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.

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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.

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Superconducting Tunnel Junction (STJ) array X-ray detector

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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.

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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

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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)

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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

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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

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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.

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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)

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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

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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 %)

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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

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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.

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