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1
Workshop on Trace Analysis
IntroductionInstrumentation
TXRF
Peter Wobrauschek
Denver X-Ray Conference 2012Email: [email protected]
X-ray source X-ray detector
sample
Interaction:
Photoeffectelastic scatteringinelastic scattering
X-ray tubeSynchrotron radiation
energy dispersive
E(keV)C.Streli 2002
Principle of EDXRS
X-ray tube
Synchrotron radiation
Multielement spectrum Multielement spectrum evaluted
2
xx+dx
x = d
x = 0
dx
Detektor Anregungsquelle
ϕ
dž 1
dž 2
ψ
I (E Kα
i ) =x=0
x=d∫ I0 (E)
E =Eabsi
E =Emax
∫ ⋅ G1 ⋅ρ
sin ϕ⋅
τKi (E )
ρ⋅ωK
i ⋅pαi ⋅ci ⋅ V i (E ) ⋅
⋅e−
μ (E)ρ⋅sin ϕ
+μ(EKα
i )ρ⋅sin ψ
⎛
⎝ ⎜
⎞
⎠ ⎟ ⋅ρ⋅x
⋅ G2 ⋅ f (EKαi ) ⋅ ε(E Kα
i ) ⋅dx ⋅ dE
Quantitative XRF fundamental equations I
SourceDetectorConcentration
Simple Net peak area and Background calculation
Nb=Y(a)+Y(b) * [xb-xa+1]/2
Number of background countsNb
NbNgross = sum (i) y(i) i=xa,xb
Nnet = Ngross - Nb
Nnet
Software for background stripping and deconvolution of overlapping peaks
LLD =3⋅ NB
NN⋅msample =
3⋅ IB / t
S(cps / ng)
Definition of detection limit
Reduction of detection limits:
increase in intensityreduction of background
increase of measuring time
The lower the detection limits the better you can do trace analysis
Aim of all spectral modification elements:Reduction of background due to scattering
Physical reasons: elastic and inelastic scattering on sample and substrate
Detector reasons:Detector shelf
Incomplete charge collectionCompton backscatter from detector crystal
Intensity
Energy
3
Influence on sensitivity
•Physical strategies:
•choice of the anode material
•selective excitation -• characteristic E slightly
above absorption edge E
•spectral distribution of the source
• tuning monoenergetic• high-energy cutoff• selected filters for low-energy
photons suppression
•Total reflection of x-rays ⇐ 2·S
•Technical strategies:
•distance reduction source-sample- detector
•detector area larger
•X-ray optics
•source-power increase
• 2 kW standing anode• 18 kW rotating anode
•synchrotron radiation•multiple tube (future idea)
Reduction of the Background
• Modify primary spectrum (high energy cut off) by glassreflector adjusting total reflection conditions
• Multilayer monochromator purely monoenergetic beam
• Linear polarized beam - Barkla or Bragg polarizer in triaxial geometry (90°-90°-90°)
• Optimum conditions – monochromatic synchrotronradiation
X-ray Sources
Standing Anodes – Fluorescence and Diffraction tubes
Anode Materials: Cr,Cu,Mo,W, Ag
Special for light element W-M line or Si anode
Focal size 0.04x12mm² line 0.4x0.8 mm² point
Power from few W to 3 kW
Special tubes. Thin window transmission of low E photons – Transmission anode tubes end window
Windowless tubes W- Si anode light element excitation
Rotating anode up to 90 kW
Synchrotron radiation
Femtosecond pulsed laser induced x-rays
high harmonics or plasma source andLiquid Metal Jet
X-ray tube and housing
4
Low power tube
Product Neptune - Water Cooled) XTF6000 XTF5011/75Max kV 50.0 kV 60 kV 50.0 kVMax mA 5.0 mA 1.0 mA 1.5 mAMax Power 100 Watts 50 Watts 75 WattsFocal Spot size (nominal)* 100 microns* (W) <80 microns* (W) <85 microns* (W)Target types W, Mo,Cu W,Rh W,Rh,Mo,Cr,Pd +Exit window type Beryllium Beryllium BerylliumExit window thickness 75 µm to 254 µm 80 µm to 254 µm 75 µm to 254 µmCone angle 25 degrees 20 degrees 25 degrees
www.oxfordxtg.com
Rotating anode X-ray tubes
www.rigaku.com/protein/ruh3r.html
1515
Conventional Target Technology
• Solid target close to melting• Possible Improvements
• More efficient cooling• Smaller spots• Fast rotation
• Unfortunately now very close to fundamental limits
Rotating anodeSolid anode
1616
Key Concept – The Metal Jet Anode
~ 200 µm
~ 75 m/s
5
1717
Brightness Comparison
Approximately 10× brightness compared to a solid anode in the ~5 – 40 µm focal-spot-size range
1818
Power Limitation of MetalJet
Metal vapour
1919
Window Vapour Mitigation
127 µm Beryllium vacuum window
20 µm heated carbon foil that re-evaporates contamination. 2020
Available Alloys and Their X-ray Spectra
• Alloys molten at room temperature are used in the first generation sources. Future versions may include alloys with higher melting points.
• Gallium-rich alloy has emission similar to copper• Indium-rich alloy has emission similar to silver
Element Kα1 Kα2 Lα1 Lα1
Gallium 9.25 keV 9.22 keV 1.10 keV 1.10 keV
Indium 24.21 keV 24.00 keV 3.29 keV 3.28 keV
6
2121
Acknowledgement to Bjoern Hansson EXCILLUM
Aereal view of ESRF
Synchrotron radiation ED detector LN2 cooled Si(Li)
Neue Arbeiten am Atominstitut WS07/08 | Jan.
7
Ketek Si drift detector Peltier cooling
Finger length 100 mm
High voltage, DPP,
Peltier cooling
Multichannel analyzer
USB connector
All in one small box and
Very compact
Ultimative
Source and detector in onebox HANDHELD
MANY COMPANIES
WORLDWIDE
Detector Transmittance with Ultra Thin Window
TXRF 2009, Gothenburg, Sweden | | Seite 26
Light element detector
Gresham Sirius Detector
TXRF 2009, Gothenburg, Sweden | | Seite 28
8
High ThroughputAMPTEK
Filtered radiation
Background Reduction
DetectorDetector
XX--Ray Ray SourceSource
Source FilterSource Filter
Simple effective
Scheme polarization plane - detectorposition
E-vector
linearpolarizedx-rays
plane of polarization
detector
atomicdipolessample
anisotropicdipoleemissioncharacteristic
isotropic emission offluorescence radiation
EDXRF with linear polarized radiation
Polarized radiation
www.panalytical.com
Epsilon 5
detektor
sample
polarizer
source
9
Polarizer materials
• Light elements – pressed pellets• Boron Carbide B4C• Alu• Plexiglass• Graphite• Al2O3
• Bragg crystals for 45° Bragg angle • Cu (113) for Cu Ka radiation• HOP-G highly oriented pyrolithic graphite
Polarization
Direct excitation (2-dimensional) Polarised excitation (3-dimensional)
Effect of using linear polarization
From:
Vert:: detdownlooking
Hor: detsidelooking
A. Knöchel et al Fresenius 1990
Pol planebeam
Detector
beam
Synchrotron radiation
Secondary target
Improved Fluorescence and lowerbackground
Sample
X-Ray TubeDetector
Secondary Target
ENERGY (keV)
Tube Target peak
With Zn Secondary Target
FeRegion
Continuum Radiation
Comparison of optimized direct-filtered excitation with secondary target excitation for minor elements
10
37Trace element workshop 2012
Polarization and selective excitation
From: handbook of X-ray spectrometry, Marcel Dekker 1993, 2002
unfiltered
Mo sec.targ.
HOPG Bragg polarizer
White(left) versus monoenergetic excitation (right)
White excitation Monoenergetic excitation
0 5 0 0 0 1 0 0 0 0 1 5 0 0 0 2 0 0 0 0 2 5 0 0 0 3 0 0 0 0
0 . 0
0 . 2
0 . 4
0 . 6
0 . 8
1 . 0
ar
bi
tr
ar
yu
ni
ts
D O R I S I I I L
C u t O f f S i O2
M L W\
C 4 0
S i 1 1 1
E n e r g y [ e V ]
Crystal versus Multilayer monochromator
COMPARISON OF SPECTRAL MODIFICATION
Selective Excitation:
Ti-K 4.96keV
Ba Liii 5.24keV
Cu-K 8.98keV
11
Panalytical Epsilon 5
• Commercial EDXRF Spectrometer
• X-ray Tube: W-Sc Anode• 9 Secondary Targets• Ge-Detector• Permanent vacuum during
measurements• Generator: max. 100kV,
24mA, 600W3D optical path(www.panalytical.com)
Sample Cup
X‐ray Tube
Secondary Targets
Detector
Epsilon 5 and Angelika Maderitsch
Sample measured and operating conditions
• Sample Pellet: 2g Soil 7 + 1g HWC (Wax-Binder)
• Measured with Panalytical Epsilon 5
• Dual anode: Sc/W 100kV/6mA 600 W
• Used secondary targets: Al2O3, Zr, Ge
• Detector: Ge(HP) 30mm² x 5mm, Be window 8 µm
• Measuring time: 1020 s
Soil 7- Al2O3
12
Pb - Zr sec target compared with Barkla Al2O3 Fe - Ge-, Zr - sec target compared with Barkla Al2O3
Peak to Background Ratio for Pb
Secondary Target
Netto Counts (cps)
Background (cps) Ratio
Al2O3 4,93 3,5 1,41
Zr 43,8 13,28 3,42
Peak to Background Ratio for Fe
Secondary Target
Netto Counts (cps) Background (cps) Ratio
Al2O3 586,56 8,556 68,55
Zr 3785,79 86,36 43,84
Ge 7720 73,31 105,30
13
Peak to Background Ratio for Ti
Secondary Target
Netto Counts (cps) Background (cps) Ratio
Al2O3 11,78 6,0 1,96
Zr 23,97 123,53 0,19
Ge 180,06 16,16 11,14
Triaxial Geometry polarizing primary radiation
x-ray tube Rh-anode
50kV 1.5 mA
SecondaryTarget:
2mm Mo
sample
Si drift detector
Vortex50mm²
Scheme of triaxial geometry
X-ray properties of combination Rh anodeMo secondary target
•Rh K-alpha 20.165 keV K-beta 22.72keV
• plus bremsstrahlung excitation
•Mo K- absorption edge 20.001 keV highest photo cross section
•Mo K-alpha 17.48 keV K-beta 19.6 keV
•Pb LIII edge 13.04 keV LII edge 15.2 keV
•Source Oxford XTF 50kV 1.5 mA Rh target US$ 3700
•Matusada HXR x-ray generator + PC controller US$ 4600
•Portable system - air cooling
Final Setup
detector
Tube Al holder
shieldingsample
14
Animal Bone Sample
Animal Bone Standard Reference Material IAEA (H-5)(3.1 µg/g Pb) powder pressed to a pellet
0 2 4 6 8 10 12 14 16 18 20
100
1000
10000
Pb Lβ
Pb Lα
cts
/ 100
0s
Energy [keV]
animal bone Excitation:Rh anode material49.5 kV / 1.5 mA1000s livetime
Detector:Radiant VORTEX50 mm2 SDD
LLD: 0.7 µg/g Pb
Orchard Leaves Sample
Orchard Leaves Standard Reference Material NIST SRM 1571(45µg/g) powder pressed to a pellet
0 2 4 6 8 10 12 14 16 18 20100
1000
10000
100000
Pb Lβ
Pb LαAs Kα
cts
/ 100
0s
Energy [keV]
orchard leavesExcitation:Rh anode material49.5 kV / 1.5 mA1000s livetime
Detector:Radiant VORTEX50 mm2 SDD
LLD: 0.4 µg/g Pb
CONCLUSIONS
• Using x-ray physical effects as:• Filters – polarizers – monochromators and
secondary targets• Background conditions are improvesd• Increase of primary flux by power or x-ray optics
improves sensitivity• Detector and electronics allow high countrate
processing• All ideas are perfectly applied for trace element
analysis to achieve improved detection limits
Acknowledgement• Nina Cernohlawek• Bernhard Pemmer• Angelika Maderitsch• Team of Atominstitut X-ray group
15
TXRF – A VERSATILE TOOL FOR TRACE ELEMENT ANALYSIS
A REVIEW
Peter WobrauschekAtominstitut, TU Wien
OUTLINE• TXRF: fundamentals of total reflection X-ray fluorescence analysis• Chemical analysis at pg levels by tube excitation• Chemical analysis at fg levels by synchrotron radiation excitation
• GI-XRF: grazing incidence XRF• Layered structures above surface: thickness, density, element• Implants: concentration profiles, element
• SR-TXRF: synchrotron radiation induced TXRF
• Low Z element determination
E D X R F
Standard XRF Micro XRFµ-XRF
Total Reflection XRFTXRF
Absorption Spectroscopyin fluorescence mode (XAS)
EDXRF at the Atominstitut
Advantages
• background reduction
• double excitation of sample
• small distance sampledetector (~1mm) large solid angle
• angle dependence of fluorescence signal
information about type of sample (bulk, particle, film, implantation)
ρδ ⋅≈⋅≈ΦEcrit
7.202
φcrit [mrad], E [keV], ρ [g⋅cm-3]
)()(
2
1
EfKEfK
⋅=⋅=
βδδ ~ 10-6 … dispersion:
β ~ 10-8 … absorption:
fi … atomic scattering factorr0 … classical elektron radiusλ … wavelength of incident beamNA … Avogadro‘s constantA … atomic weigthρ … density [g⋅cm-3]
ρπλ
⋅⋅⋅
=A
NrK A
2
20
Fundamentals of TXRF
Total (external) reflection of X-Rays
ϕ critical(Si, 17.5 keV) ≈ 0.1° ≈ 1.75 mrad(Si, 500 eV) ≈ 3.7° ≈ 64.6 mrad(Si,12.2 keV) ≈ 0.15° ≈ 2.6 mrad
βδ in −−= 1X-rayrange:
16
βδ in −−= 1
ρδ ⋅≈⋅≈ΦEcrit
7.202
φcrit [mrad], E [keV], ρ [g⋅cm-3]
Transmission / Reflection Coefficient17.5 keV, Si-reflector
0,001
0,01
0,1
1
0 0,5 1 1,5 2
T R
φ/φcrit
Penetration depth (nm)17.5 keV in Si
1
10
100
1000
10000
0 0,5 1 1,5 2 2,5 3 3,5 4angle(mrad)
pene
trat
ion
dept
h(n
m)
φ
crit
Fundamentals of TXRF Principle of XRF and TXRF
© Siegfried Pahlke 06.2004
Comparison between Conventional XRF and TXRF shows Difference in the Geometric Grouping of Excitation and Detection Units.
X-Ray Tube Wafer Detector
Fluorescence radiation
X-Ray-Fluorescence (XRF)
X-Ray Tube Wafer
Detector
total reflected beam
Fluorescence radiation
Total-Reflection X-Ray-Fluorescence (TXRF)
Principle of TXRF
NativeSiO2
Silicon Wafer !
Penetration Depth about 5 to 10 nm
1 mrad incident angle of beam
In Total Reflection Conditions
© Siegfried Pahlke 06.2004
X-ray intensity in the proximity of the surface
λ
φ
I0 IR
D
)sin(2 ϕλ
⋅=D
-100
-50
0
50
100
01234
airsilicon
X-Ray Intensity
dist
ance
from
surfa
ce[n
m]
ϕ = ϕcrit/2
ϕ = ϕcrit
ϕ = 2ϕcrit
Long and transv. Coh restricts volume
17
Fundamentals of TXRF
0 1 2 3 4 50.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
fluore
scence
inte
nsi
ty [
a.u.]
incidence angle [mrad]
residueon surface surface layer
bulk
Angle dependence of fluorescence signal information about type of sample:
• bulk
• particle (residue on surface)
• film, implantation (surface layer)
Analytical features
• only small sample volumes required (ng some µl)
• detection limits in the pg range (for medium Z elements using X-Raytubes)
• Simple quantification ( thin film approximation) by adding an internal standard
Fundamentals of TXRF
Environment:
water: rain, river, sea, drinking water,
waste water.
air: aerosols, airborne particles, dust,
fly ash.
soil: sediments, sewage sludge.
plant material: algae, hay, leaves, lichen, moss,
needles roots, wood.
foodstuff: fish, flour, fruits, crab, mussel,
mushrooms nuts, vegetables, wine,
tea.
various: coal, peat.
Medicine / Biology / Pharmacology:
body fluids: blood, serum, urine, amniotic fluid.
tissue: hair, kidney, liver, lung, nails, stomach,
colon.
various: enzymes, polysaccharides, glucose,
proteins, cosmetics, bio-films.
Industrial/Technical applications:
surface analysis: Si-wafer surfaces,
GaAs-wafer surfaces
implanted ions depth and profile variations
thin films single layers, multilayers
oil: crude oil, fuel oil, grease.
chemicals: acids, bases, salts, solvents.
fusion/fission research: transmutational elements in
Al + Cu, Iodine in water
Mineralogy:
ores, rocks, minerals, rare earth elements.
Fine Arts / Archeological / Forensic:
pigments, paintings, varnish.
bronzes, pottery, jewelry.
textile fibers, glass, cognac, dollar bills, gunshot residue,
drugs, tapes, sperm, finger prints.
C.Streli 2003
Applications of TXRF
68Trace element workshop 2012
TXRF• TXRF for chemical analysis using internal standard for quantification
• TXRF of wafer surface analysis using external calibration and absolute quantification
•GI-XRF (changing angle of incidence) for depth profiling and thin layer analysis
• Synchrotron radiation induced TXRF (SRTXRF) for ultra trace analysis
• SRTXRF-XANES for speciation at trace levels
18
69Trace element workshop 2012
Kapton window
Fine focusX-ray tube
entrance slit50 µm
multilayermonochromator
beam exitwindow
detector
sample reflector
Pb-shielding
vacuum chamberpump
Scheme of the Atominstitut standard vacuum TXRF spectrometer
schema stanford chamber 70Trace element workshop 2012
Measurements with TXRF Aerosol on Si wafer
Experiments with Mo anode tubeML monochromator, 80mm² Si(Li)50kV 40 mA 1000sDet Lim: pg range
71Trace element workshop 2012
WOBISTRAX:Schematic view
72Trace element workshop 2012
WOBISTRAX
19
73Trace element workshop 2012
Detection limits: AXIL deconvolution
0
200
400
600
800
1000
1200
0 5 10 15 20
S9RBV1~1.txt
cts/
chn
Energy(keV)
Si
Rb
LLD= 0.7 pg
Mo/50/40/100900 pg Rb
Standard : Mo-tube
Mo anode
50 kV, 40 mA, 100 s
900 pg Rb
Analysis of tap water
75Trace element workshop 2012
Low Z performance
0
500
1000
1500
2000
2500
3000
0 1 2 3 4 5 6
AL1_01.txt
cts/
chn
Energy(kev)
Al
Si
K
Ca
Sc
Cr-scatter
Cr/30/50/100Jamcham75 ng Al, 3 ng Sc
LLD(Al) = 42 pg
0
500
1000
1500
2000
2500
3000
3500
0 1 2 3 4 5 6AL1_01.txt
cts/
chn
Energy(kev)
MgSi
K Ca
Sc
Cr-scatterCr/30/50/100Jamcham75 ng Mg, 3 ng Sc
LLD(Mg)= 156 pg
0
1000
2000
3000
4000
5000
0 1 2 3 4 5 6AL1_01.txt
cts/
chn
Energy(kev)
Na
Si
ClCa
Sc
Cr-scatter
Cr/30/50/100Jamcham90 ng Na, 3 ng Sc
S
LLD( Na) = 900 pg
E lem en t S (cp s/n g) L LD ( p g ) N a 0 .3 900 M g 1 156 A l 2 42
Cr- anode X-ray tube, 30 kV, 50 mA
KETEK 10mm2 8um Be window
• Straight TXRF – no sample preparation– Inspected area: 0.5 cm2– Mapping of wafer surface possible– Surface layer response of contamination– LLD: E10 atoms/cm2
• VPD ( vapor phase decomposition)- TXRF– Decomposition of native oxide layer with HF vapor and collection
of impurities in one drop – inspected area: wafer size!– LLD: E8 atoms/cm2
• VPT (vapor phase treatment) –TXRF– Decomposition of native oxide layer with HF, no collection – leads to residue response of contamination– Mapping still possible– LLD: 2E9 atoms/cm2
Wafer analysis with TXRF
20
Straight TXRF- VPD- VPT
Shimazaki, A.; Ito, S.; Miyazaki, K.; Matsumura, T.;
Semiconductor Manufacturing, 2005. ISSM 2005, IEEE International Symposium on 13-15 Sept. 2005 Page(s):456 - 459
Digital Object Identifier 10.1109/ISSM.2005.1513404
Wafer mapping
TXRF spectrometers• Chemical analysis:
– Picofox: www.bruker-axs.de/s2picofox.html?&L=1– TX2000: www.italstructures.com/tx2000.htm– WOBISTRAX: www.ati.ac.at/index.php?id=142&L=1– Nanohunter: www.rigaku.com/xrf/nanohunter-show.html– Handy-type TXRF: int.icc.kyoto-u.ac.jp/?p=924,
www.ourstex.co.jp/product.html– Fisichem-TXRF: www.fisichem.com
• Wafer analysis:– Rigaku: www.rigaku.com/semi/txrf-v300.html
THANKS FOR YOUR
ATTENTION
THE END