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Chemical and Electronic State X-ray Emission Analysis using SEM Equipped with
Superconducting Energy Dispersive Spectroscopy for Carbon Fibers and Resins in
CFRP
M. Ukibe, G. Fujii, S. Shiki, M. OhkuboAIST
2017/10/04 IMASM-4
Back groundDevelopment of next-generation materials• wide-band-gap semiconductors (SiC, GaN), etc• Heat resistance streel, High strength steel, etc
1. Distribution in nano scale(10-100 nm)Methods:EPMA, PIXE, TOF-SIMS, SEM-EDX
2. Local structure and electronic stateMethods:XAFS(EXAFS, XANES), XES
Light trace elements (Li, B, C, N, O)
The performance of conventional detectors is insufficient.
Fluorescence of light elements is in a soft X-ray range.
2017/10/04 IMASM-4
Comparison of soft X-ray spectrometers
Thro
ughp
ut
Energy resolution GoodBad
Low
HighEnergy-dispersive type
Wave-dispersive typeSilicon driftDetector(SDD)(10-1 sr, 50 eV)
Diffracting crystal (WDS)(10-5 sr, <10 eV)
Oxford instruments
Targetperformance
Superconducting- tunnel-junction(STJ)(10-3 sr, 5 eV)
JEOLLtd.
2017/10/04 IMASM-4
Superconducting Tunnel Junction (STJ) array X-ray detector
2017/10/04 IMASM-4
Overview of an STJ X-ray detector
Performance for Soft X-rays• High energy resolution due to small energy gap (2.6 meV): < 10eV
(Theoretical limit: 2 eV @ O-Kα)• Fast response (High count rate): ~µsec (> kcps/pixel )• High sensitivity : No surface dead layer• Room temp. electronics for readout
Cross section
Array format for high throughput analysis1mm2 :100µm×100µm×100 pixels
Top100 µm
X-ray
Nb:300nm
Nb:50nm
<1µm
Nb/Al/AlOx/Al/Nb
Substrate(Si)
Upper (Nb) lead <1µm
Insulation layer (SiO2)
Bottom (Nb) lead
NbAlAlOXSiO2
• Nb/Al/AlOx/Al/Nb:300/70/70/50nm
• Pixel size: 100 x 100 µm2
• Operating temperature : 0.3 K
• Absorption coefficient 220-740eV >90%
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Large scaled STJ array
512-pixel STJ array 1024-pixel STJ array
• Fabrication yields is the same as that for 100 pixel array• X-ray detection performance is the same that for 100 pixel array.
2017/10/04 IMASM-4
STJ detector performance ~ energy resolution ~
• Emission spectroscopy• High sensitivity for diluted elements (< 100 ppm)
Response for monoenergetic X-ray
Enough for element identification
Natural broadening due to chemical
conditions B-Kα10 eV
C-Kα12 eV
N-Kα10 eV
O-Kα14 eV
Real sample
BN
• High energy resolution : 4.1 eV(Best), 6.7 eV(Ave.)@ 400eV• High PB ratio : 1,000 ~ 10,000
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STJ detector performance ~ count rate ~
Real time signal processing
200 k cps
XAFS at SR facilities, elemental mapping based on SEM and PIXE at ion beam accelerators.
The same energy resolution up to ~100 kcps
100-ch system
2017/10/04 IMASM-4
STJ detector performance ~ sensitivity ~
0
1000
2000
3000
0 100 200 300 400 500 600 700 800
Cou
nts/
Ch
Energy(eV)
Al-Lα
O-kα
C-kα
Low energy X-ray < 100 eV can be detected.
0
100
200
300
0 50 100 150 200
Cou
nts/
Ch
Energy(eV)
Al-Lα10 eV FWHM
Al-Lα (73 eV), Li-Kα (54 eV)
Sample : AlVacc: 1 keV
AIST confidential
2017/10/04 IMASM-4
SEM system equipped with STJ array soft X-ray detector
(SC-SEM)
2017/10/04 IMASM-4
Setup of SC-SEM
Polycapillary
Beam size~5 mm
Cryogen-free 3He cryostat
Coaxial cables
Charge-sensitive amplifiers
100-pixel STJ array
Sample
Digital signal processor
PC
R.T.50 K
4 K0.3 K
Thermal window
Polycapillary collimating lensSE detector
Synchronization signalFE-SEM
2D mapSE
X-ray spectrum
• Spatial resolution(SE image) : 1.5 nm@ Vacc:15 kV, 4.0 nm@Vacc:1 kV• Throughput of STJ array : 1 mSr @400 eV• Elemental mappings can be performed.
O-K
Beamspot
2017/10/04 IMASM-4
SC-SEM
FE-SEM
X-rayElectron beam
Objective lens
Polycapillary collimating X-ray lens SE detector
Si(Li) X-ray detector
System controller
CryostatSTJ array detector
2017/10/04 IMASM-4
The comparison with other spectrometers
SDD*1 STJ WDS
Energy resolutionfor N-Kα (eV) 70 10 9
Detection efficiency*2 90*3 30 7
*1: Active area 30 mm2
*2: Scaled by count@15 kV (cps/nA)*3: In case of a low transmission window
System performance of SC-SEM• Energy resolution : Equal to WDS and ~ 10 times higher than SDD.• Detection efficiency: 5 times higher than WDS for multi elements
simultaneously. (Parallel detection is possible)2017/10/04 IMASM-4
Chemical state analysis with SC-SEM~ Emission spectroscopy ~
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Carbon fiber reinforced plastic (CFRP)
SEM image : Cross section of a CFRP
Carbon fiberEpoxy resin
Carbon fiber
Epoxy resin
1
0.01
0.0001
Fluorescent X-ray spectrum
Cou
nts
Energy(eV)
Carbon fiberEpoxy resin
C-Kα peaks(normalized)Tail area of C-Kα peaks
Carbon fiberEpoxy resin
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SiCCarbon fiber (CFRP)
Comparison of C-Kα between a carbon fiber and SiC
There are difference at the edge and the satellite of the peak.
Peak shapes depend on chemical state.
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σ bond
π bond
diamond
Reference data obtained by another spectrometer
Problems1. Low throughput : Measurement time at 1 point : ~ 30 min
Current SC-SEM can distinguish the difference of C-Kα peak shape between the several chemical states.
It is difficult to take a chemical state map of CFRP within a reasonable time.
It is necessary to improve the detection efficiency of SC-SEM.
2. Energy resolution(∆E)
In order to make the difference between them obvious, it is necessary to improve ∆E of the STJ array detector.
It is necessary to solve the above problems to perform the chemical state analysis in detail.
∆x at low acceleration voltage(1 kV) of current SC-SEM is limited to 4 nm.
3. Spatial resolution (∆x)
In order to improve ∆x (~ 1 nm), a low acceleration voltage SEM should be installed.
2017/10/04 IMASM-4
Future plan for improving the performance of SC-SEM
1. Increase of the detection efficiency (D.E.), the counting rate capability2. Improvement of the energy resolution (∆E)3. Improvement of the spatial resolution (∆x)
2017/10/04 IMASM-4
1. Future plan ~ increase of D.E., counting rate capability ~
In order to increase D.E. and the counting rate capability, X-ray optics and STJ array detector will be replaced.
Now Future Improvement
X-ray optics Low solid angle+ Collimating
High solid angle + Focusing
~100 times larger (D.E.)
STJ array 100 pixels 1000 pixels 10 times larger(Counting rate capability)
An STJ array detector in improved SC-SEM can exhibit a high energy resolution and a high detection efficiency, simultaneously. • Energy resolution : Equal to WDS and ~ 10 times higher than SDD.• Detection efficiency : 1000 times larger than WDS.• Counting rate capability : Several times larger than SDD
2017/10/04 IMASM-4
2. Future plan ~ increase of ∆E ~
In order to increase ∆E, the design of the STJ will be changed.
An additional aperture is made on the top of the STJ.
Without aperture
STJ
Active areaa whole electrode
STJ
With aperture
Active area a part of a electrode
X µm
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Improvement by the additional aperture
STJ
X µm
The dependence of ∆E on the aperture size
0.0 µm12.5 µm25.0 µm37.5 µm
The dependence of X-ray peak shape on the aperture size
100 µm
∆E is improved by reducing the active area size.
0 µm
12.5 µm25.0 µm
37.5 µm
The reduction rate of 44%
∆E is improved 20 eV from 32 eV.(The Improvement rate is 160 %)
2017/10/04 IMASM-4
2017 2018 2019FY
Roadmap of SC-SEM performance
1.Increase of the detection efficiency (D.E.), the counting rate capability
2.Improvement of ∆E
SC-SEMVer.2.0
? Counting rate capability:10times larger
D.E.:100 times larger
Installation of a high performance X-ray optics
? Installation of 1000 pixel STJ array
∆x:~ 1nm @ 1keV
∆E:~1.5 times higherMaking Nb STJ with an additional aperture
Replacement to a low acceleration voltage SEM
Chemical state Mapping of CFRP.
SC-SEMVer.2.5
Emission spectroscopy of carbon in detail
SC-SEMVer.3.0
Emission spectroscopy of carbon with a high spatial
resolution
3. Improvement of ∆x
2017/10/04 IMASM-4
Summary
We have succeeded in realizing STJ array soft X-raydetectors with a high energy resolution (~5 eV) anddetection efficiency( ~10-3 sr).
A SEM-EDS analyzer utilizing an STJ array (SC-SEM)has been developed.
SC-SEM can distinguish the difference of the C-Kα peakshapes between different chemical states.
In the future, SC-SEM can exhibit a high energyresolution of the WDS and a high throughput of the SDD,simultaneously and obtain the clear chemical state mapof CFRPs with high spatial resolution, sensitivity andthroughput.
2017/10/04 IMASM-4