Download - X-ray Microanalysis
X-ray Microanalysis
The fluorescent production of X-rays by electrons is one of the most important interactions available in the SEM because it permits chemical (atomic) identification and quantitative analysis to be performed
About 60% of all SEMs are now equipped for X-ray microanalysis
Characteristic X-rays
Characteristic X-rays are formed by ionization of inner shell electrons. The inner shell electron is ejected and an outer shell electron replaces it. The energy difference is released as an X-ray
incidentelectron
scatteredelectron
ejectedelectron
K -emittedX-ray
K-shell
L-shell
M-shell
X-ray peaks
The characteristic X-ray signals appear as peaks (‘lines’) superimposed on the continuum, These peaks have fixed energies
Mosley’s law
Mosley showed that the wavelength of the characteristic X-rays is unique to the atom from which they come
This is the basis of microanalysis
Wavelength and Energy
Å 12.26EkeV
X-rays can be identified by either their wavelength or by their energy E
These two quantities are related by this relationship, so either can be used
Mosley’s law
K-lines come from 1st shell (1s)
L-lines come from 2nd shell (2s)
M-lines come from 3rd shell (2p)
Each family of lines obeys Mosley’s law
K-lines
K-lines are the easiest to identify and highest in energy
Gaussian shapeK and Kcome
together as a pair
L-lines
Often occur in groups of three or four lines so shape can vary
Can overlap K-linesImportant for analysis
of elements Z>40
Silver L-line cluster
M-lines ... and N- and O - lines
are very complex Not all lines are
shown on all analyzer systems so check with standards if in doubt
Avoid use if at all possible! However at low energies they must be used. Lead and gold are best analyzed with the M lines
Fluorescent Yield
Not all ionizations produce X-rays
The fractional yield (the fluorescent yield) is called
varies rapidly with atomic number Z and is low for low Z
0 20 40 60 80
ATOMIC NUMBER
1.0
0.5
0
K
L
M
Measuring X-rays
Wavelength Dispersive Spectrometers measure by diffraction from a crystal. Accurate but slow and low sensitivity
Energy Dispersive Spectrometers measure photon energy. Fast, convenient, good sensitivity but has limitations in energy resolution
The Energy Dispersive Spectrometer
A solid state device - Si(Li) P-I-N diode
Converts X-ray energy to charge. The output voltage step is exactly proportional to the deposited X-ray energy
Measures the photon in about 100microseconds so can process 1000 or more photons/second
BiasWindow
PIN diode
Capacitor C
Voltage=Q/CXray generates electron/hole pairs (3.6eV / pair)
Charge ~ Xray energy
The EDS detector The cryostat cools the
pre-amp electronics and detector diode
The window protects the detector from the SEM vacuum, BSE, and visible light
Beware of ground loops, noise (TV monitors) , lights in the chamber (the ChamberScope !)
System peaks
X-rays are also produced by electrons hitting the lens, the aperture and the chamber walls.
To keep these system peaks to an acceptable level a collimator must look at the point where the beam hits the surface.
EDSsample
Lens
aperture
Chamber wall
Detector position
The working distance must be set to the correct value in order to maximize count rate and minimize the systems background
12 mm in the S47003020100
0
100000
200000
Count rate vs Working Distance 35 degree TOA @20keV
Working Distance (mm)
Cou
nt
Rate
Deadtime
Processing and displaying pulse takes some finite time
MCAs (multi-channel analyzer) only handle one pulse at a time so some pulses will be missed
This ‘deadtime’ must be allowed for in quantitative analysis
If N pulses are processed/sec
and each takes then
Dead time N
Live time 1 N
Fractional loss N
1 N
How much deadtime?
Deadtime increases with count rate (beam current and energy) and process time (set by operator)
Values greater than 25% may allow 2 or more pulses to hit detector at same time giving ‘sum’ peak.
Values >50% waste time and may cause artifacts
MCA parameters
During spectrum acquisition the operator has control of a variety of parameters
The most important of these are the beam current, which controls the input count rate, and the pulse processing time
The processing time must be set with care to achieve optimum results
Count throughput
For spectra choose a low count rate, and a long process time to give best resolution
For x-ray mapping choose the highest beam current and the shortest process time to give highest throughput
Resolution
The spatial resolution and depth penetration of a microanalysis is set by beam energy and material
Typically of order of 1 micron but can be much less if E is close to Ecrit
Monte Carlo models are a valuable aid in understanding the lateral and depth resolution of X-ray microanalysis
Reading the spectrum
GOLDEN RULE - identify the highest energy peaks first
Then find all other family members of this peak i.e the L,M lines
Then identify the next highest energy peak
If a peak cannot be identified..
Is it a sum peak ? (look for dominant peaks at lower energies, one half of the energy.)
Is it an escape peak ? (look for a strong peak 1.8keV higher in energy)
Is the system calibrated properly? Is it really a line? - is it of the right width,
does it have the right shape, are there enough counts to be sure ? How would we know?
Detectable limits
For an X-ray line to be statistically valid it must exceed the noise (randomness) in the corresponding background region of the spectrum by a suitably large factor
Rule of thumb the peak should be twice the background to be considered valid
2x5x
10x
Visibility and peak height
10x
1x?
Counting statistics
The signal is equal to the peak integral - background
Poisson statistics apply to the data so the noise estimate = (background)1/2
For 95% confidence we need the signal to be 3 standard deviations over noise
Peak>3.(background)1/2
Peak
Background
Integral
ENERGY
Bkg =25 Peak>15Bkg =100 Peak>30Bkg =250 Peak>50
i.e minimum size of peak falls as fraction of BKG as count rises
Detection limits
This statistical limit determines the lowest concentration of an element that might be detectable (MDL - the minimum detectable limit)
For an EDS system this is typically in the range 1-5% depending on the overall count acquired in the spectrum and on the actual elements involved
Optimizing MDL
Count for as long as possibleSince P/B (peak to background) rises with
beam energy use the highest keV possibleSet MCA process time for highest detector
energy resolutionMaximize take-off angle where possibleMinimize system peaks, spurious signal
Trace detection ?
EDS is not a trace detection technique - needs a 10x improvement to achieve even parts per thousand level
But minimum detectable mass (MDM) is very good (10-12 to 10-15 grams) for this technique
Best with inhomogeneous samples
Low Energy Microanalysis
The reduction in interaction volume makes possible high spatial resolution microanalysis even from solid samples
Lower cps and lower dead timesX-ray generation in silicon at 3keV
Microanalytical Performance
Count rates are lower than at conventional beam energies
K lines are better than M lines. L lines are lowest in yield
Beam energy will determine which elements can be analyzed
151050.01
.1
1
10
Si K-line
Cu L-line
Au M-line
Energy (keV)
Cou
nts
/pA
/sec
H He
Li Be B C N O F Ne
Na Mg Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I
Ba
Xe
La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra AcCe Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
E0 = 10 keV
U0 > 1.25
K-shell
L-shell
M-shell
Not detected
Cs
Elements accessible to X-ray Microanalysis at 10keV
H He
Li Be B C N O F Ne
Na Mg Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra AcCe Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
E0 = 5 keV
U0 > 1.25
K-shell
L-shell
M-shell
Not detected
Elements accessible to X-ray microanalysis at 5keV
Practical Problems for Low Energy EDS
All available lines are in 0-3keV range
There are more than 60 elemental lines between 0 and 2keV, and more than 30 between 2 and 4keV
Spectrometers with better than 30eV resolution are needed!
0
10
20
30
40
50
60
70
K-linesL-linesM-lines
Nu
mb
er
of
Lin
es
Energy
2keV 4keV 6keV 8keV 10keV
Distribution of X-ray lines as a function of spectral energy