Vol.9 No.3 2017
14/74

Research paper : High quality and large-area graphene synthesis with a high growth rate using plasma-enhanced CVD (M. Hasegawa et al.)−127−Synthesiology - English edition Vol.9 No.3 (2017) case of Ar/H2 plasma pretreatment, the broad peaks (934.5 eV and 942.5 eV) of Cu 2p3/2 of the bivalent copper oxide compounds disappeared. The peaks due to the monovalent copper oxide Cu2O still remained in the vicinity of 932.5 eV and 952.5 eV.[29]–[33] From these results, we confirmed that the surface of the as-received copper foil substrate is covered with the bivalent copper oxide on Cu2O/Cu. The bivalent copper oxide was completely eliminated from the substrate and the monovalent copper oxide was not removed by the Ar/H2 plasma pretreatment. On the other hand, in the spectrum obtained after the He/H2 plasma pretreatment, the peaks attributed to monovalent copper oxide, Cu2O, and the bivalent compounds Cu(OH)2 and CuO were not observed, only the peaks attributed to Cu 2p1/2 and Cu 2p3/2 of pure copper were observed. This is consistent with there being no O 1s signals related to any oxides on the copper foil substrate after the He/H2 plasma pretreatment, as shown in Fig. 3(a). This indicates that the He/H2 plasma pretreatment is very effective for removing copper oxide on the surface of copper foil substrates. Then, we examined the removal of silicon impurities on the copper foil substrate from the XPS spectrum of the Si 2p binding energy shown in Fig 3(c). A peak with Si 2p binding energy of 102 eV was observed on the surface of the as-received copper foil substrate. We surmise that siloxane compounds containing Si, such as silicone were applied as protective coating for copper foil surfaces before shipment from the factory. The Si 2p binding energy of Si compounds depends on the oxidization state of siloxy units[34][35] and silicon oxides.[36]If the number of oxygen atoms binding to Si atoms increases, the Si 2p binding energy will shift from 101 eV to 103 eV. The observed binding energy of 102 eV of Si 2p corresponds to that of poly(dimethylsiloxane) (PDMS),[34][35] as shown in Fig. 3(c). In the case of Ar/H2 pretreatment, a new peak at 103.0 eV due to Si 2p appeared, although the peak intensity of Si 2p at 102.0 eV of the as-received copper foil substrate decreased slightly. The appearance of the peak of Si 2p at 103.0 eV following the Ar/H2 plasma pretreatment occurs for two reasons. First is the oxidation of PDMS, indicating the formation of a CH3SiO3 siloxy unit by the oxidization of the CH3SiO2 (PDMS), as shown in Fig. 3(c). Second is the formation of SiO2 from etching of the quartz window by the Ar/H2 plasma. In contrast, the peak due to Si 2p disappeared completely after the He/H2 plasma pretreatment. Hence, it is found that the He/H2 plasma pretreatment effectively eliminates silicon impurities including silicon oxides on the copper foil surface and suppresses the extreme over-plasma etching of the quartz window. Furthermore, we investigated the XPS spectrum of the C 1s region in order to clarify the protective coating material on the as-received copper foil substrate, as shown in Fig. 3(d). There are three peaks at 285.0 eV, 286.5 eV, and 288.6 eV observed for the as-received copper foil. The strongest peak observed at 285.0 eV is mainly due to the C-C and C-H bonded groups in a sp3-hybridized state.[37] The shoulder peak observed at 286.5 eV is due to C-O-C bonding of the ether/phenolic components, and the highest binding energy observed at 288.5 eV is due to O=C-O bonding of the ester/carboxylic components.[37] Furthermore, we observed nitrogen atoms at 400.2 eV in the inset of the survey spectrum in Fig. 2. Again, we surmise that this is due to a corrosion inhibitor for copper foil that contains O=C-O, C-O-C, C-C, and C-H groups and N. It is well known that benzotriazole (BTAH) is used as an effective corrosion inhibitor for copper.[38][39] Although, BTAH (C6H5N3) has none of the functional groups of O=C-O and C-O-C, Finšgar et al. have observed the XPS C 1s spectrum with these groups on the surface of copper after of 1 h treatment with 3 % NaCl solution containing 10 mM BTAH.[38] They suggested that either the oxidation of carbonaceous species occurred or oxidized carbon compounds were adsorbed on the topmost surface of copper. Their spectrum closely resembles that of the as-received copper foil in Fig. 3(d). They have also observed signals of the Cu Auger L3M4,5M4,5 region of the Cu-BTAH complex at 572.6 eV at uppermost part of the copper substrate using angle resolved XPS measurement. The signal of the Cu-BTAH complex in the region of Cu Auger was not detected in our experiment, because it was not an angle-resolved measurement. After the Ar/H2 plasma pretreatment, the peak of C 1s at 285 eV observed for the non-plasma-treated substrate became sharper and the peaks at 288.6 eV and 286.5 eV disappeared. This means that BTAH is easily decomposed by the Ar/H2 plasma pretreatment. The peak of C 1s at 285.0 eV shifted to 284.5 eV after the Ar/H2 plasma pretreatment. The binding energy of 284.5 eV corresponds exactly to that of PDMS.[35] Hence, PDMS was mostly left on the copper foil substrate after the Ar/H2 plasma pretreatment. This is consistent with the existence of PDMS of O 1s (532.0 eV) in Fig. 3(a) and Si 2p (102.0 eV) in Fig. 3(c) after the Ar/H2 plasma pretreatment. After the He/H2 plasma pretreatment, the binding energy observed at 284.1 eV corresponds to that of HOPG which is composed of sp2 bonding.[40][41] These were confirmed as amorphous sp2 carbon lms from the Raman spectrum. The difference between the effect of He/H2 and Ar/H2 plasma pretreatment methods can be attributed to the difference between the sputtering yields of SiO2 for helium and argon. In the case of surface-wave plasma CVD, high-density plasma is excited in the vicinity of the quartz window, and the mixing of silicon and oxygen with the plasma by the sputtering of the quartz window is a major issue. That is, it is necessary to suppress the deposition of such impurities onto the substrate. According to the basic theory of sputtering by Sigmund, sputtering yield depends on the atomic weight and atomic number of the target and ions.[20] When the ion energy is 100–600 eV, the yield of quartz (SiO2) sputtering with argon ions is 2.5–3.8 times greater than that of helium ions.

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