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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
K-shell
incident
electron
L-s hell
M-shell
K emitted
X-ray
ejected
electron
scattered
electron
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
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 shape
K and Kcome
together as a pair
L-lines
Often occur in groups of
three or four lines so
shape can vary
Can overlap K-lines
Important 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
1.0
K
0.5
L
M
0
0
20
40
60
ATOMIC NUMBER
80
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
Window
Bias
PIN diode
Capacitor C
Xray generates
electron/hole pairs
(3.6eV / pair)
Voltage=Q/C
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
Lens
aperture
Chamber
wall
EDS
sample
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.
Detector position
20 0000
Count Rate
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 S4700
Count rate vs Working Distance
35 degree TOA @20keV
10 0000
0
0
10
20
Working Distance (mm)
30
Deadtime
If N puls es are proces s ed/s ec
and each takes then
Dead time N
Live time 1 N
N
Fractional los s
1 N
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
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
Acquisition
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 peak? - 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
1x?
2x
5x
10x
10x
Visibility and peak height
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 2 to 3 larger than
the background to be
considered valid
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 possible
Since P/B (peak to background) rises with beam
energy use the highest keV possible
Set process time for highest detector energy
resolution
Maximize take-off angle where possible
Minimize 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 times
X-ray generation in silicon
at 3keV
Microanalytical Performance
1
Counts/pA/sec
K lines are better
than L lines. M lines
are lowest in yield
Beam energy will
determine which
elements can be
analyzed
10
Si K-line
.1
Cu L-line
Au M-line
.01
0
5
10
Energy (keV)
15
Elements accessible to X-ray Microanalysis at 10keV
K-shell
E0 = 10 keV
U0 > 1.25
H
Li Be
M-shell
Not detected
Na Mg
K Ca Sc Ti
Rb Sr
L-shell
Y
B
He
C
N
O
F
Ne
Al Si
P
S
Cl Ar
V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Zr Nb Mo Tc Ru Rh Pd Ag Cd In
Cs Ba La Hf Ta W Re Os Ir
Sn Sb Te I
Xe
Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra Ac
Ce 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
Elements accessible to X-ray microanalysis at 5keV
K-shell
E0 = 5 keV
U0 > 1.25
H
Li Be
M-shell
Not detected
Na Mg
K Ca Sc Ti
Rb Sr
L-shell
Y
B
He
C
N
O
F
Al Si
P
S
Cl Ar
Ne
V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Zr Nb Mo Tc Ru Rh Pd Ag Cd In
Cs Ba La Hf Ta W Re Os Ir
Sn Sb Te I
Xe
Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra Ac
Ce 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
Practical Problems for Low
Energy EDS
Number of Lines
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!
70
K-lines
L-lines
M-lines
60
50
40
30
20
10
0
2keV
4keV
6keV
8keV
Energy
Distribution of X-ray lines as a
function of spectral energy
10keV
Microanalysis Summary
Characteristic X-rays, Mosley’s law
Fluorescent Yield
Deadtime
Count throughput
Reading the spectrum
Detectable limits
Microanalytical Performance