Photon Factory Activity Report 2018 #36 (2019)

BL-13B/2016T002, 2017PF-23

Improved Pumping Speeds of Oxygen-Free Palladium/Titanium Nonevaporable Getter
Coatings and Suppression of Outgassing by Baking under Oxygen
Tetsuya Miyazawa1, Yu Kano2, Yasuo Nakayama2, Kenichi Ozawa3, Toshiharu Iga4,Misao Yamanaka5, Ayako Hashimoto5,6, Takashi Kikuchi7, and Kazuhiko Mase1,7,*
1
SOKENDAI, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
2
Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan
3
Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro 152-8551, Japan
4
Osaka Vacuum, Ltd., 7-775 Ohtori-higashi-machi, Nishi-ku, Sakai 593-8324, Japan
5
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
6
University of Tsukuba, 1-2-1 Sengen, Tsukuba 305-0047, Japan
7
Photon Factory, Institute of Materials Structure Science, High Energy Research Organization(KEK), 1-1 Oho, Tsukuba, 305-0801, Japan
1 Introduction
Nonevaporable getter (NEG) coating is a technique inwhich the inner walls of a vacuum chamber are coated witha thin film of a NEG material [1, 2]. We recently developed
a new NEG coating consisting of an oxygen-free thin filmof Ti covered with a thin film of Pd, which we named the‘oxygen-free palladium/titanium (Pd/Ti) coating’ [3-6].
Here, we report an effective and simple method forremoving carbon contamination from the Pd/Ti surface toimprove the pumping speeds of oxygen-free Pd/Ti for H2and CO, and to reduce the partial pressures of H2, CO, H2O,and CH4 [7].
2 Experiment
The oxygen-free Pd/Ti film was deposited on SS304L
substrates or a SS304L chamber under clean ultrahighvacuum (UHV) conditions. The thicknesses of the Pd andTi thin films were approximately 50 nm and 1.3 μm,respectively. X-ray photoelectron spectroscopy (XPS)
measurements of oxygen-free-Pd/Ti coated substrate werecarried out at room temperature by using p-polarized SR
light with a photon energy (hν) of 700 eV at BL-13B ofPhoton Factory. The surface morphologies of the unheatedand UHV-heated oxygen-free Pd/Ti samples wereobserved by scanning electron microscopy (SEM). Totaland partial pressure curves of the oxygen-free Pd/Ti coatedchamber were measured using the apparatus shown in Fig.
1a. Before, during, and after UHV or O2 baking, the total
and partial pressures in the oxygen-free Pd/Ti coatedchamber with the TMP were measured by using aquadrupole mass spectrometer and the apparatus shown inFig. 1b. Pumping speeds of the oxygen-free Pd/Ti coatedchamber for H2 or CO were measured by the orificemethod [8] using the apparatus shown in Fig. 1c.
3 Results and Discussion
Samples of oxygen-free Pd/Ti that were unheated,heated in UHV at 150 °C for three hours, or heated at150 °C for three hours under an O2 pressure of 1.3 × 10−4
Pa were analyzed by XPS. We found that carbon
contamination decreased to an extent on heating in UHV,
but decreased considerably on heating in O2 (Figs. 2 and3). The graphite or graphene coverage of the unheatedsample was estimated to be 0.9 ML, whereas those of theUHV-heated and O2-heated samples were estimated to be0.3 and 0.04 ML, respectively.
Figures 4a and 4b show SEM images of the Pd surfaceof the unheated and UHV-heated (at 150 °C for 12 hours)
oxygen-free Pd/Ti samples. The Pd surface of the unheatedsample had an uneven structure with irregularities ofseveral tens to several hundreds of nanometers andnumerous steps, whereas the surface of the UHV-heatedsample had a leaf-like structure with relatively large flat
surfaces. These results are consistent with the XPS spectra,
which suggest that the number of less-coordinated Pd
atoms decreases after heating.

Fig. 1: Schematics of the apparatus for measuring (a) thetotal and partial pressures in the coated chamber before andafter valve closure; (b) the total and partial pressures in thecoated chamber without valve before, during, and after
UHV or O2 baking; and (c) the pumping speeds for thecoated chamber. Reproduced from Ref. 7, with thepermission of AIP Publishing.

Photon Factory Activity Report 2018 #36 (2019)

Fig. 2: Wide-scan XPS spectra of the unheated, UHVheated, and O2-heated oxygen-free Pd/Ti samples.
Reproduced from Ref. 7, with the permission of AIP
Publishing.

Fig. 4: SEM images of the Pd surface of the (a) unheatedand (b) UHV-heated oxygen-free Pd/Ti samples.
Reproduced from Ref. 7, with the permission of AIP
Publishing.
Figure 5 shows the total and partial pressures of theoxygen-free Pd/Ti coated chamber before and after valve
closure after UHV or O2 baking at 150 °C for 12 hours. In
the case of UHV baking, after the valve closure, the totalpressure was 4.2 × 10−6 Pa and the partial pressures of H2and CO increased to 1.4 × 10−7 and 8.9 × 10−7 Pa,
respectively, in five hours. On the other hand, in the caseof the O2 baking, the corresponding pressures wereimproved to 1.0 × 10−6, 3.6 × 10−8, and 5.8 × 10−8 Pa in fivehours. These improvements are much larger than would be
expected from the graphite or graphene coverage (0.3 MLfor UHV-heated and 0.04 ML for O2-heated samples). The
results therefore indicate that removal of carbon
contamination not only improves the pumping speeds ofoxygen-free Pd/Ti thin films for H2 or CO, but also
suppresses outgassing from the chamber.
Figure 6 shows the partial pressures in the oxygen-freePd/Ti coated chamber before, during, and after UHV andO2 baking. During UHV baking, the partial pressure ofCO2 was 1 × 10−6 to 2 × 10−8 Pa, whereas that during theO2 baking was 6 × 10−6 to 3 × 10−7 Pa. The partial pressuresof CO2 during the O2 baking were larger by a factor of6−15 than those during the UHV baking. This suggests O2
reacts with the carbon on the Pd surface to form CO 2.
During UHV baking, the partial pressure of H2O was 2 ×10−5 to 9 × 10−7 Pa, whereas that during the O2 baking was
3 × 10−5 to 1 × 10−6 Pa. The partial pressures of H2O during
the O2 baking was larger by a factor of 1.1−1.5 than thatduring the UHV baking. This result suggests that H on Pd
surface was removed by catalytic chemical reactions suchasFig. 3: Measured, fitted, and deconvoluted peaks of (a) Pd
3d5/2 and (b) C 1s. Reproduced from Ref. 7, with thepermission of AIP Publishing.

1 Pdcat
2Had + O2 → H2 O2

Photon Factory Activity Report 2018 #36 (2019)

during O2 baking. Removal of H and C adsorbed on theinner surfaces of the chamber seems to be responsible forthe reduction of the partial pressures of H2, CO, H2O, andCH4 after the O2 baking.
Figure 7 shows the measured pumping speeds of theoxygen-free Pd/Ti coated chamber for H2 and CO as afunction of the pumped quantity after UHV or O2 baking.
The apparatus used is shown in Fig. 1c. Pumping speedsfor H2 after UHV or O2 baking were measured as 510–120
and 990–400 L s−1, respectively in the pumped-quantity
range 0.01–10 Pa L. Those for CO were determined to be960–40 and 1250–140 L s−1, respectively in the pumpedquantity range 0.005–1 Pa L. The initial pumping speeds
were improved by factors of 1.9 for H 2 and 1.3 for CO.
These results indicate that O2 baking improves thepumping speeds for H2 and CO owing to removal of carbon
contaminants. These results demonstrate that O2 baking isuseful for removing the carbon contaminants from oxygenfree Pd/Ti and for recovering pumping speeds of oxygenfree Pd/Ti for H2 and CO.

Fig. 6: Partial-pressure measurements in the oxygen-free
Pd/Ti coated chamber during (a) UHV or (b) O 2 baking.
Five minutes after starting RP, the TMP was started, andthe QMS was started about 13 minutes later. The time
when pressure measurements commenced is taken as theorigin of the time axis. (c) Enlarged partial pressure curves
before and after the UHV or O2 baking off. The dominantgas species are indicated in parentheses. The apparatus
used is shown in Fig. 1b. Reproduced from Ref. 7, with thepermission of AIP Publishing.

Fig. 5: Total and partial pressure curves of the oxygen-free
Pd/Ti coated chamber before and after closing thepneumatic UHV gate valve 12 hours after (a) UHV or (b)O2 baking at 150 °C for 12 hours. The time of closure ofthe valve is taken as the origin of the time axis. (c)Enlarged pressure curves before and after valve closure.
The dominant gas species are indicated in parentheses.
Partial pressures of residual gases other than m/z = 2 (H2)or m/z = 28 (CO) are not shown. The apparatus used isshown in Fig. 1a. Reproduced from Ref. 7, with thepermission of AIP Publishing.

Fig. 7: Measured pumping speeds of the oxygen-free Pd/Ti
coated chamber for H2 or CO after UHV or O2 baking at
150 °C for 12 hours. The apparatus used is shown in Fig.
1c. Reproduced from Ref. 7, with the permission of AIP
Publishing.

Photon Factory Activity Report 2018 #36 (2019)

Acknowledgement
The authors are grateful to Takahiro Koyama, Shigeji
Sugimoto, Masashi Iguchi (Osaka Vacuum, Ltd.),
Mitsuyoshi Sato (Scienta Omicron), Hiromu Nishiguchi,
Eriko Kazama (Baroque International Inc.), Yasunori
Tanimoto (KEK-ACCL), Akio Toyoshima, and Hirokazu
Tanaka (KEK-IMMS), for their invaluable advice andsupport. This work was partly supported by a Grant-in-Aid
for scientific research (JSPS KAKENHI Grant Number
JP17K05067) and TIA-Kakehashi grants (TK17-013 andTK18-014). Part of the present research was conducted incollaboration with Osaka Vacuum, Ltd. Part of this work
was carried out by using the facility of NIMS TEM station.
This work was partially supported by the Global ResearchCenter for Environment and Energy based on
Nanomaterials Science.
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[2] C. Benvenuti et al., Vacuum 50, 57 (1998).
[3] T. Miyazawa et al., Vac. Surf. Sci. 61, 227 (2018).
[4] T. Miyazawa et al., J. Vac. Sci. Technol. A 36, 051601(2018).
[5] T. Miyazawa et al., AIP Conf. Proc. 2054, 060045(2019)[6] T. Kikuchi et al., AIP Conf. Proc. 2054, 060046 (2019).
[7] T. Miyazawa et al., J. Vac. Sci. Technol. A 37, 02160(2019).
[8] M. H. Hablanian, J. Vac. Sci. Technol. A 5, 2552 (1987).
* mase@post.kek.jp