富山大学水素同位体科学研究センター研究報告 23 : 17-27, 2003.

Ｎｂ表面に偏析した酸素の水素イオンによるスパッタリング
波多野雄治1), Andrei Busnyuk2), Alexander Livshits2), 中村幸男3),
大藪修義3), 渡辺国昭1)(1)

富山大学水素同位体科学研究センター〒930-8555 富山市五福 3190

(2)

Bonch-Bruyevich University of Telecommunications
61 Moika, St. Petersberg 191186, Russia(3)

核融合科学研究所〒509-5292 岐阜県土岐市下石町 322-6

Sputtering of Oxygen on Nb Surface by Hydrogen Ions
Yuji Hatano1), Andrei Busnyuk2), Alexander Livshits2), Yukio Nakamura3),
Nobuyoshi Ohyabu3) and Kuniaki Watanabe1)(1)Hydrogen Isotope Research Center, Toyama University
Gofuku 3190, Toyama 930-8555, Japan(2)

Bonch-Bruyevich University of Telecommunications
61 Moika, St. Petersberg 191186, Russia(3)

National Institute for Fusion Science
Oroshi-cho, Toki 509-5292, Japan(Received March 29, 2004; accepted March 15, 2005)
Abstract
The rate of oxygen removal from an oxygen-doped Nb membrane by hydrogen ion
sputtering was examined by measuring the change in the sticking coefficient of H2, α. First,
the temperature dependence of α was measured at various oxygen concentrations toinvestigate correlation between bulk oxygen concentration and α. Then the oxygen
concentration in the membrane was adjusted to 1.5 at%, and the surface of the membrane wasirradiated by hydrogen ions of 600 eV at 1113 K. The change in α with sputtering time wasmeasured, and the reduction in bulk oxygen concentration was estimated from the extent of17波多野雄治・Andrei Busnyuk・Alexander Livshits・中村幸男・大藪修義・渡辺国昭

change in α. The oxygen concentration decreased with increasing sputtering time bycontinuous surface segregation of oxygen and sputtering. The sputtering yield of oxygen on
the Nb membrane surface was evaluated to range from 0.012 to 0.036.

1. Introduction
Metallic membranes covered by adlayers of non-metallic impurities such as oxygen andsulfur have enormous permeability for atoms and ions of hydrogen isotopes [1]. Hence, suchmembranes can be applied for particle control in fusion devices [2-5]. The high permeability
is due to the suppression of reemission of hydrogen isotopes from incident surfaces by thenon-metallic impurities. Therefore, reduction in the impurity coverage by sputtering degrades
the permeation capabilities. In other words, compensation for removed impurities is necessary
to sustain the performance of membranes under sputtering.
The present authors have reported that oxygen doping in the bulk of a Nb membrane iseffective to sustain the permeation capability under sputtering due to continuous surface
segregation of oxygen [6-9]. Prolonged sputtering, however, led to the degradation of
permeability owing to the reduction in the oxygen concentration in the membrane [9]. Thisobservation indicates that periodic oxygen doping is required as a maintenance work to keep
the good permeability. A longer intervening period between oxygen doping treatments is
better for the application viewpoint. Namely, the interval should be enough longer than that ofdischarge time. Otherwise, the permeation membranes could not be used for particle control.
One of the key factors determining the intervening period between oxygen doping treatments
is the sputtering yield of oxygen on membrane surfaces.
The purpose of the present study is to develop an experimental technique to evaluate thesputtering yields of impurity elements segregated to the surface of membrane materials. First,
a Nb membrane was loaded with oxygen to various bulk concentrations, and the correlation
between bulk oxygen concentration and the sticking coefficient of H2, α, was examined by18

Ｎｂ表面に偏析した酸素の水素イオンによるスパッタリング

permeation experiments on account that α could provide a measure of non-destructive
evaluation of the bulk oxygen concentration. Then, the bulk oxygen concentration was
adjusted to 1.5 %, and sputtering was carried out by hydrogen plasma at 1113 K. The
sputtering was periodically interrupted, and α was measured to evaluate the reduction in bulk
oxygen concentration with sputtering time. The sputtering yield was estimated from thenumber of incident hydrogen ions and that of removed oxygen atoms. The value of sputtering
yield thus obtained was compared with results of computer simulation with EDDY (computer
simulation of Erosion and Deposition based on DYnamic model) code.

2. Experimental
The details of the permeation apparatus used in the present study are reported
elsewhere [10], and only a brief description is given here. The permeation apparatus consistsof two ultra-high vacuum chambers, i. e. upstream and downstream chambers, separated by atubular specimen membrane. Both the upstream and downstream chambers are evacuated byturbo-molecular pumps (TMPs) and sputter-ion pumps (IPs) and equipped with analysis
instruments such as pressure gauges and mass spectrometers. The upstream chambercomprises the main discharge chamber and auxiliary chamber, and Ta filaments are installedin the former chamber as electron sources for discharge. The latter is equipped with theabove-mentioned analysis instruments. The discharge is carried out by applying a voltage
between the filaments serving as cathode and the chamber wall serving as anode. The kinetic
energy of incident ions onto the membrane surface was controlled by a bias voltage between
the chamber wall and the specimen membrane. The specimen membrane in tubular form isheated ohmically, and the temperature of the membrane is measured with a thermocouple
inserted in the membrane.
The specimen used was the membrane of polycrystalline Nb of 0.1 mm thickness.
Before the measurement of sputtering yield, the correlation between bulk oxygen

19波多野雄治・Andrei Busnyuk・Alexander Livshits・中村幸男・大藪修義・渡辺国昭

concentration and the sticking coefficient of H2, α, was examined by permeation experiments
at various oxygen concentrations under symmetric membrane condition, i. e. therecombination coefficients of H2 on the upstream and downstream surfaces are equal to eachother. Under such condition, α can be evaluated from the permeation flux φp by the following
relation: α = 2φp / φH2, where φH2 is incident flux of H2 on the upstream surface. The details of
permeation experiments are described elsewhere [11].
First, oxygen was removed from the membrane at around 1000 K by sputtering withhydrogen ions at H2 pressures from 0.3 to 0.6 Pa and bias voltages from 300 to 600 V. The ion
current density was kept at 2 mA•cm-2; this ion current density corresponds to the ion flux of
1×1016 cm-2•s-1. The sticking coefficient α which was 2.1×10-4 at 970 K before sputteringreached the maximum value, 2.6×10-3, after sputtering for 320 min. Such increase in α wasascribed to oxygen removal form the surface and bulk of membrane caused by continuous
surface segregation of oxygen and sputtering as described in Section 3. The value of α after
sputtering indicates that the oxygen concentration was about 0.03 at%. This oxygen
concentration is considered to be determined by a balance between the rates of oxygen
removal by sputtering and deposition by oxygen-containing impurity species in the plasma.
Then, oxygen was introduced into the membrane step by step up to 1.5 at%, and thetemperature dependence of α was measured at each step of oxygen doping. The method of
oxygen introduction is given in a previous paper [12].
After the introduction of oxygen up to 1.5 at%, the membrane was sputtered byhydrogen plasma at 1113 K. The discharge was periodically interrupted, and the change in thebulk oxygen concentration was examined by measuring α. During the sputtering, the bias
voltage and ion current density were adjusted to 600 eV and 1.1 mA•cm-2; this ion current
density corresponds to the ion flux of 7×1015 cm-2•s-1.
The surface concentration of oxygen was examined by Auger electron spectroscopy
in the Department of Advanced Energy Engineering Science, Kyushu University. Sheet type20

Ｎｂ表面に偏析した酸素の水素イオンによるスパッタリング

specimens containing 1 and 2 at% of oxygen were prepared, and Auger spectra were
measured at 1113 K, i. e. the same temperature as the above-mentioned sputteringexperiments. The energy of primary electron beam was adjusted to 3 keV. The pressure ofresidual gas during the measurements was less than 1×10-7 Pa.

3. Results and discussion
Figure 1 shows the temperature dependence of α after sputtering for given periods oftime along with that observed for specimens of various oxygen concentrations. The open
symbols indicate the values of α before sputtering. It is evident that α has strong dependence
on the bulk oxygen concentration COX; it decreased radically with increasing COX. Hence, it
was concluded that non-destructive evaluation of bulk oxygen concentration is possible bymeasuring α. This result is consistent with the observation in the previous study [12] that theH2 sticking coefficient, α

1000

800

600

T (oC)

400

500

COX
525 min

10-3

0.03%
0.038%
480 min

10-4

450 min

0.062%
0.088%
0.13%

555 min

0.23%

10-5
240 minSputtering time

10-6

Fig.1

0.8

1.0
1.2
1000/T (K-1)

0.5%

0.71%
1%
1.5%

1.4

1.6

Change in α with bulk oxygen concentration and sputtering time.
21波多野雄治・Andrei Busnyuk・Alexander Livshits・中村幸男・大藪修義・渡辺国昭

surface recombination coefficient of deuterium decreased with increasing bulk oxygen
concentration. The mechanism underlying the correlation between α and COX will be
discussed in a separate paper.
As mentioned in the previous section, the sputtering was started at COX = 1.5 at%.
The sticking coefficient α increased with sputtering time as shown by the filled circles inFig. 1. Such increase in α can be ascribed to the removal of oxygen not only from the surface
but also from the bulk caused by continuous surface segregation of oxygen and sputtering.
This is because no significant change in α was observed between respective runs of the
sputtering experiments; α should decrease in the intervals of sputtering owing to theresegregation of oxygen if only surface concentration of oxygen was reduced and COX kept
the initial value.
The change in COX with sputtering time tS is shown in Fig. 2. The bulk oxygen
concentration decreased with increasing tS. This plot is roughly approximated by a single

Bulk oxygen concentration, COX (at.%)straight line, indicating that the sputtering yield and oxygen surface coverage kept almost

1.6

Hydrogen ions energy: 600 eV
Incident flux of ions: 7x1015 cm-2s-1
Temperature: 1113 K

1.4
1.2
1.0
0.8
0.6

Sputtering yield
β = 0.036

0.4
0.2
0.0

Fig. 20100 200 300 400 500
Sputtering time, ts (min)600

Change in bulk oxygen concentration with sputtering time.
22

Ｎｂ表面に偏析した酸素の水素イオンによるスパッタリング

constant value in the range of COX from 1.5 to 0.058 at %. The sputtering yield of oxygen β
can be evaluated from the slope of the plot by the following equation:-dCOX / dts = β •φin •N / f •h(1)

where φin is the incident flux of ions, f the factor to convert the bulk oxygen concentrationfrom at% to atoms•cm-3, h the thickness of membrane, and N the number of protons
contained in incident ions, i. e. N = 1 for H+, 2 for H2+ and 3 for H3+. By assuming N = 1, βwas evaluated to be 0.036. The abundance ratio of H+, H2+ and H3+ in the plasma, however,
was not measured because operation of mass spectrometer during discharge was difficult due
to its geometrical configuration. Hence, the value of β can range from 0.012 (for each protonin H3+) to 0.036 (for H+). Another factor bringing uncertainty in β is the emission of
secondary electron by ion bombardment. Namely, the secondary electron emission leads tothe overestimation of incident ion flux. Ion beam experiments independently carried out in
Bonch-Bruyevich University indicated that the emission yield of secondary electron was
0.2-0.3 for 20 keV H3+ (0.07-0.1 for each proton). Therefore, the error induced by secondary
electron emission is considered to be about 10 %.
Figure 3 shows a typical example of Auger electron spectra obtained at 1113 K for
Nb specimen containing 1 at% of oxygen. Only peaks of Nb and oxygen were observed, andno other impurity was detected. The peak-to-peak height ratio of Nb (167 eV) and oxygen(512 eV) was measured at several different points randomly-selected on the specimen surface.
It ranged from 0.29 to 0.36, and the average value was 0.33. By assuming that oxygen waspresent only at the top layer, the oxygen coverage θOX was evaluated from the average valueof peak-to-peak height ratio by taking account of the escape depth of Auger electrons from Nb.
Namely, θOX was determined by the following equation:

θ OX ≅ [O] /[Nb ] =I (O) S Nb
I ( Nb) S O

1
,
−x
∞
) dx
∫0 exp(λ cos δ(2)where I(O) / I(Nb) is the peak-to-peak height ratio, SNb and SO are the atomic sensitivity

23

dN / dE

波多野雄治・Andrei Busnyuk・Alexander Livshits・中村幸男・大藪修義・渡辺国昭

Nb
O

Nb

0

100 200 300 400 500
Auger electron energy (eV)600

Fig. 3 Typical example of Auger spectra obtained at 1113 K for the specimen containing
1 at% of oxygen.
factors of Nb (1.3284) and oxygen (1.2571) at 3 keV of primary electron beam energy [13],x is the depth from the surface, λ the mean escape depth of Auger electrons from Nb (167 eV)in Nb, and δ the angle between detector and normal direction to the specimen surface. The
escape depth λ was evaluated by the equation proposed by Seah and Dench [14]. The value ofθOX thus evaluated was close to unity. The oxygen coverage on the specimen containing 2 at%of oxygen was also estimated to be 〜 1. Hence, it was concluded that θOX on the surface ofspecimen used for the sputtering experiments, COX in which was 1.5 at%, was also close tounity. It is appropriate to consider that θOX kept the value close to unity even under sputtering,
because no significant reduction in plasma-driven permeation rate was observed during thedischarge. Namely, the above-mentioned sputtering yield, 0.012-0.036, corresponds to thevalue at θOX 〜 1. It appears that the rate of oxygen diffusion was high enough incomparison with that of sputtering under the present conditions.
The sputtering yield obtained experimentally was compared with the results ofcalculation with EDDY code. The physical sputtering yield can be evaluated with this code by24

Ｎｂ表面に偏析した酸素の水素イオンによるスパッタリング

taking account of both elastic and inelastic collisions. The details of this code are described
elsewhere [15]. In the calculation, the oxygen concentration on the surface was adjusted to 50
at% as mentioned above and that in bulk to 1 at%. The binding energy of oxygen on thesurface was adjusted to 7.2 eV as a summation of the heat of adsorption of O2 molecule on thesurface of group 5 metals (4.6 eV for 1/2O2) [16] and the dissociation energy of O2 (2.6 eV
for 1/2O2) [17]. The dominant incident ion is assumed to be H+. The calculation was carriedout with various incident energies from 200 to 600 eV to simulate the incidence of 600 eV H+,
H2+ and H3+ ions and incident angles from 0° to 89° against the normal direction to thesurface. The sputtering yield was more sensitive to the incident angle than the incident energy,
and it ranged from 0.0038 to 0.0058 at 0° and 0.036 to 0.050 at 89°. The values of sputtering
yield obtained experimentally, 0.012-0.036, fall in this range. Therefore, it is appropriate toconsider that both the experiments and calculations are reliable, and physical sputtering is thedominant process of oxygen removal from the membrane surface under the present conditions.
The characterization of incident ions, i. e. incident angle distribution and the abundance ratioof H+, H2+ and H3+ ions, is necessary for more detailed discussion.

4. Conclusions(1) The sticking coefficient of H2, α, on Nb surface sensitively decreased with increasing

bulk oxygen concentration and could be used as a reliable measure for non-destructive
evaluation of bulk oxygen concentration.
(2) The surface oxygen coverage was close to unity at 1113 K and 1-2 at% of bulk

concentration.
(3) The sputtering yield of oxygen on Nb surface by hydrogen ions was estimated to be

0.012-0.036 under the bias voltage of 600 eV.
(4) The results of computer simulation with EDDY (computer simulation of Erosion and
Deposition based on DYnamic model) code was in reasonable agreement with the25

波多野雄治・Andrei Busnyuk・Alexander Livshits・中村幸男・大藪修義・渡辺国昭

experimentally-obtained sputtering yield.

Acknowledgements
This work has been supported in part by a Grant-in-Aid for Young Scientists (A) of
Ministry of Education, Culture, Sports, Science and Technology of Japan, No. 14702068, andthe NIFS LHD Project Research Collaboration. The authors express their sincere thanks toProfessor K. Hashizume and Mr. Y. Iditi for the use of a scanning Auger electron microscope
and the help in measurements. The computer simulation was carried out by EDDY code in the
PWI simulation code library of the National Institute for Fusion Science, Japan.
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