam energy, we can take care
of that by lowering the beam energy. By
inclining the surface and then Auger electron
spectroscopy can also be used for
qualitative and quantitative spectroscopy.
It can also be used for the depth profiling
part and basically this Auger electron
spectroscopy, we realize that we have basically
Auger process arising from three
electrons one is the core level electron,
which is basically knocked off. Then, we have
one electron jumping from higher energy shell
to a lower energy shell and then third
thing that energy is absorbed by a electron,
which basically is emitted via overcoming
the binding energy that is called a Auger
electron.
We saw that how the overall instrumentation
is also done that we have incident electron
beam energy source and then we have something
called analyzer that basically allows
only a certain energy levels to pass through
it. That allows only a certain electrons to
come out with certain energy levels and that
is being detected by detector, where some
dynodes or photo multiplier tubes and that
thing is again plotted as curve with respect
to
energy. So, we have signal count with respect
to energy and to make it much more
precise, we take a differentiated form with
respect to d n by d e with respect to energy
and that is much more sensitive and much more
repeatable.
Then, we also saw that the Auger electron
spectroscopy can also be utilized for depth
profiling via doing the surface by applying
certain ion beam and then etching the outer
surface. Then, we will be seeing what is happening
beneath the surface to comment on
what is the overall composition of the overall
sample which was under consideration. It
can also tell you what is happening beneath
the surface and so on, so basically we end
our lecture here.
Thanks a lot.
