Transcript Chemical analysis by EDX

Nathalie Siredey-Schwaller | 09/01/2014
Chemical analysis with EDX
On Supra40
How to get good quality
quantitative chemical analyses
- Understanding the involved
physical phenomena
- Choice of conditions of analyses
- Artefacts and misinterpretations
- Quantitative mapping
- Case of light elements
Basic cares : sample preparation
•
Sample should be an electric conductor
– Good electric conductivity of mounting
– Good electric junction between sample and microscope : use of silver glue. Pay
attention to the release of solvent.
– Beware of oxyde layer at the surface of sample / cleanliness of the surface.
•
Surface of the sample : analysis depth is only a few microns
– Surface should be out of contamination / clean
– A precise quantification needs a flat and horizontal surface (phirhoZ method).
• Measuring beam current : with a Cu tape put near the sample.
This allows to measure the system factor. Sample spectra could then been compared to
standards spectra.
Consequences of bad-electrical conductivity
Deviation of beam
Before recording
spectrum
After 5 min analysis
Chemical signal changes
Duane-Hunt limit should be
identical to primary electron
energy
Conditions for acquiring EDX signal
•
Position of sample
26
25.5
signal
25
intensity
(kcps) 24.5
24
EDX detector
23.5
7
8
9
10
working distance
11
Sample
WD = 8.5 to 9 mm, magnification > 300
•
No object between sample and detector
Détecteur
Echantillons
No analyses
inside the
hole
Sas d’entrée
•
Infra-red camera is « off » : it induces noise signal
How to choose voltage and beam aperture ?
• Voltage has to be chosen according to chemical elements expected
Enough energy to eject an orbital electron of the element  electron reorganization and emission of characteristic X-rays : K or L, (M) spectra
The efficiency is the best for Eprimary beam = 2*Eionization
– Analysis of light elements with small voltage (better signal/background ratio)
– The highest-Z element to measure determines the voltage.
– The voltage is more specially chosen according small content amount
elements
– Massic absorption coefficients for K spectra are the better known.
–  Usual values are15 keV, 20 keV
• Beam current has to be chosen according to detector
settings
– Detector with high spectral resolution only accepts small intensity signal
– Mapping needs good X-ray statistic per points, so high intensity signal
Measuring beam current / intensity of the signal
•
•
•
You have to do it in order to be able to compare your sample to standards.
Measuring System Factor SF
Conditions of measurments : on copper :
– Focus to the good working distance
– Tuning of the primary electronic beam : deviation (wooble), stigmatism.
– Check the tilt angle (must be 0°and not 70°)
– Choose the right device settings for the detector
– Calibrate. Check if spectral position of Cu-Ka peak is OK.
Peak maxima match
with color element
lines
– Once this is done, nor the focus, nor the electron beam should be changed.
Choice of device settings
•
•
•
Detector has dead time, during which X-ray signal cannot be count. For good
statistics, dead time shall not to exceed 10%
60 kcps setting = better spectral resolution, higher dead time ( less
acceptable intensity)
275 kcps setting has less spectral resolution but accepts more signal.
 use of 60 kcps (40 keV) setting when peaks are close together, or overlap
 use 275 kcps (20 keV) when high statistic are needed (mapping)
60 kcps
275 kcps
Conditions of spectra acquisition
•
•
Analysed area should be homogeneous
What is a point spectrum ?
– Existence of an emitting volume
rho (g/cm3)
E0 (keV)
z max (µm)
2
(Al2O3)
15
3.3
5
(Fe3O4)
15
1.3
8
(Fe)
15
0.8
20 (Pb, Au)
15
0.3
Theses phases cannot be analyzed
separately
Size of analyzed particle is about 1
micron. At 25 kV, part of the Al matrix
actually contributed to the signal.
Acquisition of a spectrum
•
Acquisition time :
–
–
–
•
higher is, better is signal / background ratio. X-ray emission
is a statistic phenomenon : relative error is proportionnal to
1/√N.. Better is 106 counts.
Small as possible if existence of C-contamination of the
sample or if sample is a bad electrical conductor.
When mapping, time spent on one point should be as less
as possible  use of high beam current
Identification of elements :
–
–
–
–
An element is identified by all characteristic X-rays. Color
lines indicate the relative intensities.
All peaks should be explained
Beware of peaks overlapping
Beware of artefacts due to detector saturation : 2 photons
E1, E2 are count together as one photon E=E1+E2. Exists
in case of high signals.
Overlapping of peaks
Surface C, O
contamination
peaks
Double-photon peak
Quantitative analysis of a spectrum
Two possible methods :
• P/B ZAF method, without standards
• Phirho(Z) method, with standards. Better in case of
special conditions
– Open library with the same device settings
– Load the method of analysis
– Quantify
• Check quantitative analysis
– Built spectrum should be identical to real spectrum
– Total unnormalized massic sum should be equal to 100 %
• Quite good quality analysis if result is between 96% and 104%
• If result is less then 100% : maybe some elements have been forgotten
P/B ZAF method
• ZAF :
– Z = « atomic number » effect
Ionization cross section, ability to stop the electrons according to the material.
– A = absorption
Depends on X-ray energy, density, depth of emission
– F = fluorescence effect between elements
One photon emitted by element A is absorbed by element B, inducing fluorescence. Photon from
A is lost, one photon from B is observed.
• Intensity of X-ray characteristics peaks is compared
to intensity of background
 no standards are needed
F(rz) method
• The function f(rZ) is calculated. This function describes
emission of photons X, for a given material. It depends on :
–Density r of the material
–Depth Z, from where the emission occurs
Z and A are simultaneously determined. It is then obtained curves,
describing the emitted and emergent signals, according to the
depth.
• Fluorescence between elements is a phenomenon distinct
from emission. Its calculation is similar to the previous case (P/B
ZAF).
•
Analyze with standards :
For each elemental X-ray characteristic peak, sample signal is
compared to standard signal (known composition or pure standard).
“k” is a corrective term from Z.A.F. corrections. “k” depends of amounts
of all the elements existing in the sample and is only obtained after an
iterative process.
Csample
Cs tandard
kratio 
k
I sample
I s tandard
I sample
I s tandard ( pure )
Perfect match between real
spectrum and built one
Except for these energies
(C contamination)
Total sum is almost 100%
Loading of the standards
- Choice of the method : triangle near « quantify ».
- May be loaded from recorded files, may be created, may be saved.
The objects
•
One special point may be choose (beware of emitting volume !)
•
Lines may be analyzed
– Acquisition :
• Choose the number of points in the line. Distance between points should be higher than
size of emitting volume.
• Thanks to the global spectrum, determination of all the chemical elements.
• Determination of the measuring time. According to the signal intensity. At least egal to
number of points x 20 000 counts.
• Possibility of increasing width of the line
– Quantification : choose the appropriate method.
– Saving : in the project or in a file .txt, readable by Excel.
Indicates the intensity
of signal = number of
counts per second.
Use this indication to
determine the minimum
analysis time on each
point
The « hypermap » object
•
The aim is to record a mapping of a sample area. This mapping may be
subsequently analyzed.
•
Acquiring a mapping :
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–
–
–
–
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Use of a high beam current in order to reduce analysis time. Consequently, use
appropriate detector settings (130 kcps, 275 kcps)
Determination of the total points number
Determination of the total analysis time= nber of points * 20 000 counts
Select the chemical elements to display.
To begin, click on « Acquire » (parameters = right-hand triangle). To stop, click on the
same button. The mapping is realized with a lot of quick scanning. This allows to reduce
C contamination, charge effect. The mapping becomes more and more precise with time:
signal / background ratio increases.
Save the mapping : in a separate file : .rtb or .bcf, usually quite big . In this file are
recorded the full spectra of each point.
Quantitative analysis of mapping : Qmap
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May be time-consuming, so to be done as a post-process.
Select the quantification method (triangle near « Qmap »).
To begin : click on « Qmap ».
Results may be displayed on maps, false color maps, or recorded on “.txt” files (1 for
each elements)  readable by Excel
Qualitative mapping before quantitative analysis
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•
•
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This mapping was realized on Ka-Si
peak energy
This is not a Si-content mapping: at
this energy, some others peaks
overlap
Also beware, in qualitative mapping,
of variation in background signal
A
quantitative
mapping
is
mandatory
N1 = Number of counts / second
N2 = Number of points / line
Minimum = 100 points
S = Size of the image = HxV
N2 = should be chosen to be H / d
d = distance between two points (1,2
or 3 microns).
N3 = (H/d) x (V/d) is the total number
of points in the mapping
Time of the mapping, for 20 000
counts per points is (in seconds) :
N3 x 20 000 / N1
Chemical analysis including one light element
•
Preliminary considerations :
Detector should have a good spectral resolution
Characteristics X-rays of light elements have small energy. In this range, there is C contamination,
usually of high number of X-ray characteristics peaks, bad transmission of the detector window. In
order to increase signal/background ratio, primary electron energy should be small, which is
usually incompatible with the other chemical elements
•
The best method is to measure out by difference, once all other chemical elements have been
analyzed.
This needs a careful analyzing of the other elements
– PhirhoZ method with standards
– Very good standards (at least 1 million counts standards)
– Very good statistics of the spectrum (at least 1 million counts standards)
– Check on an area not containing light element that total unnormalized sum is 100±1 %.
Select « measure out by difference » inside the method