J. Plasma Fusion Res. SERIES, Vol. 10 (2013)

Hydrogen incorporation into tungsten deposits growing
under hydrogen and argon mixed plasma
Kazunari KATAYAMA, Yasuhito OHNISHI, Satoshi FUKADA and Masabumi NISHIKAWA
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University,
6-10-1, Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan(Received: 10 May 2012 / Accepted: 9 October 2012)

Tungsten deposits were produced by exposing a tungsten tube to hydrogen and argon mixed plasma in different
deposition conditions. The erosion rate of the tungsten tube decreased with increasing hydrogen concentration in
the working gas. This indicates that the erosion was mainly caused by argon ion incident. In order to releasehydrogen incorporated into the tungsten deposit, the tungsten deposit has to be heated up to 600 oC. While thetungsten deposition rate was promoted by the increase of negative target bias supplied to the tungsten tube, theinfluence of the target bias on hydrogen retention in the tungsten deposit was different in the deposition condition.
However, it was found that hydrogen retention increases with the ratio of the energetic hydrogen flux reflected on
the tungsten tube to the tungsten deposition flux. The flux ratio of hydrogen to tungsten toward the growing surfaceof the tungsten deposit is an important parameter to evaluate the amount of hydrogen incorporated into tungsten
deposits growing under the plasma containing hydrogen ions.
Keywords: tungsten, deposition, hydrogen retention, sputteringhydrogen and argon mixed plasma was investigated.

1. Introduction

It is important to understand the accumulation
phenomena of hydrogen isotopes in a plasma confinement
vessel of a fusion reactor from viewpoints of tritiumeconomy and safety. Plasma-facing wall is eroded byenergetic particle incident and sputtered atoms would form
deposition layers. It has been known that carbon deposition
layer can contain a large amount of hydrogen isotopes.
Tungsten is a candidate material for plasma-facing wall
because of low solubility and low sputtering yield forhydrogen isotopes. In a long time operation of a fusion
reactor, a certain amount of tungsten deposits would beformed in the plasma confinement vessel. Erosion anddeposition will be promoted if a noble gas such as argon isintroduced around plasma-facing wall for a reduction ofheat load. It has been reported by the present authors that alarge amount of hydrogen isotopes are incorporated into
tungsten deposition layer formed by hydrogen isotope
plasma sputtering [1,2]. The empirical equation has been
proposed from a systematic study of the influence of thedeposition conditions on the deuterium retention by using
magnetron sputtering and linear plasma devices [3]. The
influence of oxygen on hydrogen isotope retention intungsten deposition layers has been investigated [4,5].
Nevertheless, experimental data for hydrogen isotope
retention in tungsten deposits are still insufficient. In thisstudy, the influence of the deposition condition onhydrogen incorporation in tungsten deposits growing under

2. Experimental

A schematic diagram of the experimental apparatus isshown in Fig.1. A cylindrical quartz tube, 13 mm in outer
diameter, 10 mm in inner diameter and 400 mm in length,
which has a horizontal branch tubule, 6 mm in outer
diameter, 4 mm in inner diameter and 30 mm in length,
was used as a plasma sputtering chamber. A tungsten tube,
1.4 mm in outer diameter, 0.2 mm in thickness and 99.95%
in purity (Nilaco Co.), was inserted to the branch tubule asa sputtering target. A copper tube, 3 mm in outer diameter,
was rolled on the outer surface of the quartz tube in theupstream side as an inductive coil. The distance from theedge of the coil to the tungsten tube was set to be 35 mm.
In order to collect a part of sputtered tungsten, a quartz
substrate, 10 × 5 mm2 in size and 1 mm in thickness, wasplaced beneath the tungsten tube. The detail of thesputtering-deposition region is illustrated in Fig.2. The
distance from the center of the quartz tube to the surface ofthe substrate was 3.3 mm. The angle T is about 113o whichis 31 % of the all circumferences. The length of the quartz
substrate is 5 mm which is 50 % of the tungsten tube
length in the longitudinal direction. Therefore theplasma-facing area of the quartz substrate views
approximately 16 % of the plasma-facing area of thetungsten tube. The mixture gas of hydrogen and argon wasintroduced via a mass flow controller (SEC-E40, HORIBA

author’s e-mail: kadzu@nucl.kyushu-u.ac.jp

89©2013 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchK. Katayama et al., Hydrogen Incorporation into Tungsten Deposits Growing under Hydrogen and Argon Mixed Plasma

supplied in the tungsten tube by a DC power supply(PWR400H, KIKUSUI Co.). The target current wasmeasured by a digital multi-meter (PC520M, SANWAInc.). Evacuation of the quartz tube was performed by arotary pump (GLD-201B, ULVAC Inc.). The working gas
pressure during sputtering-deposition process wasmonitored by a Pirani gauge (GP-1G, sencer:WPB-10-034,
ULVAC Inc.). After the process, the absolute pressure wasmeasured by a capacitance manometer (Baratron 626B,MKS Ltd.) in same deposition condition. The true flowrate of the mixture gas was obtained from the absolutepressure in the quartz tube and the exhaust velocity by therotary pump. The temperature in the vicinity of the quartz
substrate during sputtering-deposition process wasmeasured by a thermocouple fixed on the outer surface ofthe quartz tube. In order to confirm the temperature inside
the quartz tube, a thermocouple was inserted from thedownstream side of the quartz tube to the position of thesubstrate. Both temperatures were almost same. The
deposition conditions were summarized in Table 1. In casesthat temperature is just 100 oC, the temperature wascontrolled by a ribbon heater. The sputtering-deposition
region was wrapped by the ribbon heater except in Ex.1and Ex.2. The temperatures in Ex.1 and Ex.2 are lower
than that in the others because there is no effect of heatretention by the ribbon heater.
The amount of deposited tungsten was obtained from theweight change of the substrate before and after
sputtering-deposition process. The weight change wasmeasured by an electric balance with sensitivity of 0.01 mg(CP225D, Sartorius Inc.), whereas the original weight ofthe substrate was about 100 mg. The cross section
observation of the tungsten deposit was performed by thescanning electron microscopy (SEM:SU8000, HITACHICo.). The weight percent of oxygen in the tungsten deposit
was analyzed by energy dispersive X-ray equipment(EDX:Genesis2000, EDAX Inc.). SEM and EDX used inthis study were installed at the Center of Advanced

Fig.1 The schematic diagram of the experimental apparatus.

Fig.2 The illustration of the sputtering-deposition region.
(a) : a view from the gas flow direction. (b) : a view from thecross-section direction of the W tube.

Ltd.) calibrated with pure hydrogen gas. Hydrogenconcentrations in the mixture gas used in this study were 2,20, 50 and 80 %. A packed bed of adsorbent, MS5A wasinstalled between the gas cylinder and the mass flow
controller to exclude a small amount of impurity water
vapor in the gas cylinder. Plasma was inductively
generated inside of the quartz tube by supplying the RF
power of 40 W or 100 W with 13.56 MHz (NR05N-06,NRF Co.) to the copper coil via matching box(NM05AL-05, NRF Co.). In order to promote erosion ofthe tungsten tube, a negative bias of 200 V or 300V wasTable 1 The summary of deposition conditions.

H2[%]

Flow rate[sccm]

Pressure[Pa]

RF power[W]

Targetbias [-V]

Flux to target[ions/m2/s]

Temperature[oC]

Deposition
period [h]

Ex.1

2

3.8

7.6

40200

1.30㬍1020

58

20

Ex.2

2

3.8

7.6

40300

1.53㬍1020

58

17Ex.320

6.7

7.1

100

200

20

1.31㬍10104

20

300

1.96㬍1020

94

21100

14

Ex.4

20

6.7

7.1

100

Ex.5

20

3.1

4.6

100

300

1.62㬍1020

Ex.6

20

6.7

7.1

100

300

2.72㬍1020

96

36.5

Ex.7

20

3.1

4.6

40

200

1.23㬍1020

100

24

Ex.8

50

3.8

5.4

100

200

20

2.61㬍10146

25

106

46

156

30

Ex.9

50

3.8

5.4

100

300

2.77㬍1020

Ex.10

80

2.5

4.2

100

200

3.99㬍1020

90

K. Katayama et al., Hydrogen Incorporation into Tungsten Deposits Growing under Hydrogen and Argon Mixed Plasma

0.35-300V

0.30

61500

1000-200V42500

Deposition rate [mg/h]+8+Erosion yield [mg/h/A]

2000

0.40

10RF 40W RF 100W

Ion density ratio, H2 /Ar [-]

2500

0.25

H2/Ar ratio
2%
20%
50%
80%

0.20
0.15

Deposition rate
= 0.12
Erosion rate

0.10
0.05

00204060
H2/(Ar+H2) [%]

80

00.00
0.0

0.5

1.0

1.5 2.0 2.5 3.0Erosion rata [mg/h]

3.5

4.0

Fig.3 The dependence of hydrogen concentration in theFig.4 The deposition rate on the substrate with the erosion

working gas on the erosion yield of the tungsten tube and on

rate of the tungsten tube.

the ratio of hydrogen ions to argon ions.

increasing hydrogen concentration. Additionally, theweight loss of the tungsten tube was undetectable inseveral-hour exposure under pure hydrogen plasma. These
results indicate that the erosion of tungsten was caused byargon ion incident. Impinging energy of argon ionsobtained from negative bias is mitigated by collisions withneutral molecules. Therefore the erosion yield isconsidered to decrease with decreasing the argon iondensity. The erosion yield at a negative bias of 300 V wasquite lager than that at a negative bias of 200 V in 2 % and
20 % of hydrogen concentration but these are close in
50 % of hydrogen concentration. This indicates that thenegative bias effect on erosion is large when argon ion
density is high.
The deposition rate against the erosion rate in eachdeposition condition is plotted to Fig.4. The ratio of thedeposition rate to the erosion rate was 0.12 on the average.
This value is smaller than 0.16 which is estimatedgeometrically. This difference is due to the erosion of thetungsten tube in the branch tubule, the slip of the substratefrom the designed position and the scattering of sputteredtungsten atom by collision with neutral molecules.
SEM image of the cross-section of the tungsten deposit
formed under hydrogen and argon mixed plasma, 20 % inhydrogen concentration, 7.1 Pa in working gas pressure,
100 W in RF power, 300 V in negative bias, 96 oC in
temperature and 36.5 h in deposition period (Ex.6) isshown in Fig.5. The quartz substrate was broken in half
and the surface was observed. It was found that thetungsten deposit has a columnar structure. The weightpercent of oxygen in the tungsten deposit formed underhydrogen and argon mixed plasma, 20 % in hydrogenconcentration, 4.8 Pa in working gas pressure, 40 W in RFpower, 200 V in negative bias and 100 oC in temperature(Ex.7), is 2.0 wt%.
An example of hydrogen release behavior from tungsten

Instrumental Analysis, Kyushu University.
Ion densities at the vicinity of the substrate under purehydrogen plasma and pure argon plasma were measured by
the double probe method. The probes were inserted fromthe branch tubule to plasma without the target tube.
Additionally, the emission intensities from H2+ and Ar+ in
plasma at the vicinity of the substrate were measured forpure hydrogen, pure argon and hydrogen-argon mixed
plasma by a plasma process monitor (PlasCalc-2000,Mikropack). Ion densities of hydrogen and argon in themixed plasma were estimated from the double probe
measurement and the emission intensity measurement
assuming the emission intensity is proportional to the ion
density and H2+ is dominant in hydrogen ions.
After sputtering-deposition process, the substrates wereplaced into a separate quartz tube connected to a gas
chromatograph (GC:GC-8A, SHIMAZU Co.). The quartz
tube was filled with argon gas and heated from 200 oC to800 oC in 200 oC step by an electric furnace. Argon gas
was introduced into the reaction tube every 20 minutes inorder to transport the released hydrogen into GC. Since theamount of hydrogen released at 800 oC was comparable tothe background level, the hydrogen released by 600 oC wasregarded as the hydrogen incorporated in the tungsten
deposit.

3. Results and discussions

Fig.3 shows the dependence of hydrogen concentrationin the working gas on erosion yield of the tungsten tube
which was derived from dividing the erosion rate (mg/h)by the target current (A). The right axis indicates the ratioof hydrogen ion density and argon ion density under eachdeposition condition. The hydrogen ion ratio increased
approximately proportionally to hydrogen concentration. Itwas observed that the erosion yield decreases with91

K. Katayama et al., Hydrogen Incorporation into Tungsten Deposits Growing under Hydrogen and Argon Mixed Plasma㩷㩷

1.53.03Pm

800-200V (Total:1.22×10 H2-mol/g)

700-5-300V (Total:4.99×10 H2-mol/g)

87600
Temperature

500

5400

4300

3200

20204060-40.0

6.9 u10 3 f H f W 0.42 ,(1)where 7.3 ≤ fH/fW ≤ 4780, 65 ≤ En ≤ 652 eV, 331 ≤Temperature ≤ 429 K. En is the average energy of thereflected particles on the target. The value of En was
estimated using the following formula [6]:10010H DWo6300

between the target and the substrate. Therefore, the bias ofthe target is considered to be one parameter that affects onhydrogen retention in tungsten deposits forming bysputtering method. However, the result shown in Fig.6
suggests that other parameter also contributes to hydrogenretention. The present authors considered that the ratio ofhydrogen flux to tungsten flux from the target to thegrowing surface, fH/fW contributes to hydrogen retention.
The hydrogen flux reflected from the tungsten target wasestimated from the hydrogen ion flux and the fraction ofbackscattered particles, RN [6]. The hydrogen ion flux wasestimated from the target current and the ratio of hydrogenion density to argon ion density. The tungsten flux wasestimated from the deposition rate. The relationshipbetween the ratio of hydrogen flux to tungsten flux andhydrogen retention as H/W is shown in Fig.8. The amountof tungsten deposited on the substrate was obtained fromthe deposition weight. Data from the parallel plate RF
sputtering device [2,7] are also indicated. Although thetungsten deposit reported in [2] contained not only
deuterium but also hydrogen, the reflected particle isassumed to be only deuterium because the working gas is
pure deuterium. The expression obtained from fitting all of
data is as follows:

Temperature [ C]

-5H2 release rate [10 mol/g/(20min)]

9300 200
Target voltage [-V]

200

0.5

and deposition rate for three deposition conditions.

deposits is shown in Fig.6. For all samples, the release
behavior of hydrogen was similar. The hydrogen releasebegan at 200 oC and continued until 600 oC. The highest
release rate was observed at 400 oC. It was found that ahigh temperature of 600 oC is necessary to removehydrogen incorporated in the tungsten deposit.
The dependence of negative bias on hydrogen retention(mol-H2/g) and deposition rate (g/hour) are shown in Fig.7
for three deposition conditions. It was observed that thedeposition rate at 300 V is obviously higher than that at200 V. This means that the impinging energy of ions to thetungsten tube at 300 V is higher than that at 200 V. On theother hand, the change of hydrogen retention with negative
bias was different in the deposition condition. A part ofhydrogen and argon impinging to the target is considered
to be reflected and implanted into the growing surface ofthe tungsten deposit. These reflected particles areconsidered to have relatively-high energy as comparedwith ions which impinge to the growing surface of thetungsten deposit with the sheath potential although its
energy is reduced by the collision with neutral molecules-4

2%H2/Ar, RF 40W, 7.6Pa
20%H2/Ar, RF100W, 7.1Pa
50%H2/Ar, RF100W, 5.4Pa

Fig.7 The dependence of target bias on hydrogen retention

formed in Ex.6.

2%H2, RF40W, 7.6Pa

1.0

0.5

0.0

Fig.5 SEM image of the cross-section of the tungsten deposit

101.5Deposition rate [×10 g/h]

2.01.0-4

H2 retention [×10 mol-H2/g]

2.5

E n ( E0 )080 100 120 140 160 180
Time [min]E0RE ( E0 )R N ( E0 ),(2)where E0 is the energy of the ions impinging on thetungsten target, RE is the energy reflection coefficient. TheFig.6 Hydrogen release behavior from the tungsten deposit
formed in Ex.1 and Ex.2.

92

K. Katayama et al., Hydrogen Incorporation into Tungsten Deposits Growing under Hydrogen and Argon Mixed Plasma

4. Conclusions-3

The influence of deposition condition on hydrogenincorporation into tungsten deposits growing underhydrogen and argon mixed plasma was investigated byexposing a tungsten tube to the plasma. The erosion of thetungsten tube is mainly caused by the incident of argon
ions. The tungsten deposit has a columnar structure. Inorder to remove the hydrogen incorporated into thetungsten deposit, a high temperature of 600 oC is necessary.
While the tungsten deposition rate is promoted by theincrease of negative target bias from 200 V to 300 V, theinfluence of the target bias on hydrogen retention isdifferent in the deposition condition. Hydrogenconcentration in the tungsten deposit increases with theratio of the flux of the hydrogen reflecting from thetungsten target to tungsten deposition flux. The flux ratio ofhydrogen and tungsten toward the growing surface of thetungsten deposit is an important parameter to evaluatehydrogen accumulation amounts.

0.42(H + D)/W = 6.9×10 (fH/fw)(fitting all of data)(H+D) / W [-]10652eV/D [2]-1580eV/D [2]480eV/H [7]1010-295eV 65eV-3

110: 7.6Pa, 2%H2: 7.1Pa, 20%H2: 4.6Pa, 20%H2: 5.4Pa, 50%H2: 4.2Pa, 80%H2o: 10Pa, 100%H2, 68 C
o: 2Pa, 100%D2, 71 C
o: 10Pa, 100%H2, 111 C1001000
H flux / W flux [-]10000

Fig.8 The dependence of the ratio of hydrogen flux to tungsten
flux on hydrogen isotope retention.

Acknowledgments

This work was supported by a Grant-in Aid for Young
Scientists (B) from the Japan Society for the Promotionof Science (KAKENHI21760691).

obtained value, 0.42 is close to the value of 0.4±0.1
reported by De Temmerman and Doerner [3], where 2 ≤(fH/fW) ≤ 500, 60 ≤ En ≤ 280eV, 293 ≤ Temperature ≤ 600 K.
The dependence of En on the retention was not observed inthis work while they suggest in [3] that the retention is
proportional to En1.88. Strictly speaking, the impinging
energy of reflected hydrogen to the growing surface of thetungsten deposit is abated by collisions with neutral
molecules between the target to the substrate. That is to say,hydrogen retention depends on the working gas pressure as
has been reported in [2]. Especially, the influence of thepressure would be large in the parallel plate RF sputtering
device [7] because the distance from the target to thesubstrate is long. The impinging energy of hydrogenisotope to the growing surface in the parallel plate RF
sputtering device might be comparable with that in thedevice used in this work. Hydrogen retention depends on
temperature as has been reported in [3,4,8]. For the datasummarized in Fig.8, these effects would be able to ignore
because the temperatures are relatively low and the range
is narrow.

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[2]
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[7]
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