Eds On Sem Primer
Eds On Sem Primer
Eds On Sem Primer
This primer is intended as background for the EDS Analysis on the SEM course offered by the
University of Minnesotas Characterization Facility. You must learn this material prior to the hands-on
training sessions. It is also assumed that you have a working familiarity with the content in the
Scanning Electron Microscopy Primer. Good sources for further information are: Scanning Electron
Microscopy and X-Ray Microanalysis by Joseph Goldstein et al; and, X-ray and Image Analysis in
Electron Microscopy by John J. Friel.
Initial EDS analysis usually involves the generation of an X-ray spectrum from the entire scan area of
the SEM. Below is a secondary electron image of a polished geological specimen and the corresponding
X-ray spectra that was generated from the entire scan area. The Y-axis shows the counts (number of X-
rays received and processed by the detector) and the X-axis shows the energy level of those counts. The
EDS software we have, Noran System Six (NSS), is quite good at associating the energy level of the
X-rays with the elements and shell levels that generated them.
keep the electron beam stationary on a spot or series of spots and generate spectra that will provide
more localized elemental information.
Obviously all of these constraints are important to understand; however, spatial resolution of the signal
is often surprising to newcomers. The units of spatial resolution are microns -- not nanometers. This is
due to the fact that X-rays are generated from very deep in the interaction volume. Also, it is common
to use intermediate (1520 keV) accelerating voltages to ensure the peaks one wishes to record. The
size of the interaction volume icreases with accelerating voltage. EDS is certainly not a surface analysis
technique. It doesnt take much magnification in the SEM to reach the point where the pixel size on the
specimen approaches this dimension.
You may want to consider Wavelength Dispersive Spectroscopy if you are in need of better: limits of
detection (30300 ppm; 1% wt%); performance for light elements; energy resolution (10 eV [FWHM]
at Mn K); precision and accuracy.
Characteristic X-rays result when the beam electrons eject inner shell electrons of the specimen
atoms.
Continuum (Bremsstrahlung) X-rays result when the beam electrons interact with the nucleus of
the specimen atoms.
Characteristic X-rays reveal themselves as peaks imposed upon a background of Continuum X-rays.
The energy and wavelength of an X-ray are related by the following equation: (nm) = 1.2398 / E
(keV). The most energetic continuum X-rays will have the minimum wavelength, termed the Duane-
Hunt limit. The Duane-Hunt limit is the energy value where the X-ray continuum background goes to
zero.
The intensity of the continuum background increases with probe current, atomic number and
accelerating voltage.
The most probable transition when a K-shell vacancy is created is the L to K transition, because these
are adjacent energy shells. Therefore K radiation will always be more intense than K radiation. It also
follows that K radiation will be of higher energy than K radiation, in as much as the energy difference
between the M and K shells (K radiation) is greater than the energy difference between the L and K
shells (K radiation).
To ionize an atom, the incoming electron or ionizing radiation must possess a minimum amount of
energy. That energy is the binding energy of the particular inner shell electron, which is a specific,
characteristic energy for each electron in the atom. The binding energy for a K-shell electron, for
example, is greater than that of an L-shell electron, since the K-shell electron is closer to the nucleus and
more tightly bound. Therefore, if sufficient energy exists in the incident beam to excite K X-rays, L and
M X-rays will also be excited if these shells and the one above them are occupied. Likewise, if
sufficient energy exists in the incident beam to produce K X-rays, K X-rays should be produced as
well.
The following common families of lines can be used by the microscopist in peak identification:
K : K = 10 : 1
L : L1 : L2 : L = 10 : 7 : 2 : 1
M : M = 10 : 6
For example, if a K line is identified in a spectra then a K line should exist as well and have
approximately one tenth the counts of the K line.
The beam energy necessary for ionization is always slightly greater in energy than the corresponding X-
ray emission line and is termed the critical ionization energy. In practice, one must exceed this critical
ionization energy by a comfortable margin, preferably by a factor of 1.5 to 3, to efficiently excite the X-
ray line with an electron beam. The term for this factor is overvoltage.
Moseleys Law
The energy of the characteristic radiation within a
given series of lines varies monotonically with
atomic number. This is Moseleys Law:
light elements
will emit X-rays
of the K series
only;
intermediate
elements will
emit X-rays of
the L series or K
and L series;
heavy elements
will emit X-rays
of the M series
or L and M
series.
Peak to Background Ratio (P/B): The most important factor in determining the limits of detection in
EDS analysis is the presence of the continuum background [1].
This equation would seem to imply that the best P/B would occur at the highest accelerating voltages.
However, very high accelerating voltages not only decrease spatial resolution, they also increase the
absorption of X-rays within the specimen before they are measured by the detector (more on this below).
An overvoltage of 1.5 - 3 is a good compromise.
The Figures [4] below show the spatial resolution (distance from incident beam from which the signal is
derived) and depth of signal for the elements Aluminum and Gold at the accelerating voltages of 5 kV
and 15 kV. Take some time to put the relationships together between: incident beam energy;
accelerating voltage; and, atomic number. Or better yet download a free electron flight simulator
program such as Casino[5] and experiment on your own.
The important point here is that the voltage pulse produced is proportional to the energy of the
incoming X-ray photon.
an X-ray Detector which detects and converts X-ray into electronic signals;
a Pulse Processor which measures the electronic signals to determine the energy of each X-ray
detected; and,
a Multiple Channel Analyzer which displays and interprets the X-ray data.
4. Crystal: The material used for the crystal is silicon (Si), into
which is drifted lithium (Li) to compensate for small levels of
impurity. When an incident X-ray strikes the detector crystal its
energy is absorbed by a series of ionizations within the
semiconductor to create a number of electron-hole pairs. An
electron-hole pair is created for every 3.76 eV of incoming X
radiation. Thus, for example, a Ni K X-ray photon (7,471 eV)
will produce a current of 1,966 electrons. The electrons are
raised into the conduction band of the semiconductor and are
free to move within the crystal lattice. When an electron is
raised into the conduction band it leaves behind a hole, which
behaves like a free positive charge within the crystal. A high
bias voltage, applied between electrical contacts on the front face and back of the crystal, then sweeps
the electrons and holes to these opposite electrodes, producing a charge signal, the size of which is
directly proportional to the energy of the incident X-ray.
Pulse Processor
The signal (voltage step) from the preamplifier is transformed
into a voltage pulse that is suitable for the multi channel analyzer.
Shaping and noise reduction of the signal are achieved by digital
computation. The noise on the voltage ramp from the detector is
effectively filtered out by averaging the signal. The time over
which the waveform is averaged is called the process time (Tp).
Tp is under control of the operator. The longer the Tp, the lower
the noise. If noise is minimized, the resolution of the peak
displayed in the spectrum is improved, and it becomes easier to
separate or resolve, from another peak that is close in energy.
By controlling the rate of water pouring into the bucket (spot size) you are controlling deadtime.
The analogy breaks down concerning why the output rate would actually decrease after a given input
rate and not be constantbut hopefully you get the point.
The MCA takes the data from the pulse processor and displays it as a histogram of intensity (number of
counts) vs voltage. The voltage range (for ex., 20 keV) displayed on the x-axis is divided into a number
(1024, 2048, etc) of channels each corresponding to a given energy range (for example, 5,280 eV 5,300
eV). The MCA takes the peak height of each voltage pulse, converts it into a digital value, and puts it
into the appropriate channel. Thus a count is registered at that energy level.
Spectral Artifacts
There are a number of artifacts possible with EDS, but most of them are related to detector electronics
and are rarely seen in a properly functioning system. Two artifacts that are commonly seen are pulse
pileup (sum) peaks and escape peaks [4]. We can choose to show/not show these with our NSS software
Accelerating voltage:
One must exceed the critical ionization energy of the element(s) of interest by a factor of 1.5 to 3 to
efficiently excite the X-ray line(s) with an electron beam. Exploratory analysis is often conducted with
an accelerating voltage in the 1520 keV range since a broad array of elements will be detected. An
addition spectrum collected at 5-10 keV could help to avoid missing low atomic number elements at low
concentrations. The accelerating voltage can subsequently be tailored to the elements and shell levels of
your specimen. Keep in mind that accelerating voltage and atomic number are two factors that
determine the spatial resolution of and depth of signal from your specimen.
Process time:
There are a range of available process times to select from
on the NSS software. These are referred to as Pulse
Processor Time Constants and can be found in the
Acquisition Properties dialogue box. The range is: high
resolution/low throughput (65 us time constant) --high
throughput/low resolution (4 us time constant).
Probe current:
The probe current can be adjusted to maximize the throughput at a given time constant for a given
specimen. Deadtimes in the 30-50 % range should work well. Remember that changing probe currents
will necessitate realignment of the microscope.
Three broad categories are used when referring to the concentrations of elements present in a sample:
1. Major Components - More than 10 wt %;
2. Minor Components - 1-10 wt %;
3. Trace Elements - Less than 1 wt %.
The apparent weights of peak members in a family provide an important source of information in
identifying elements.
Begin with the most intense line towards the high-energy region of the spectrum where lines within a
family are well separated. If it is above 3.5 keV, it must be either a K or L line. Using the KLM
markers, compare the location of the peak to that of the marker. Check the relative intensities. If you
identified a K line, then the K line should be about 10% of the K intensity. K and K lines are
typically resolved at sulfur and above. Below that, K is so small that K and K collectively show
themselves as one symmetrical peak. If a K line is identified, look for L lines, if they occur for that
element. This will aid in identifying the lines at low energy later.
If the line chosen does not correspond to a K line, try an L series. If the line is identified as the L,
several other lines both above and below the L should be present and correspond to the element.
The EDS system should be able to display them at their correct relative intensities. M lines do not
exist above 3.2 keV, so one would not look for them at this point.
Characterization Facility, University of MinnesotaTwin Cities
While working with the most intense line, look for escape and
sum peaks. If they are not found for this line, they are unlikely
to cause interferences. If they are present, keep looking for
them after other identifications.
After all major and minor peaks are located, trace elements can be attempted. In this case, the
problem is to confidently find a peak above background. Only the major peak of a series will be
visible, and that may be lost in the background. If a peak cannot be identified with some certainty,
the first step is to collect more counts. If it still cannot be confirmed, it is either not present or
present below the limits of detectability and WDS is warranted.
Measured intensities from the specimen need to be corrected for a host of matrix effects (ZAF):
Two factors must be considered regarding atomic number: the backscatter coefficient and stopping
power. The backscatter coefficient increases with atomic numberleading to the premature loss of
beam electrons prior to ionization resulting in X-ray production. The rate of energy loss due to inelastic
interaction increases with decreasing atomic numberleading to the same result. These two factors
tend to cancel one another.
Absorption is usually the biggest factor that must be considered in the measurement of composition by
x-ray microanalysis. As an X-ray travels through the sample, it may be absorbed, giving up its energy
entirely to an electron and ejecting the electron from its orbital. The probability that an X-ray will be
absorbed depends on its energy and the energy with which the electron is bound to its nucleus. The
probability of absorption increases as the X-ray energy approaches this binding energy from above and
reaches a maximum when the
X-ray energy is just greater
than the binding energy. At
this point there is a
discontinuity (absorption
edge) in the probability curve.
Lower energy X-rays no
longer have sufficient energy
to overcome the binding
energy and the probability of
absorption drops to a lower
value. The probability of
absorption then increases
again as the X-ray energy
approaches the binding energy of a more loosely bound electron. An absorption curve [1] for a given
element includes an absorption edge for each electron shell.
As X-rays are generated deeper in the specimen, progressively greater fractions are lost to absorption.
The ratio of the measured X-ray intensity to the generated X-ray intensity at some position in the sample
is dependent on the: mass absorption coefficient; specimen density; and path length. The probability of
X-ray absorption as a function of path length through the sample is given by Beers Law:
When an X-ray is absorbed by a sample atom, the absorbing atom is left in an excited state. It
subsequently relaxes, emitting its own characteristic X-rays (secondary fluorescence). Since an X-ray
can be absorbed only in an interaction with an electron having a binding energy less than the energy of
the absorbed X-ray, the energy of the secondary fluorescence is necessarily less than the energy of the
primary X-ray. For example, in a Cu-Fe sample, Cu K radiation (8.04 keV) is of sufficient energy to
excite Fe K radiation (Kab = 7.11 keV). As a consequence, the measured iron intensity would be
enhanced due to fluorescence by copper, while the copper intensity would be suppressed due to strong
absorption by iron. In practice, secondary fluorescence is only significant if the characteristic energy is
within approximately 3 keV of the critical ionization energy. The fluorescence effect can be calculated
with sufficient accuracy and it is usually the least important of the three factors.
Obtain standards for each of the elements identified in the qualitative analysis. Standards must be
homogeneous at the microscopic level. Both specimen and standards must be flat polished with
scratches less than 0.1 um
Obtain the x-ray spectrum of the specimen and standards containing the elements that have been
identified in the specimen. All measurements for a given element, in both the specimen and the
standards, should be made at the same: Deadtime; spectrometer take-off angle; calibration;
resolution; beam energy; and beam current.
Process the spectra of the specimen and standards to remove the continuum X-ray background signal
so that the measured intensities consist only of the characteristic signal. The background could be a
significant fraction of the characteristic peaks for minor and trace constituents
Develop the x-ray intensity ratios (k value) using the specimen intensity Ii and the standard intensity
I(i) for each element present in the sample. The measured intensity ratios should be equivalent to the
ratios of mass or weight fractions:
In standardless quantitative analysis (what NSS terms without standards), the intensity that forms
the denominator of the k ratio, I(i) , is provided by calculation rather than direct measurement.
Typically the standards are derived from a suite of experimental standards measurements (a
standards data base) performed by the manufacturer and subsequently adjusted for the characteristics
of the local instrument.
This equation must be applied separately for each element present in the sample.