High Sensitivity EPMA:Past, Present and Future

John DonovanCAMCOR

University of Oregon

(541) 346-4632donovan@uoregon.edu

camcor.uoregon.edu

The Past: EPMA/SEM (from Goldstein, et. al. 1988):60 sec P (cps/10-8 A) P/B CDL(ppm)

Si K EDS 5400 97 580WDS 40 1513 1,710

Fe K EDS 3000 57 1,000WDS 12 614 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 CDL(ppm)

EDS Na K 32.2 2.8 1,950Mg K 111.6 6.4 1,020Al K 103.9 5.7 690Si K 623.5 22.8 720Ca K 169.5 8.5 850

WDS Na K 549 83 210Mg K 2183 135 120Al K 2063 128 80Si K 13390 362 90Ca K 2415 295 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 peaksWDS : 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

4

Accuracy (not precision) in characterizing the continuum becomes the limiting factor...

29000 30000 31000 32000 33000 34000Spectrom eter 2 LPET position (s in theta * 10 5)

2.2

2.4

2.6

2.8

3

3.2

Ti K

c

ps/

nA

0

400

800

1200

1600

2000

Ti K

c

ps/n

A

20keV, 100nA, 20um beam

S iO 2 220 sec/po int

T iO 2 40 sec/po int

Continuum Artifacts

Other Artifacts: “Holes” in the Continuum

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

PPM wt.

Reality Check: Accuracy at the 400 PPM Level?

0 400 800 1200 1600

0.02

0.04

0.06

0.08

0 400 800 1200 1600

R elative D istance (um )

0.02

0.04

0.06

0.08

Ti W

eig

ht %

R efug io -1 7 Tra ve rse 2T i Ka Spec 2 (LPET)

T i Ka Spec 4 (PET)

0 400 800 1200 1600

0.02

0.04

0.06

0.08

0 400 800 1200 1600

R ela tive D istance (um )

0.02

0.04

0.06

0.08

Ti W

eig

ht %

Uncorrected for B lank Iteration

Corrected for B lank Iteration

unk

std

std

levelmeasstdunkcorr ZAF

ZAF

C

CCIII

][

][*

)(*

Note: Blank level (Clevel) can be non-zero

1 10 100 1000 10000

O n-Peak Integration T im e (sec)

0.0000

0.0020

0.0040

0.0060

0.0080

0.0100

0.0120

0.0140

0.0160

0.0180

3 S

igm

a R

epor

ted

Det

ectio

n (w

t. %

)Ti in S iO2 , 2 0 ke V , 2 0 0 n A , 2 0 u mC a lc. fro m L o ve a n d Sco tt (1 9 8 3 )

Spectrom eter 1 (P ET)

Spectrom eter 2 (LP ET)

Spectrom eter 3 (LP ET)

Spectrom eter 4 (P ET)

Spectrom eter 5 (P ET)

Aggregate In tensity O ption

PET

LPET

5 Spectrometers

Still… We Need to Improve Sensitivity as well..

1 10 100 1000 10000

O n-Peak In tegration T im e (sec)

1E-005

0.0001

0.001

0.01

0.1

Det

ectio

n Li

mit

(t-t

ests

) (w

t. %

)

Ti in S iO2 , 2 0 ke V , 2 0 0 n A , 2 0 u mC a lc. fro m Go ld ste in , e t a l., (1 9 9 2 )

Spectrom eter 1 (99% C I)

Spectrom eter 1 (95% C I)

Spectrom eter 1 (90% C I)

Spectrom eter 1 (80% C I)

Spectrom eter 1 (60% C I)

Aggregate In tensity (60% C I)

(normal PET crystal)

1 ppm

170 ppmNIST SDD

EDS is Dead!

5 Spectrometers

WDS Analysis of Hg (polymer door frames from suspected Mexican facility)

TakeOff = 40.0 KiloVolt = 20.0 Beam Current = 50

Un 6 std-flex

Results in Elemental Weight PercentsELEM: Hg Pb CrTIME: 240.00 240.00 240.00AVER: .09916 -.03815 -.00240SDEV: .07870 .06759 .00271

Detection limit at 99 % ConfidenceELEM: Hg Pb CrAVER: .00485 .00551 .00280

990 PPM of Hg easily detected, with 48 PPM sensitivity

Checked with EDS-count 200 sec- several nA -no Hg found

Checked with WDS- count 20 sec- 50 nA- Hg found

Why?

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)

Mercer- Butte Ti % error

-200

-150

-100

-50

0

50

100

150

200

-0.002 0 0.002 0.004 0.006 0.008 0.01

wt% Ti in quartz

Ti %

err

or

3/8/2007 unk

3-8-2007 synth

5/7/2007 unk

5-7-07 synth

7/11/2007 unk

7-11-07 synth

7/24/2007 unk

7-24-07 synth

7/25/2007 unk

7-25-07 synth

9/19/2007 unk

9-19-07 synth

5/28/2008 unk

5-28-08 synth

8/14/2008 unk

8-14-08 synth

10/1/2008 unk

10-1-08 synth

15 keV, 200nA, 600 sec on-peak, 600 second off peak, Ti Ka, LPET + PET (aggregate intensities)

So what exactly can WDS do on a “typical” quantitative analysis?

Mercer- Butte Zr % error

-200

-150

-100

-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

Zr

% e

rror

8/20/2008 unk

8-20-08 synth

10/1/2008 unk

10-1-08 synth

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...

•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

SiO2 Glass SiO2 Quartz

F K in VG2 Glass (1800 secs total count time)

Correcting for Intensity Loss (and Gain)Results in Oxide Weight Percents

ELEM: Na2O SiO2 Al2O3 MgO TiO2 MnO P2O5 Cl FeO K2O CaO O H2O SUM 169 1.140 72.895 12.112 .065 .080 .052 .007 .174 .502 4.323 .823 -.039 7.867 100.000 170 1.267 72.815 11.824 .069 .143 .032 -.009 .172 .512 4.536 .869 -.039 7.809 100.000

AVER: 1.204 72.855 11.968 .067 .112 .042 -.001 .173 .507 4.429 .846 -.039 7.838 100.000SDEV: .090 .056 .204 .003 .045 .014 .011 .001 .007 .150 .032 .000 .041SERR: .064 .040 .144 .002 .032 .010 .008 .001 .005 .106 .023 .000 .029%RSD: 7.5 .1 1.7 4.2 40.4 33.9 -806.3 .4 1.3 3.4 3.8 -.4 .5VOL%: 96.461 -2.091 -1.673 ---- ---- ---- ---- ---- 1.218 60.289 ---- ---- ----DEV%: 18.1 .6 .8 ---- ---- ---- ---- ---- 5.0 6.1 ---- ---- ----VOLF: LINEAR LINEAR LINEAR ---- ---- ---- ---- ---- LINEAR LINEAR ---- ---- ----

But Not Always What You Expect!

Hyper-exponential Loss

Results in Oxide Weight PercentsELEM: Na2O SiO2 Al2O3 MgO TiO2 MnO P2O5 Cl FeO K2O CaO O H2O SUM 169 1.790 72.897 12.121 .065 .080 .052 .007 .173 .501 4.318 .823 -.039 7.213 100.000 170 1.969 72.817 11.833 .069 .143 .032 -.009 .172 .511 4.530 .868 -.039 7.103 100.000

AVER: 1.879 72.857 11.977 .067 .111 .042 -.001 .173 .506 4.424 .845 -.039 7.158 100.000SDEV: .127 .056 .203 .003 .045 .014 .011 .001 .007 .150 .032 .000 .078SERR: .090 .040 .144 .002 .032 .010 .008 .001 .005 .106 .023 .000 .055%RSD: 6.7 .1 1.7 4.2 40.4 33.9 -806.4 .4 1.3 3.4 3.8 -.4 1.1

VOL%: 201.072 -2.091 -1.673 ---- ---- ---- ---- ---- 1.218 60.289 ---- ---- ----DEV%: 4.0 .6 .8 ---- ---- ---- ---- ---- 5.0 6.1 ---- ---- ----VOLF: QUADRA LINEAR LINEAR ---- ---- ---- ---- ---- LINEAR LINEAR ---- ---- ----

Two exponential processes with different decay

constants overlapping in

time (?)

The Present “state-of-the-art”: MultiPoint Backgrounds:Combined Qualitative and Quantitative acquisition

ThSiO4 (Pb free, i.e., “blank”)

Th Mz1 and Mz2

Fluorescence from Si KCharacteristic 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...

~50 nm Fe, Nb, Se film on Si wafer (20 keV, 30 nA)

Is that my signal? Nope… Si sum peak!

•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!

0 100 200 300 400 500

Relative Distance (um)

0

0.004

0.008

0.012

0.016

0.02

0.024

Wt %

Hf

Hf La, LIF, 20 keV, 100 nA640 sec on-peak/ 640 sec off-peak

The increase in signal as a 10 m diameter beam is scanned over a region containing a monolayer of Hf atoms deposited on a Si substrate using a

Cameca SX50

EPMA WDS Monolayer Detection Demonstrated...

0.1 nm thick, 10 um dia.

100 nm thick, 0.01 um dia.

Requires a 1000 foldimprovement in

sensitivity!

(1,000,000 x fewer atoms

but1000 x

thicker film)

How do we improve sensitivity 1000 fold?

•Utilizing high energy emission lines with higher fluorescent yieldse.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 keVNb K is 16.58 keV

In 2 cm of Ar37% 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

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.

Transmission Electron Probe Nano Analyzer (TEPNA)

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-MSLarger error bars for EPMA reflect actual small scale compositional variation.

Current/Future Capabilities of High SensitivityEPMA/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