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Chemical State Analysis of Tungsten and Tungsten Oxides Using an Electron Probe

Microanalyzer

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2004 Jpn. J. Appl. Phys. 43 7292

(http://iopscience.iop.org/1347-4065/43/10R/7292)

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Chemical State Analysis of Tungsten and Tungsten Oxides

Using an Electron Probe Microanalyzer

Masahiro KATOH� and Yohei TAKEDA

Material Research Dept., A.L.M.T. Corp. Iwasekoshi-machi 2, Toyama-shi, Toyama 931-8543, Japan

(Received January 6, 2004; revised May 21, 2004; accepted July 23, 2004; published October 8, 2004)

In this research, the discrimination of tungsten and different oxidization states of its oxide has been studied using an electronprobe microanalyzer (EPMA), and the results have been compared with those obtained by X-ray photoelectron spectroscopy(XPS). As a result, the relationship between the oxidization state of tungsten oxide and the intensity ratio of W M�=M� in theX-ray spectra of EPMA has been clarified in small-area analysis. We can discriminate W, WO2, W18O49 (WO2:72) and WO3

using EPMA under given conditions. Comparing XPS with EPMA, a practical disadvantage of XPS is that the small-areaanalysis is sometimes difficult using the conventional XPS equipment, while the commercially available EPMA easilyachieves a few micrometer resolution. XPS measurement is sensitive in the surface state; EPMA can be used to measurewithout being influenced by surface oxidation. [DOI: 10.1143/JJAP.43.7292]

KEYWORDS: chemical state analysis, tungsten, tungsten oxides, EPMA, XPS

1. Introduction

Tungsten has high density, melting point, hardness andelasticity; therefore tungsten products are widely used forhigh-accuracy and highly functional parts such as electricdevice and heat-resistant materials. Tungsten is oxidizedeasily and results in several states of intermediate oxideduring its manufacture. An analysis method of micrometerresolution is required to discriminate different states oftungsten oxide in intermediate tungsten oxides and tungstenproducts.

The state analysis technique using an electron probemicroanalyzer (EPMA) is very useful not only for elucidat-ing the behavior of intermediates for manufacturing-processimprovement but also for evaluating oxidization damage tothe tungsten products under operation conditions. However,there is hardly any research using EPMA in distinguishingbetween different states of tungsten oxide by the small-areaanalysis.

Most research studies that determine the oxidization stateof various elements are carried out by X-ray photoelectronspectroscopy (XPS). Several XPS studies of tungsten and itsoxides have been performed; these included studies of UVreduction by Fleisch and Mains,1) catalyst activity by Katribet al.,2) and characterization of scanning-tunneling-micros-copy tips by Lisowski et al.3)

Many studies to determine the oxidization states ofvarious elements have been carried out using X-ray emissionspectra. In particular, a large number of studies of iron havebeen performed using EPMA. Concerning Fe, Fe2þ, andFe3þ, the peak intensity ratio of L�=L� studied by Albee andChodos,4) the peak shift of L� studied by Fujimori et al.,5)

and the peak shift of L� studied by Fischer6) were reported.There are a large number of reports on the chemical effectson the L� and L� spectra of Cu, Cu2O and CuO.7,8) The L�and L� X-ray fluorescence spectra of various coppercompounds and alloys have been measured by Kawai etal.8) to determine the effect of self-absorption on IðL�Þ=IðL�Þ intensity ratio. Tsuji9) reported that localized surfaceanalysis is possible in grazing-exit EPMA.

For pure tungsten and W2C, Nemoshkalenco et al.10)

reported the analysis method for the emission spectrum ofOIIINVI,VII caused by high-energy-electron bombardment.Vlaicu et al.11) reported that the M-shell satellite structure ofW X-ray emission lines excited by electron bombardmentwas investigated by a high-resolution single-crystal spec-trometer, and the measured emission energy was comparedwith the calculated transition energy.

In this paper, the chemical state discrimination of thetungsten oxide is carried out using EPMA and its result iscompared with XPS.

2. Experimental

W, WO2, W18O49 (WO2:72), and WO3, which werefabricated by A.L.M.T. Corp., Japan, were prepared assamples of tungsten and its oxides. These tungsten and itsoxide powders were of 99.99% purity except for oxygen.The oxygen contents of the samples were analyzed using themass decreasing method by hydrogen reduction. The crystalstructure of each tungsten oxide was identified by X-raydiffraction analysis.

All the sample powders were fixed on carbon adhesiontape and analyzed by XPS and EPMA. The XPS spectra oftungsten and its oxides were measured using ShimadzuESCA1000 with Al K� (energy ¼ 1486:6 eV) radiation. Allthe binding energies were referenced to C1s at 284.6 eV ofeach carbon tape. The binding energies of W4f peaks weremeasured from 46.0 eV to 26.0 eV with a step of 0.1 eV, andthe duration of one channel was 0.2 s. In this experiment, thesurface of the tungsten was cleaned by argon etching (2 kVand 20mA for 1min). Argon etching was not performed fortungsten oxide samples. The measurement region was 1mmin diameter defined by the size of the incident X-ray beam.

The X-ray line profiles were measured using a ShimadzuEPMA8705 with wavelength dispersive X-ray spectrometers(WDX). To avoid specimen charge-up, gold was coated ontothe samples up to a thickness of approximately 10 nm withan ion-coating machine. The measurement conditions inEPMA were sample current of 20 nA, acceleration voltage of15 kV and beam size of 10 mm diameter. The X-rays emittedfrom the samples were directed to a detector during thescanning of the analyzing crystal at a speed of 0.01 �A/min.The Roland circle radius was 101.6mm.

To check the self-absorption effect of this heavy-element-�E-mail address: masahiro-katoh@allied-material.co.jp

Japanese Journal of Applied Physics

Vol. 43, No. 10, 2004, pp. 7292–7295

#2004 The Japan Society of Applied Physics

7292

containing sample, other experiments were carried out inEPMA. To change X-ray absorption distance, we changedacceleration voltage from 10 kV to 20 kV on powdersamples. As another technique to change absorption dis-tance, we measured characteristic X-rays by changing theexit angle. The X-ray detection angle (analyzing crystal) isfixed at 52.5� on this EPMA. A tungsten plate sample wasused only on this measurement with tilt jigs.

3. Results and Discussion

The discrimination of tungsten and its oxides wereperformed by the measurements of the W4f peaks usingthe XPS. The spectra are shown in Fig. 1. For tungsten, twosharp peaks of W4f were observed at 31.2 eV and 33.4 eV.For WO3, two sharp peaks were observed at 35.6 eV and37.8 eV. For WO2, four broad peaks were observed at 31 eV,32 eV, 35 eV, and 37 eV. These peaks were related to thoseof W and WO3.

For metal tungsten, four W4f peaks of tungsten oxide dueto surface oxidization were observed before argon etching inFig. 2. After the etching, two W4f peaks of tungstendisappeared and the other two peaks clearly existed. SinceXPS measurement is sensitive in the surface state, it has theproblem in that profiles the same as those of bulk tungstenoxides and surface-oxidized tungsten in air. Argon etching isgenerally performed in XPS vacuum chambers for oxidizedsurfaces. Although the oxidized surface was clearly removed

by argon etching, tungsten oxides were deoxidized by theetching. For example, argon-etched WO3 has four W4fpeaks, the same as WO2.

The generation domain of X-rays in EPMA is small at thetungsten surface and X-rays are mainly generated inside at adepth of 200 nm at an acceleration voltage of 15 kV from theresults of Monte Carlo simulation using calculation code ofMC Demo.12) The generation of photoemission within 1 nmdepth (3 monolayers) in XPS is mainly due to the kineticenergy of the emitted photoelectrons that limits the depth.EPMA can be used to measure small areas, without beinginfluenced by surface oxidization and the 10 nm gold coatingfor electrical conduction of a sample.

Figure 3 shows the EPMA spectra of tungsten rangingbetween 1.2 �A and 1.5 �A measured using a lithium fluoride(LiF) crystal. Completely identical profiles of WO2 andWO3 were obtained.

We measured the 23.7 �A O K� peak using a W/Si layeredstructure analyzer (LSA) crystal. This peak is broad and wedid not detect any difference between the oxides, because ofthe resolution of the crystal. Takahashi and Okumura13)

distinguished the degree of oxidization with O K� of CuOand Cu2O using a thallium acid phthalate (TAP) crystal;Fialin and Remond14) also observed O K� peaks using W/Siand Ni/C LSA crystals of the MgO–Al2O3–SiO2 system foroxygen estimation.

EPMA spectra of tungsten were measured using apentaerythritol (PET) crystal from 6.6 �A to 7.1 �A at anacceleration voltage of 15 kV (Fig. 4). There are no dif-ferences in full width at half maximum or peak position ofspectra of tungsten and its oxides. The peak wavelengths ofW M� and M� were 6.985 �A and 6.757 �A, and their energieswere 1.775 keV and 1.835 keV, respectively. However, itbecame clear that M�=M� peak intensity ratio for the X-rayspectra is related to the oxidization state of tungsten.Figure 5 shows W M� line profiles normalized by M� at anacceleration voltage of 15 kV. When the amount of oxygencombined with tungsten increases, M�=M� intensity ratioincreases. There is a strong relationship between theoxidization state of tungsten oxides and M�=M� intensityratio in the EPMA X-ray spectrum. The M�=M� intensityratios at an acceleration voltage of 15 kV for W, WO2,

26.030.034.038.042.046.0

W

WO2

WO3

31.2

eV

33.4

eV

35.6

eV

37.8

eV

Inte

nsity

/ ar

b. u

nits

Binding Energy / eV

Fig. 1. W4f XPS spectra of W, WO2 and WO3.

26.030.034.038.042.046.0

W

31.2

eV

33.4

eV

surface oxidation peaks

Inte

nsity

/ ar

b. u

nits

Binding Energy / eV

after etching

before etching

Fig. 2. W4f XPS spectra of tungsten before and after argon etching.

Wavelength / Å1.3 Å1.4 Å1.5 Å

L 1

1.282 Å L 2

1.245 Å

L β 4 L 3

Lα 1

1.476 Å

L α 2

1.487 Å

X-r

ay I

nten

sity

/ arb

. uni

ts

X-ray Energy / keV9.58.5 9.0 10.0

LIII absorption edge1.21 Å

β

β

β

Fig. 3. W L EPMA spectra profiles of tungsten at an acceleration voltage

of 15 kV.

Jpn. J. Appl. Phys., Vol. 43, No. 10 (2004) M. KATOH and Y. TAKEDA 7293

WO2:72 and WO3 are 0:44� 0:02, 0:49� 0:02, 0:54� 0:01and 0:56� 0:01, respectively. The results are shown inFig. 6.

The tungsten characteristic radiations M�1, M�2 and M�contribute to electron transitions of M5–N7, M5–N6 and M4–N6, respectively. On the other hand, the W4f spin orbitmeasured by XPS is the highest energy electron orbital of

the N shell. These characteristic radiations and spin orbitalsare sensitive to oxidization.

We attempted to discriminate a mixture of knowntungsten oxides. Figure 7 shows the scanning electronmicroscopy (SEM) image of mixed WO2 and WO3 powders.In addition, M�=M� intensity ratios are indicated atrepresentative places at an acceleration voltage of 15 kV.The morphology of the powder is different in the SEMimage between WO2 and WO3, and it corresponds well tothe present analysis.

From consideration of the relationship between theoxidization state of tungsten oxide and M�=M� peakintensity ratio, it is found that the expected order forM�=M� intensity ratio is

low W concentration > high W concentration: ð1Þ

Since the W MV absorption edge is 1.814 keV as shown inFig. 4, M�=M� intensity ratio is expected to increase astungsten concentration decreases. To change X-ray absorp-tion distance, we changed acceleration voltage from 10 kV to20 kV. The results are shown in Fig. 8. M�=M� intensityratio increases as the acceleration voltage decreases (i.e.,X-ray absorption distance decreases) for all the samples.

The result of changing the X-ray exit angle is shown inFig. 9 at an acceleration voltage of 15 kV. Absorptiondistance increases when the tilting angle approaches theX-ray detection angle of 52.5�, and M�=M� intensity ratiodecreases.

These results show that the self-absorption effect is

WWO2

WO3

6.7Å6.8Å

M/M

αIn

tens

ity

Rat

io

Wavelength / Å

0

0.4

1.841.83

X-ray Energy / keV1.85

0.6

0.2

β

Fig. 5. W M� line profiles normalized by M�. (15 kV acceleration

voltage)

0.40

0.45

0.50

0.55

0.60

+0 +2 +4 +6

Tungsten Oxidation Number

M/M

α I

nten

sity

Rat

io WO3WO2.72

WO2

W

β

Fig. 6. Relationship between tungsten oxidization number and M�=M�

intensity ratio. (15 kV acceleration voltage)

Mβ

β

/Mα=0.555

WO3

M /Mα=0.487

WO220µ m

Fig. 7. SEM image of mixed WO2 and WO3 powders. In addition,

M�=M� intensity ratios are indicated. (15 kV acceleration voltage)

X-r

ay I

nten

sity

/ arb

. uni

ts

WWO2

WO3

Wavelength/ Å

Mα

Mβ6.985 Å

FWHM 0.01 Å 6.757 Å

FWHM0.01 Å

7.1 7.0 6.9 6.8 6.7 6.6

1.76 1.78 1.80

X-ray Energy / keV

1.82 1.84 1.86

MV absorption edge1.814 keV6.83 Å

Fig. 4. W M line profiles normalized by M�. (15 kV acceleration voltage)

0.35

0.40

0.45

0.50

0.55

0.60

5 10 15 20 25

Acceleration Voltage / V

M/ M

α In

tens

ity R

atio

WO3

WO2.72

WO2

W

β

Fig. 8. Relationship between M�=M� intensity ratio and acceleration

voltage of EPMA.

7294 Jpn. J. Appl. Phys., Vol. 43, No. 10 (2004) M. KATOH and Y. TAKEDA

strongly related to the M�=M� intensity ratios. Total massattenuation cross sections (�=�) of M� and M� are1:18� 103 cm2/g and 3:61� 103 cm2/g, respectively.Atomic masses of W, WO2 and WO3 are 183.84, 215.86and 231.86, respectively. The density of WO3 is 79% or lessin comparison with W, and the effect of the self-absorptionseems to decrease in proportion to the density. It is necessaryto sufficiently examine the shape and composition of thesamples in order to analyze the chemical state using thecharacteristic X-ray intensity ratio.

4. Conclusions

A state analysis of tungsten and its oxides was carried outusing XPS and EPMA, and we obtained the followingresults.

It became clear that the state analysis by EPMA is suitablefor the discrimination of tungsten and its oxides by thesmall-area analysis of interfusion particles and the tungstenbulk surface because there is minimal influence of the

oxidized sample surface layer on tungsten. Since it can onlybe used to analyze very shallow sample surfaces, XPScannot discriminate between bulk tungsten oxide and sur-face-oxidized tungsten in air. Another practical disadvantageof XPS is that the microarea analysis is sometimes difficultusing the conventional XPS equipment, while the commer-cially available EPMA easily achieves a few micrometerresolution.

Acknowledgements

We are grateful to Dr. Tomoaki Futakuchi of ToyamaIndustrial Technology Center for helpful advice and assis-tance in analysis by XPS. We also thank Dr. YoshiharuYamamoto for kind support and advice. One of the authors(M. K.) also appreciates helpful discussions with andsuggestions from Dr. Satoshi Iida of Toyama University.

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0.35

0.40

0.45

0.50

0.55

0.60

-60 -40 -20 0 20 40 60

Tilt Angle / degree

M/ M

Inte

nsity

Rat

ioWDXdetection angle 52.5°

Electron Beamβ

α

Fig. 9. Relationship between M�=M� intensity ratio and tilt angle of

tungsten plate. (15 kV acceleration voltage)

Jpn. J. Appl. Phys., Vol. 43, No. 10 (2004) M. KATOH and Y. TAKEDA 7295