Transcript High Sensitivity EPMA, Past, Present and Future

High Sensitivity EPMA:
Past, Present and Future
John Donovan
CAMCOR
University of Oregon
(541) 346-4632
[email protected]
camcor.uoregon.edu
The Past: EPMA/SEM (from Goldstein, et. al. 1988):
60 sec
Si K
Fe K
EDS
WDS
EDS
WDS
P (cps/10-8 A)
5400
40
3000
12
P/B
CDL(ppm)
97
1513
57
614
580
1,710
1,000
4,900
Comparison of EDS to WDS, Equal Beam Current, pure Si and Fe, 10-11 A
(0.01 nA), 25 keV
Peak cps
P/B
EDS
Na K
Mg K
Al K
Si K
Ca K
32.2
111.6
103.9
623.5
169.5
2.8
6.4
5.7
22.8
8.5
CDL(ppm)
1,950
1,020
690
720
850
WDS
Na K
Mg K
Al K
Si K
Ca K
549
2183
2063
13390
2415
83
135
128
362
295
210
120
80
90
90
Comparison of EDS to WDS, Optimized Conditions, 15 keV, 180 seconds counting time:
EDS : 2 x 10-9 A (2 nA) to give 2K cps spectrum to avoid sum peaks
WDS : 3 x 10 -8 A (30 nA) to give 13K cps on Si spectrometer (< 1 % dt)
More recent data for EPMA/SEM:
“The detection limit cannot
be reduced indefinitely by
accumulating more counts,
however, because systematic
errors in the background
correction eventually become
significant.
- Stephen Reed
Accuracy (not precision) in
characterizing the continuum
becomes the limiting factor...
4
Other Artifacts: “Holes” in the Continuum
3.2
2000
20keV, 100nA, 20um beam
SiO2 220 sec/point
TiO2 40 sec/point
2.8
2.6
1600
1200
800
Continuum Artifacts
2.4
400
2.2
0
29000
30000
31000
32000
33000
Ti K cps/nA
Ti K cps/nA
3
34000
PPM wt.
5
Spectrometer 2 LPET position (sin theta * 10 )
Spectrometer-Crystal
1-PET
2-LPET
3-LPET
4-PET
4-PET
Set 1, Ti Concentration Average (without blank correction)
-0.8
-12.3
-29.5
4.7
-3.4
Set 2, Ti Concentration Average (without blank correction)
1.2
-11.4
-29.7
7.3
-1.4
Set 1, Measured Deviation (1 )
5.8
2.3
1.2
6.5
2.3
Set 2, Measured Deviation (1 )
3.9
2.7
3.5
5.2
1.4
Set 1, Ti Concentration Average (with blank correction)
-0.6
0.5
1.6
-1.2
-0.6
Set 2, Ti Concentration Average (with blank correction)
3.5
2.3
1.2
4.0
3.4
Reality Check: Accuracy at the 400 PPM Level?
0.08
Refugio-17 Traverse 2
Ti Ka Spec 2 (LPET)
Ti Ka Spec 4 (PET)
0.06
Uncorrected for Blank Iteration
0.04
Ti Weight %
Ti Weight %
0.08
Corrected for Blank Iteration
0.06
0.04
0.02
0.02
0
400
800
1200
Relative Distance (um)
I corr  I unk
1600
0
400
800
1200
Relative Distance (um)
(Cmeas  Clevel ) [ ZAF ]std
 I std *
*
Cstd
[ ZAF ]unk
Note: Blank level (Clevel) can be non-zero
1600
Still… We Need to Improve Sensitivity as well..
0.0180
PET
0.0160
3 Sigma Reported Detection (wt. %)
Ti in SiO2, 20 keV, 200 nA, 20 um
Calc. from Love and Scott (1983)
Spectrometer 1 (PET)
Spectrometer 2 (LPET)
Spectrometer 3 (LPET)
Spectrometer 4 (PET)
Spectrometer 5 (PET)
Aggregate Intensity Option
0.0140
0.0120
0.0100
0.0080
LPET
0.0060
0.0040
0.0020
5 Spectrometers
0.0000
1
10
100
1000
On-Peak Integration Time (sec)
10000
0.1
Detection Limit (t-tests) (wt. %)
NIST SDD
Ti in SiO2, 20 keV, 200 nA, 20 um
Calc. from Goldstein, et al., (1992)
Spectrometer 1 (99% CI)
Spectrometer 1 (95% CI)
Spectrometer 1 (90% CI)
Spectrometer 1 (80% CI)
Spectrometer 1 (60% CI)
Aggregate Intensity (60% CI)
170 ppm
0.01
(normal PET crystal)
0.001
5 Spectrometers
1 ppm
0.0001
EDS is Dead!
1E-005
1
10
100
1000
On-Peak Integration Time (sec)
10000
WDS Analysis of Hg
(polymer door frames from suspected Mexican facility)
Checked with EDS
-count 200 sec
- several nA
-no Hg found
Checked with WDS
- count 20 sec
- 50 nA
- Hg found
TakeOff = 40.0 KiloVolt = 20.0 Beam Current = 50
Un
6 std-flex
Results in Elemental Weight
Percents
ELEM:
Hg
Pb
Cr
TIME:
240.00 240.00 240.00
AVER:
.09916 -.03815 -.00240
SDEV:
.07870 .06759 .00271
Detection limit at 99 % Confidence
ELEM:
Hg
Pb
Cr
AVER:
.00485 .00551 .00280
Why?
990 PPM of Hg easily detected, with 48 PPM sensitivity
Polymer door frames (2000s) from suspected Mexican facility,
Check with EDS for 100 sec, 20 keV, 50 nA... nothing...
100 sec counting time
Still nothing...
500 sec counting time
Hg peaks barely visible…
1000 sec counting time, with Be window inserted (to remove C and O)
So what exactly can WDS do on a “typical” quantitative analysis?
Mercer- Butte Ti % error
3/8/2007 unk
3-8-2007 synth
200
5/7/2007 unk
5-7-07 synth
150
7/11/2007 unk
7-11-07 synth
100
7/24/2007 unk
7-24-07 synth
Ti % error
50
7/25/2007 unk
7-25-07 synth
0
9/19/2007 unk
9-19-07 synth
-50
5/28/2008 unk
5-28-08 synth
-100
8/14/2008 unk
8-14-08 synth
-150
10/1/2008 unk
10-1-08 synth
-200
-0.002
0
0.002
0.004
0.006
0.008
0.01
wt% Ti in quartz
15 keV, 200nA, 600 sec on-peak, 600 second off peak, Ti Ka, LPET + PET (aggregate intensities)
Mercer- Butte Zr % error
200
150
8/20/2008 unk
8-20-08 synth
100
10/1/2008 unk
10-1-08 synth
Zr % error
50
0
-50
-100
-150
-200
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0.05
wt% Zr in rutile
15 keV, 200nA, 300 sec on-peak, 300 second off peak, Zr La, LPET + PET (aggregate intensities)
Every sample is beam sensitive
-at a sufficiently high beam current...
SiO2 Glass
SiO2 Quartz
•Usually thermally insulating samples (e.g., non conductors…)
•Classical beam sensitive samples (e.g., alkali, hydrous glasses)
•Orientation dependent intensity changes over time (e.g., apatites)
•Trace element measurements (high beam currents, long counting)
•Use alternating on and off-peak measurements (constant delta)
•Extrapolate to zero time intensities
•Use a “blank” correction to apply a systematic error offset
F K in VG2 Glass (1800 secs total count time)
Correcting for Intensity Loss (and Gain)
Results in Oxide Weight Percents
ELEM:
169
170
AVER:
SDEV:
SERR:
%RSD:
VOL%:
DEV%:
VOLF:
Na2O
1.140
1.267
SiO2
72.895
72.815
Al2O3
12.112
11.824
MgO
.065
.069
TiO2
.080
.143
MnO
.052
.032
P2O5
.007
-.009
Cl
.174
.172
FeO
.502
.512
K2O
4.323
4.536
CaO
.823
.869
O
-.039
-.039
H2O
SUM
7.867 100.000
7.809 100.000
1.204
.090
.064
7.5
96.461
18.1
LINEAR
72.855
.056
.040
.1
-2.091
.6
LINEAR
11.968
.204
.144
1.7
-1.673
.8
LINEAR
.067
.003
.002
4.2
----------
.112
.045
.032
40.4
----------
.042
.014
.010
33.9
----------
-.001
.011
.008
-806.3
----------
.173
.001
.001
.4
----------
.507
.007
.005
1.3
1.218
5.0
LINEAR
4.429
.150
.106
3.4
60.289
6.1
LINEAR
.846
.032
.023
3.8
----------
-.039
.000
.000
-.4
----------
7.838 100.000
.041
.029
.5
----------
But Not Always What You Expect!
Hyper-exponential Loss
Two exponential
processes with
different decay
constants
overlapping in
time (?)
Results in Oxide Weight Percents
ELEM:
Na2O
SiO2
Al2O3
169
1.790 72.897 12.121
170
1.969 72.817 11.833
MgO
.065
.069
TiO2
.080
.143
MnO
.052
.032
P2O5
.007
-.009
Cl
.173
.172
FeO
.501
.511
K2O
4.318
4.530
CaO
.823
.868
O
-.039
-.039
H2O
SUM
7.213 100.000
7.103 100.000
AVER:
SDEV:
SERR:
%RSD:
1.879
.127
.090
6.7
72.857
.056
.040
.1
11.977
.203
.144
1.7
.067
.003
.002
4.2
.111
.045
.032
40.4
.042
.014
.010
33.9
-.001
.011
.008
-806.4
.173
.001
.001
.4
.506
.007
.005
1.3
4.424
.150
.106
3.4
.845
.032
.023
3.8
-.039
.000
.000
-.4
7.158 100.000
.078
.055
1.1
VOL%:
DEV%:
VOLF:
201.072
4.0
QUADRA
-2.091
.6
LINEAR
-1.673
.8
LINEAR
----------
----------
----------
----------
----------
1.218
5.0
LINEAR
60.289
6.1
LINEAR
----------
----------
----------
The Present “state-of-the-art”: MultiPoint Backgrounds:
Combined Qualitative and Quantitative acquisition
Th Mz1 and Mz2
ThSiO4 (Pb free, i.e., “blank”)
Fluorescence from Si K
Characteristic
Fluorescence from Al Continuum
~100 PPM Si
Back To The Future: A proposal for a TEPNA instrument
The Transmission
Electron Probe Nano
Analyzer integrates
several new detection
technologies to optimize
compositional
characterization with a
target spatial resolution of
~10 nm for “as deposited”
films and particles in the
range of tens to hundreds
of nanometers in
thickness, while still
attached to electron
opaque substrates.
50 nm Bi2Te3, 30 keV, 20 nm beam
Thin films are a trace element problem...
Is that my signal? Nope… Si sum peak!
~50 nm Fe, Nb, Se film on Si wafer (20 keV, 30 nA)
•18 hour integration at 30 nA can provide significant sensitivity...
•Newbury ID “blunders” are still here…
•Si sum peak identified as Sn…
•Nb peak not identified...
Even though the Nb Ka peak is visible...
Like thin films, nano-particles also present a sensitivity problem...
WO3 nano-particles on Si
Si sum peak is not 3 sigma, but neither are the W peaks!
EPMA WDS Monolayer Detection Demonstrated...
0.1 nm thick, 10 um dia.
0.024
Hf La, LIF, 20 keV, 100 nA
640 sec on-peak/ 640 sec off-peak
0.02
Requires a 1000 fold
improvement in
sensitivity!
0.016
Wt % Hf
100 nm thick, 0.01 um dia.
0.012
0.008
0.004
(1,000,000 x
fewer atoms
but
1000 x
thicker film)
0
0
100
200
300
400
500
Relative Distance (um)
The increase in signal as a 10 mm diameter beam is scanned over a region
containing a monolayer of Hf atoms deposited on a Si substrate using a
Cameca SX50
How do we improve sensitivity 1000 fold?
•Utilizing high energy emission lines with higher fluorescent yields
e.g., Nb L = 3.5%, Nb K = 74% (20-30 fold improvement)
•Energy filtering of Be exit windows for high energy emission lines (?)
•Why not do it now?
Goldstein et. al. 1992
Highest effective fluorescent yields are found for element emission
lines whose absorption edges are higher than 8 keV
Zn K is 9.659 keV
Nb K is 16.58 keV
In 2 cm of Ar
37% of Zn K trans.
86% of Nb K trans.
In 2 cm of Xe
.05% of Zn K trans.
59% of Nb K trans.
While still retaining
soft x-ray sensitivity!
Other sensitivity improvements are possible...
•Small FC and/or large area crystals (3 to 4 fold improvement)
•Multiple WDS spectrometers in “aggregate” mode
- 2 to 5 fold improvement using only software
•Increased counting time/beam current in electron “transmission mode”
- 30 keV beam through 100 nm of FeS2 loses ~30 eV of energy
- assume 2 to 5 fold improvement by increasing time/current
•Reduced continuum signal using “faraday cup” TEM grid holders
- preliminary measurements show a 30% reduction in continuum
Transmission Electron Probe Nano Analyzer (TEPNA)
An electron beam instrument that integrates several innovations to
optimize compositional characterization with a target spatial
resolution of ~20 nm for samples in the range of tens to hundreds
of nanometers in thickness on various electron opaque substrates.
The TEPNA complements existing analytical techniques by
providing an unmet need for quantitative compositional analysis
conveniently intermediate between that currently achieved by
wavelength dispersive x-ray (WDX) electron probe micro analysis
(EPMA) and energy dispersive x-ray (EDX) analytical electron
microscopy (AEM).
EPMA vs. LA-ICP-MS
Larger error bars for EPMA reflect
actual small scale compositional
variation.
Current/Future Capabilities of High Sensitivity
EPMA/TEPNA WDS:
Bulk Analysis: presently single digit PPM sensitivity (and accuracy)
-AFTER correction of various continuum artifacts, e.g., “blank”
Thin Film/Particle Analysis: feasible now for major/minor elements
-with typical ~1 um beam diameters on samples >50 nm thick
-requires 10-1000 more sensitivity for <50 nm beam
TEPNA: Transmission Electron Probe Nano Analyzer
- utilize high energy 20-50 nm electron beam (transmission mode)
- high fluorescent yield lines (> 8 keV)
- tandem gas flow/SDD photon counters (full energy sensitivity)
- large area/small FC crystals/spectrometers
- aggregate intensities in software