Hello everyone, I hope everyone's doing
okay
in these troubling times. Today we're
going to be talking about energy
dispersive spectroscopy of X-rays, so
it's a
technique used both in SEM and in
TEM.
So today we're going to be covering
sort of what is
EDS, also the components of
the EDS detection system,
how x-rays are produced and
the nature of the x-rays. We're going to
talk about
the SEM settings that we use for doing
EDS
and within the EDS system itself, what
settings we use
in the software in order to get the
results that we
want. So and I'll also talk about EDS
resolution,
some things about sample preparation,
and I'll give a few examples. I'll try to
leave enough time that we can get
in plenty of questions, if needed.
You can see here a nice picture
showing
some EDS mapping and we're seeing
different elements.
And it's a mix map so we can see the
different colors,
so it's orange because there's a mixture
of red and yellow.
So what is EDS? So there are common
acronyms that are used for EDS such as
EDS< which we've been saying,
which I'll use for most of the
presentation. EDX,
and a fewer number of people refer to it
as XEDS.
It identifies the chemical elements
present within a
specific sort of volume. We don't get any
valence state
information that you do with some other
techniques.
It is non-destructive but because of
electron beam energies that are involved
here, it may cause damage to certain
samples
or cause contamination such as
carbon deposition. It can be both
qualitative and quantitative,
and as I said previously it's both an
SEM
and a TEM technique, in the case of
doing maps and line scans it's a
scanning
transmission electron microscopy
technique, so that's why we have the
S there, in the description. These sort of data that you normally
get out
are spectra which can be from spots or
areas,
maps or line scans.
Because of the nature of x-ray
production
we don't get any detection of hydrogen
or helium. Lithium
you need to have the right type of
equipment to do them
and conditions. EDS
doesn't require any special preparation
beyond what you normally have to do
with either SEM or TEM
but you do need to be more careful about
preparation
when you're doing quantitative analysis.
So EDS detectors, there
are two main types that we see.
There's a silicon lithium detector
which is cooled with liquid nitrogen, you
can see down here in the
bottom right hand corner that type of
detector.
One thing I will mention at this point,
we pretty well have exclusively
Oxford instruments, EDS
systems, so it's going to tend to be a
lot focused on that particular
type of system, but we can talk about
the other systems that do exist out
there. So
up here you can see the second type of
detector which is a
silicon drift detector, Pit has a
palche
cooling unit so it's a thermal
electrically cooled.
And next you can see a picture which
is showing you the sort of composition
of the tip down at the bottom,
where you have the different components
of the EDS detector so there's a
collimator
which is used for sort of focusing
collecting
the x-rays. Because we don't want
the electrons
going into the actual
sensor, we have an electron trap
so that the back scattered electrons,
especially, don't get into the actual
detector.
Now the windows that are in front of the
detector
can come in various types,
mostly these days you'll see probably
polymer
windows, but they're not like a big open
piece of polymer you'll see that they'll
have slits in them where they
you have a coating over the polymer and
you'll have the slits through which the
x-rays can go.
There also exists out there silicon
nitride, thin
silicon nitride windows as well,
and on very old systems
you'll see beryllium windows.
The big disadvantage with these systems
is that
you can only with a beryllium windows
detect
heavier elements such as sodium and
above
with the EDS detector. There are also
versions of
detectors that have turrets on them,
which allow you to switch between sort
of windowless
and having a thin window.
There also are detectors that are
strictly windowless,
which don't have any thing between the
vacuum of the microscope and the
seat and the sensor itself.
Back in behind the sensor, so for the
most part
sensors these days are silicon,
there are a few detectors out there that
actually are
germanium, but those are kind of, I
haven't seen
anyone that actually has one of those,
but they do exist,
and behind that we have a field effect,
a field effect transmitter,
which is used for sort of some of the
amplification that needs to be going
into the
system once it goes out into
our pulse processor and analyzer systems.
So we'll have a computer associated with the EDS,
and we'll have EDS software for doing
things.
Now when the electron beam hits the
material and in this case we're talking
about
a bulk material, if we were talking about
TEM we'd maybe be talking about the thickness of material up in this
sort of range here,
where you see sort of the secondary
electrons coming from.
So the nature of x-rays is that once
they go in here and they get produced
within the material,
they travel in straight lines, they're
electrically neutral
so electric and magnetic fields don't
have any influence on them.
They're produced at high voltages, so
you need to have sort of in the kV range
to produce
x-rays of significant energy.
The behavior of the x-rays is controlled
by
the nature of the material that you're
looking at. And the x-rays,
there's sort of three possible
interactions you can have. You can have
absorption where the x-ray gets absorbed
by the material,
dispersion where you're getting the x-rays either deflected
or actually creating new x-rays,
or transmission where they travel
through things. Now when we're talking
about
the actual production of x-rays,
if we have on the SEM an accelerating
voltage
of 15 kV we're going to produce 15
keV electrons,
the kV and material affect the depth to
which
the electrons of the primary beam
or electron beam penetrate,
and the range of x-ray energies that are
produced.
Once the beam interacts with the
specimen,
is basically from sort of zero to the
energy of the electron beam, so 15 kV
in this case. Now we can have different
types of
x-rays produced, so here you're seeing
the sort of volume
where we're getting the characteristic
x-rays, which are the x-rays
which give us the peaks which allow us
to identify elements.
Continuum x-rays come from this sort of
volume,
and those are the x-rays that are
associated with a sort of background
radiation,
or breaking radiation, and you can have
the
x-rays that are produced by either
the characteristic or background, produce
other x-rays which can come from sort of
this region.
So it's important to talk about
what actual sort of data we're
collecting,
so it's important to understand where
the x-rays actually come from and what
controls their sort of energy.
So here we have 
an atom of silicon,
and if we have the primary beam
knocking off a secondary electron
we get an x-ray produced when a higher
energy shell electron drops to a lower
energy shell.
So EDS uses these inner shells so the K-L
and -M shells, removing electron from
the K shell, the sort of
requires the highest ionization energy.
For every element each shell and
sub-shell
have specific ionization energies,
the higher the atomic number of
material
the higher the ionization energy
For a given x-ray shell.
So characteristic x-rays are produced
with these various transitions and here
you can see
if you're having the
electron dropping back down to the K
shell, where we've had our
secondary electron produced, there's a
certain energy
that's produced of the x-rays.
So background radiation or continuum
x-rays or
a fairly unpronounceable German name for
 breaking radiation,
is produced when the electrons are
affected by the nucleus
of the atom but don't
cause any electrons to be lost from the
atom.
So because this is an inelastic
collision,
some energy is lost and some of that
energy is converted to x-rays, it is
possible for all the energy to be lost.
So the energy range
of these x-rays that are produced can
vary anywhere from almost zero eV
up to the energy of the primary beam.
And here you can see what the background
radiation looks like when we're looking
at a spectrum
of x-ray intensity versus x-ray energy.
The Duane-Hunt limit is basically the
accelerating voltage electrons that are produced here. So if
we're doing 15 kV
that's our Duane-Hunt limit. So
15 kV is the Duane-Hunt limit for the
x-rays produced.
One problem we have,
is the very low energy background
x-rays tend to be absorbed within the
sample or within the detector itself,
so we don't see them as much.
Now we're talking about
the actual production of what we
actually
see, if we're considering say
silicon, we'll have
different
shells that can be,
we can ionize, so once we
ionize and electrons so, and we get a
secondary electron
produced an electron from a higher
energy stage shell can drop down and
we get an x-ray produced. So in this case
you can see the energies
of the various x-rays produced, so in
this case
this is the most common x-ray you see
with silicon,
so 1.74 is the energy
of the actual x-ray. And you'll see that
on your spectrum down here. And here you
can see the nice
background radiation here, and you can
see the characteristic x-rays
sticking up as peaks.
So one of the problems with doing this type of spectroscopy is you
do get
some of the x-rays absorbed, they don't
actually
manage to get out of the sample.
Sometimes you have a lot of surface
roughness, this can be
sometimes a problem because the higher
areas will absorb the x-rays
that are in the sort of path to the
detector.
So higher atomic numbers tend to absorb
the lower
energy x-rays so we have a lot of
problems with
fairly light elements. Also
if there's a very long path for the
x-rays to travel through the material
it's more likely that the x-ray will get
absorbed.
Now another thing that can happen is
once we produce these x-rays
they can produce secondary or
scatter x-rays with different sort of
characteristics from the
x-ray that was produced by the original
element.
This is likely to happen if the
x-ray energy is equivalent to the
critical ionization energy
of an electron in another element.
Light elements 
tend to have very low
ionization and they also have
low sort of fluorescence yields, so
we don't have it fluorescing creating
more x-rays. Now
one other thing that you have to
consider sometimes if you're looking at
x-ray efficiency, is that there's a
competing process to producing x-rays
and that is auger electron production.
So auger electron production is more likely
to occur in
low atomic number materials, so we
have to consider
that as well, so. Here we have an
illustration showing that we
can get auger electrons instead.
Now the primary electron beam will
have,
you'll set the energy of that by
using the accelerating voltage or kV.
Now the energy of the electrons in the
primary beam must be high enough
to overcome the ionization energy of
the specific element
that you're looking at within the
material and
of the particular inner shell electron.
So this is the critical
ionization energy. Now because of the
kVs that TEMs are set to this usually
isn't a problem for them, so
it's mainly a concern in SEMs or if
you're doing TEM
in the SEM.
So if we don't achieve the
critical ionization energy, you don't get
any
x-rays produced, or you, if you don't
have enough energy above this value, you
may not get a large number of them. So
here we can see in the top
 spectra
the main sort of K alpha and K beta
peaks
for the, so that's the K shell peaks
for chromium,
and down here we can see the L shell
peak for chromium.
So at 15 kV we have enough,
the primary beam electrons high
enough energy
15 above, you know you can see the
energy of the x-rays produced here.
So we have enough energy, and so we're
getting all the
these x-rays produced. However if we drop
down to 5kV
you can see here that we don't actually
get any peaks here, and you can also see
it has an effect
on the background radiation, that we're
only getting you know
5 kV sort of background radiation.
Now what's important here is
you have to have enough voltage
to get enough stimulation
of the x-rays within a material
and usually if you're looking at x-ray
production versus the voltage,
the magic optimal number
ends up being 2.7.
So you want to have it
above one times that energy so that you
just don't get a few x-rays.
So usually we're trying to get in the
region
of two times the voltage
in order to stimulate the x-rays. And
here you can see where we've done,
if we do some under stimulation, so at 25
kV both the silicon and copper
peaks are well stimulated, but at 15 kV
there's a reduction
in the amount of x-rays you get
for copper. So copper is around 8 kV,
so we may want to go a little bit higher
to get sort of proper stimulation.
So yeah, that has to be considered as
a very important factor
in looking at things, so you have to
get the kV at a value that's
high enough. Now
the beam current, or spot size, or
probe current has to be stable
during EDS acquisition. This is
especially important if you're doing
quantitative work.
The current represents the number of
electrons in the primary beam,
it's controlled by the condenser lens spot
size control.
The number of electrons the primary beam
are proportional to the number of x-rays
produced,
so basically more current you will get
more
x-ray counts. So here you can see,
same kV, 15 kV,
we're getting more x-ray counts
at 3.2 nano amps as opposed to
0.8 nano amps. Increasing the beam
current will not change the relative
heights of peaks, so
the different Tungsten and peaks will
still have the same
relative heights to each other
within the material.
I'm going to talk about dead time in a
moment, we'll talk about a few factors
first.
Once the x-rays have escaped the sample
they need to get to the detector.
So you have the detector up here,
there's a particular takeoff angle that
it's set to.
We have the sample here, so there'll be a
particular
working distance and for every EDS
arrangement
there is a set working distance where
you have the proper takeoff angle for
collecting
x-rays, and it's calibrated
under those conditions, so any variation
from that, so
closer working distance or further
working distance,
or a different angle will have an effect
on what you're actually collecting.
So tilting the sample unless it's been
calibrated for tilting the sample may
have an effect.
Also tilting the sample we run into
sometimes the danger
of backscatter electrons
hitting the detector because of fairly
high energy.
And we want to have that sort of
consideration of that. So you can see
here
this is at a working distance of four
millimeters. we have a hundred
thousand
x-ray counts, whereas if we drop down to
ten millimeters
we have thirty-five thousand x-ray
counts. So
the number of x-rays counted. In this
particular detector arrangement,
four millimeters is the working
distance that's been set
for the system, you will find on a lot of
other systems it is actually 10
millimeters.
Now it's still getting a lot of x-rays
but
that angle may not be optimum for doing
quantitative work.
Now one thing that's important is that
once the signal leaves the detector it
goes to the pulse processor
and we need to adjust the amount of time
that we're processing the signal so this
is actually an
averaging of things. So here you can see
a nice chart
that shows us sort of the different sort
of pulse processing
times or as they're called process times.
And it's not a set time you see its
number basically from one to six.
So longer processing times
we get better resolution of
peaks because they're average longer. It
also improves our resolution,
but we get a higher dead time.
Dead time is the time when the system
is not
counting the incoming x-rays but
is processing the previous collected
data.
28 to 50 percent is a reasonable dead
time,
you don't want to have less than 10
because you really don't have
the saturation of the detector with
signal,
and above 60 percent
especially with older detectors you will
run into some problems with artifacts.
It also will with the newer detectors,
you'll also start to get see some
artifacts,
it also takes much longer to collect if
you do.
The x-ray resolution that we actually
get,
so is dependent on the detector and its
electronics,
plus as you can see the process time
that we use.
Now the counts of x-rays
or x-ray intensity, you'll also see it's
sometimes listed as counts per second,
is affected by the working distance
which
would be set for the particular EDS
setup:
kV, current, and also
the detector size, so there are a
range of detector sizes,
in something like a tabletop SEM you
might have a quite small
detector, there are now quite the large
detectors available
and on some systems, like actually the
one that we're using for a lot of these
examples,
it actually has two 80 millimeter
squared
detectors. So
when we're talking about the sort of
peak height here
you shouldn't make the assumption when
we're doing SEM
that it's proportional to the
concentration
of the element. There is a tendency to
do that, so
you do tend to have more of something if
it's it tends to be
a larger peak but if you can look
here, 
when we went down here to
a lower kV
we're getting more contribution from the
carbon
that's present on the surface as a
coating, or the carbon that's within the
material,
so it does have a much taller peak
and so there's, you can't really go by
peak height, it is related,
so the concentration of elements
is related to the peak height
but you have to make, be careful about
making conclusions
about them.
We do get spectral artifacts, one
thing that's nice about modern detector
systems,
is that a lot of these are corrected
for.
However if you look up in the
two spectra that we have up here, you can
see here some,
in this case we have some,
some escape peaks, and
this case is with it uncorrected it's
not 100 percent corrected
by the system. So this is an older system,
there has been some improvement
in the newer system. So internal
fluorescence
usually only happens in the older
detector systems which are the 
silicon lithium. Escape peaks
happen when you get the actual 
x-ray not being converted to an electron
hole pair.
In the actual detector or sensor,
instead you get a silicon x-ray
generated, so
but because for all the sort of 
elements within the material
that are detected you can actually
correct for that. So
usually in the older systems you may be
able to click on a button
which will do the correction, newer
systems usually do it automatically,
along with pulse process processing.
Some peaks also
the newer software corrects, 
you'll usually see it referred to as
pulse pile up correction.
This happens when you get two x-rays
coming in simultaneously
into the x-ray detector and it records
one x-ray energy signal with both
energies.
So it could be the same x-ray or it
could be two different energy
x-rays that do this.
Higher count rates, so sometimes you see
this in the TEM
or if you have the dead time above
60
you will get 
more prevalence of the escape in some
peaks
and sometimes it will have problems
correcting for those.
I have seen where it does do over
correction,
so something to look out for when you're
doing things.
Another thing I will point out at this
time,
EDS detectors do detect infrared energy
so if you do have the chamber scope on
you may get a very high dead time
because of that and you will get a very
strange looking
x-ray spectrum.
So when we're talking about the
spatial resolution, so sort of basically
the area over which we're getting
the x-ray information
and the depth, there's a little
variation between
SEM and TEM,
there also are some detection limits.
So with SEM you're usually looking at
spatial resolutions and depth
resolutions from
you know, maybe with high atomic
number of materials,
maybe down to 0.2 microns
up to you know a few microns,
but usually it's
never more than sort of 10 microns, in most cases, so it's usually on average
a few microns.
Unfortunately in low vacuum and
environmental SEM
we get a sort of skirting effect
where this spatial resolution actually
gets significantly expanded.
So instead of the spot size of the
sort of x-rays being you know a few
microns
it may end up being
you know 50 or more microns, so it's
something you have to be careful about
when
doing SEM work.
In the TEM we also,
we have the great advantage of it's very
thin
so we're not going to have as much
volume,
also because we have higher kVs we're
not getting as much beam spread,
so we're getting spatial resolution on
the order of nanometers,
and the depth is basically the thickness
of the sample.
The detection limits are usually around
0.1 weight percent, a little bit higher for lower
atomic number elements. Beryllium
you may have some problems, unless it's
there in significant quantities,
same with boron. Detection limits for
TEM tend to be a little bit better, it
is possible to
get sort of trace element analysis
sometimes. Detection
limits are approximately the peak height,
equal to three times the standard
deviation of the background.
So if you have a very noisy background
you're going to run into sort of more
problems.
So this is why longer collection times
are better
for having better detection limits.
And you can have different variations
based on the x-ray lines,
what sort of elements are around.
So you will have different problems
with
under light element in heavy elements
so that can be an effect on things.
Accuracy will really depend on sort of
what sort of spectral processing you're
doing,
and corrections to the raw data. And
I'm not going to talk about it a lot but
You can use standards
to actually improve
quantitative detection of elements.
Reproducibility is basically based on
you know the statistics involved in
things,
more x-rays you have a more reproducible
data set.
Sample preparation, you know you
have to consider things like size,
can you fit it in so you can look at the
regions of interest,
can you go to the working distance, you
don't want it moving around,
you want to have it conductive, so you
want to have a pathway to ground.
Quite often carbon is the preferred
coating for EDS.
We have to worry about what effect
sort of heavier metal coatings have on
absorption of
sort of low energy x-rays. Carbon
though can cause
problems if you don't also correct
for it,
so there is a correction you can use for
coatings,
not always great for doing
the heavy metal ones though. You have to
make sure your sample is representative,
so you don't want to alter it with
something like etching and actually
remove
material or form oxides on the surface.
Clean, you want to have the organics
removed,
you don't want to be able to see any
sort of fingerprints.
I have seen people analyze things and
I had to tell them afterwards that you
analyzed fingerprints not
anything on your sample. You also have to
be careful about the solvents you use
sometimes,
acetone has a tendency to sort of
redeposit
something, so sometimes you're better to
use a higher quality
solvent for cleaning things.
Carbon, unfortunately pretty well
everything on
on the planet will have carbon on them.
So and also the vacuum systems you have
within
SEMs and TEMs are not perfect
so you can
quite often detect carbon even though
there is absolutely no
carbon supposed to be in the sample.
You can do things to improve this,
sometimes you might plasma
clean the sample, it's something that's
quite often done
in the TEM. Biological material because
we are looking at pretty well
everything with EDS we have to,
in order to avoid sort of charging or
change of structure
effects, we need to make sure it's
dehydrated,
if we're doing high vacuum sort of work.
But you know there are some instances
where you can do
low vacuum or ESEM of biological
materials.
One thing that's sometimes used now is
conductive liquids.
Also we need to kill off usually the
biological material, so we might have to
fix it
and sort of attach it to sort of
surfaces.
Conductive staining is sometimes used,
so
we might be adding a heavy metal
to things, like osmium or we might be
doing labeling of things, like with gold
nanoparticles.
So and we may be looking at EDS for
actually where these things have stained
or
where the labeling is to identify sort
of structures.
Now when we're talking about
quantitative analysis,
we want to make sure that the sample is
polished flat and the polish
depending on what sort of accuracy that
you need
might have to be down to the
sort of 50 nanometer
root mean squared
surface conditions, so you might have to
have an extremely fine polish
to really get good quality
quantitative analysis.
Now we have been talking about
qualitative
which is basically where we're just
identifying
the elements with the 
characteristic x-rays so there's all the
peaks that are there.
One thing to always do is to
not accept what the software is telling
you.
Identification of elements
is not a hundred percent accurate,
elements can be
missed or incorrectly identified.
You will have more problems, usually
the broader the peaks are, so if you do
have a
very low process time that sometimes
you get more problems. Also there is a
problem sometimes with
a peak overlap, so there are certain
peaks from different elements that do
overlap,
so we sometimes have problems
differentiating between those elements
or
a particular element might get buried
under another peak.
So usually if you're doing a qualitative
EDS but you are looking at something
that's unknown you want to start maybe
in the 15 to 20 kV
range so that you have enough
overvoltage for most sort of x-rays that
are produced.
You should do it before you do
quantitative EDS, just so you have a good
idea
of what actually is going on.
Quantitative EDS results in,
you can get sort of mass fractions or
weight percent
of the elements present in the sample.
The spectral process, they remove the
background x-rays, spectral artifacts,
and then they're compared with some data
that's been
measured usually from the factory
of the EDS manufacturer of reference
materials.
And those are used to actually determine
what the
quantitative results are,
this is referred to as
standardless
quantitative analysis. Some people
also refer to it as semi-quantitative
analysis, (cough)
excuse me. So 
which I would say now with
more sort of
better software and detectors
we're really I think on the older
systems you could maybe say it was
semi-quantitative,
but I think there's been quite a lot of
improvement in what you can do
that we can say that it's a quantitative
analysis
now. You can improve
the quantitative analysis if you
actually 
collect data from standards of similar
composition
on the same instrument, so this can
improve accuracy quite a bit,
but when you are doing
quantitative analysis
how homogeneous the material is
and its density and its topography will
have significant effects
on the analysis results.
So here we can see some EDS spectra
and along with this we we've also done
some quantitative
analysis. So we can see all the various
nice peaks,
so we have x-ray intensity, so here it's
in counts per second
per electron volt. You could also list it
as counts as well.
So you can see your background,
a nice little whale as it were of the
background radiation, and we've done it
with normalization the quantification, so
we've actually forced it to add up to
100 percent
for what's here. So
but some things that I will point out, so
in the
first spectrum here
this has been identified by the computer
as
phosphorus, it is actually not
phosphorus, it's actually one of the
peaks from
tungsten. It is also
identified that strontium is present,
strontium is not present in this
sample.
Even though it gives us saying there's
two percent strontium,
there's actually no strontium present.
Usually you can confirm things like
that by looking at
other peaks that are supposed to show up
for strontium.
Now sometimes you will run into the
problem
that due to the sort of sizing of the
spectrum
it doesn't label all the peaks and
things
even though you've asked it to, that happens. So down here we've properly
labeled the peak and we've got rid of
the
strontium peak, and you can see here
that we have a profile for what the
tungsten peaks their position, and also
their
relative height that normally exists.
Sometimes the relative height will
be affected if we're looking at
sometimes single crystal materials,
so that's something you have to be
careful of sometimes.
And here I've blown up with the quantas
so that you can sort of see, what it is
rather have to squint your eyes, thing down here. Oh and also it
didn't
identify the iron here, and we can see there is some iron.
Now we don't have to do the quantitative
analysis
normalized usually if we're going to
be doing quantitative analysis
where we're not doing normalization we
want to make sure that we
optimize the spectrometer.
Usually we use a calibration element,
the calibration for the spectrometer
itself usually
the companies use
manganese , I quite often use cobalt,
in the case of optimization it's usually
an element that's convenient to use, so
depending on the energy range we'll use
things like copper tape
or a piece of silicon wafer for that.
So why I'm showing this, is you can
see that we're not adding up to
exactly 100 percent here for the quantification,
also we're running into
an absorption problem here.
The absorption problem is the fact
that we have assumed
that the carbon coating is 10 nanometers
and we've corrected based on that,
but in fact the true value of the carbon
coating
on the sample is closer to 90 nanometers.
So we are getting a significant error
in the data for light elements and
then in this case we're talking about
oxygen, compared to what
we know as the certified values
for the mineral
that we're analyzing all of the.
So you can see if we have the 10
nanometer assumption this is the sort of
results we get
quantitative, and here closer to the
actual values,
manganese is off a bit, we may have to do
we may not have properly
corrected for sort of matrix
effects, so we may have to do some
standardization
to actually improve the sort of numbers
that we're getting for maybe the manganese
in this case.
Charging, so
a lot of people make the assumption
because
x-rays don't have any
sort of effect of
charging that charging isn't a problem,
but charging does affect the primary
beam.
So here we can see an example
of using a 15 kV accelerating voltage,
and you can see down here on our
spectrum
that we are not getting 
background x-rays above really
very much above 9 kV.
So we have actually decelerated the beam
due to charging and 
that is having the effect of what we're
looking at, so or maybe
you know have a 9 to 10
kV beam actually hitting
the sample and producing x-rays. So
that means that these peaks that we're
looking at for iron
are not optimal, so we may be getting an
incorrect number for iron.
Now another thing to consider if we look
at the actual sample
is that this is not a fully dense
material,
lots of holes, there's also lots of
topography,
so we're going to get some shadowing
and areas where we don't get
x-rays. So this is not 100 percent
dense, it's fairly rough
the surface, so that's going to have an
effect on our
quantitative analysis. So we may have to
do something like
crushing up the
material into powder and pressing a
pellet
to get a better reading of sort of
the
composition of the material. So
here we might have to sort of coat
it some more. It already does have a
carbon coating on it.
We might have to improve the
conductivity maybe painting closer to
the area of interest,
using low vacuum or ESEM, but you know at
the cost of
spatial resolution.
Mapping, something a lot of people
like to do
gives you nice colored pictures of
things.
I can assign different colors to
different things, so what we're
looking at here is sort of the x-ray
counts or concentration
of one element and a particular
shell of x-rays
over the area of the sample. So we could
do a smaller area,
here we're doing the sort of full field
of view that we're doing.
You don't want to do
mapping usually at fairly low
magnifications, we run into problems with
sometimes
some SEMs will have distortions, also we
run into
geometric problems just based on
sometimes you get
sample curvature
and issues like that.
So usually don't want to go
much lower than 50 times, but you may
have
some issues with your particular SEM so
you know ask about what sort of
magnification should sort of be the
minimum.
Now when we're talking about maximum you,
have to keep into
keep in mind this sort of spatial
resolution
of things. So there's no point going to
50 000 times in the SEM,
you're going to have
a lot of overlap
of data, so you know
that's not really the best thing to do.
So when we're
collecting maps, we're doing specific
time per pixel, we may have to limit it
if we're having
sort of a charging problem. We'll do a
certain number of frames, so
collection of the full field of the
field of view
multiple times, so scanning it and
when we're actually collecting this data,
if you look down here on the spectrum
we're using an energy window to
determine
what are the particular
number of counts of x-rays for that
particular element, and from that
calculating any sort of concentration as
well.
Now this can sometimes be a problem
based on what kV you're using,
in this case we're using enough
energy we're using 15 kV, so nickel
should be properly optimal,
so we should be getting good counts
there.
Line scans,
very useful 
thing, it doesn't take the time you
have to do for a map, so
maps you can you know,
might be doing if you got a really
modern system
over a very short period of time, but
sometimes mapping might be done up to
you know
tens of minutes, hours depending on how
much
data you need and what sort of things
you're looking at in the map, especially
if you're looking at things that are
fairly low concentration,
you're going to have to do longer scans with line scans and maps.
So it's important
to sort of consider what sort of
information you need,
whether a map will get you the sort of
information you want or whether a line scan
which takes less time
would be more beneficial. So same
thing,
x-ray counts and concentration.
The line actually represents a number
of data points, so you can set
how many data points you're going to be
doing
or the distance between data points
which you can
sort of keep in mind what might be your
sort of spatial resolution. So you'll
have specific
dwell times, so amount of time per point,
how many times you scan the line.
We use energy windows like we do for
mapping.
Now here instead of doing sort of the
counts
of x-rays like we did in the map, I've
actually converted to
atomic percent, so we can sort of have an
idea more
of what we're actually looking at so. And
here you can see there's a
a fairly high atomic percent of carbon.
This isn't because of the coating, this
is because this material is actually a
carbide material, so there is a lot of
carbon,
so may not have been so obvious
maybe some of the previous analysis, but
here you can see that.
And you can see the nice distribution
over the distance, so here we see in
microns
versus atomic percent, how it is across
the line.
Now this has also been processed a bit,
so there's been some smoothing,
we've also done what's called binning
and combine some pixels
or data points, as it were in this
material.
So coming to the end here,
so we'll probably stay a little bit
over time
to do some questions. I'm
just going to mention that there's a lot
of other processing you can do with
things, so
the map that I showed originally
we've actually overlaid
the different x-ray maps, so the blue
tungsten, yellow chromium,
and the red nickel, which because there's
lots of chromium
it's orange here, and you can see also
the
SEM image underneath, as well. So we've
basically mixed the images there. But
there are various other things you
can do within
software, the more modern the 
software the more things you
do, so corrections for coatings like I
showed you
in the one example, a correction
of spectral artifacts which we talked
about.
You can also do some background
subtraction,
or subtract or look at the differences
between spectrums, things like that.
You can do spectral processing for
correcting for overlapping
peaks. There's other sort of image
processing and analysis you can do.
Labeling, important to label things, so it
would be useful to label
the actual elements here and maybe
put a little chart as to what they are.
We can also use other programs for
doing things, so you might export your
data out
and do things. Or you might 
do some simulations, monte carlo
simulations
to figure out sort of where you're
actually collecting the x-rays from
to actually do something so
something. I will point out,
Goldstein's sort of the bible
of the scanning electron microscopy
and micro analysis.
The Australian MyScope website is
extremely good,
I've used that for a lot of the source
material. So thank you to them.
I would also like to put a thanks
out to Richard McLaughlin
from one of the EDS companies,
actually produces the Aztec software,
there's a lot of help and advice and
things like that.
So the casino website, so if you do want
to actually
do some calculations. NIST
down in the states produces a program
that's very useful, you may have to have some good
information about the SEM
for doing things like that, and TEM,
before you can really make
fully sort of utilization of the program
but it's very useful.
Also the people at NIST are very helpful
as well
for questions there. So
come to the end here, so
thank you for joining us and
please ask some questions. Now you can
also
send me an email
at butcher@mcmaster.ca,
I am willing to answer questions, you
know industry or academia or
general interest, no problem
answering questions about topics. Okay
so
let's have some questions.
Great, thanks Chris. So any questions
please put them in the chat. So first
question we have,
if we are trying to get accurate
quantitative analysis
do we need to worry about Zaff
corrections or does
modern software technology take care of
it well enough that it's no longer a
concern?
Yeah it's it does,
it is internalized but
zaff, on some systems it will allow
you
to switch between different systems of
correction,
we actually don't use zaff corrections
anymore,
there's a different model that's used
but it's been improved somewhat.
So it is better corrected but you're
still
dealing with, if you're doing
standardless,
not doing corrections for
matrix effects. So for instance
if you're doing standardless on our
systems
which are from oxford instruments,
the corrections for silicon are based on
running a standard
of silicon dioxide, which is not the same
material as silicon, I don't know why
they didn't use silicon,
the correction for oxygen is based on
the standard of silicon dioxide as well,
so it does create errors.
So polishing is really critically
important
for getting a nice surface, and
angle also does have an effect. And you
have to be at the optimum
position.
Great. You mentioned
that hydrogen and helium could be
detected by EDS,
could you elaborate more on that. You
just don't get x-rays.
They're too tightly bonded, the electrons, so
and you're not getting
any x-rays produced.
They have no outer shell electrons
to drop down
to the inner shell, so
there's no x-rays produced, we have to
have that
as it were knocking out for the
secondary
and that dropping down or relaxation
of the electron structure of the atom in
order to get x-rays produced.
Great, on a similar note and you sort of
just answered this, but
at the beginning of the lecture you said
that lithium
can be detected with the right equipment
and conditions,
what do you mean? Can you please explain
more?
Okay, so lithium is a very low energy
x-ray, so
in systems where you have windows
the x-rays from lithium are basically
don't get to the detector,
so you have to have a windowless system.
You also have to have the resolution of
the spectrometer,
so you might have to have a specific
spectrometer
that has high resolution in order to
collect the lithium.
Also lithium is something that
electron beams can sometimes cause
migration
of or loss, you can actually evaporate
a lithium
in the TEM, so you have to be
careful with the electron beam
on things. So you have to have proper
sort of
instrumentation, you have to have proper
conditions. You may see over time
that actually the amount of lithium
decreases
depending on what lithium compound
you're looking at.
Okay, this is more kind of a specific
question.
What would be the average interaction
volume
from EDX say at the accelerating voltage
of 15
kV? It's dependent on material
and density. You can do a monte carlo
simulation to calculate that.
There are some formulas that you can
look up for things.
For instance to sort of get an idea of
that, sometimes you'll see
charts that'll give you an idea of
things. It becomes more complicated
when you're dealing with things that are
not
single elements so mixtures of things.
