Transcript Document 7340674

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 Kcome
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