A. Sagara

K. Nishimura

M. Nishikawa

N. Noda

T. Inoue

Y. Ueda

National Institute for Fusion Science (NIFS-Repository)

§10. Retention Characteristics of Boron Thin Fi lms under High Flux Ion Irradiation Ueda, Y., Nishikawa, M. (Fac. Engineering, Osaka Univ.), Noda, N., Sagara, A., Nishimura, K., Inoue, T. In LHD experiments, boron coating method is used for impurity control, mainly oxygen. Hydrogen .ions or heli~m ions are implanted and retained in boron thm films dUrIng discharges. This hydrogen or helium retention could affect particle balance of core plasma. Therefore, it is i~portant to understand retention characteristics of boron fIlms under LHD edge plasma conditions. . In this work, retention characteristics of boron thm films produced by vacuum deposition or ion plating technique are studied by the high flux ion beam test dev.ice (HiFIT) . The HiFIT device is equipped with an ECR IOn source with triode spherical electrodes, which focuses broad ion beam onto the target materials to obtain high ion flux up to about 3 x 1021 H/m2 for 3 ke V H3 +. The details of this device was described elsewherel). In this work, boron thin films were irradiated by 1 ke V H3 + ion beam at normal 24 2 b' . incidence with fluence of 1.3 x 10 Him. Car on ImpUrIty concentration in the beams was controlled by putting graphite plates in the ion source chamber to supply carbon atoms as hydrocarbon molecules produced by chemical sputtering. Oxygen impurity concentration was always a~out O.OS %, which was independent of carbon concentratIOn. The other impurity concentration was less than a detection limit (- 0.0 I %). Irradiation temperature was about 80°C. Elevation speed of sample temperature for TDS was 1 Kls. Boron thin films were made by vacuum evaporation with a conventional 270° deflection type EB gun (4kV, SOO mA). The substrate were Mo plates with the dimensions of 9.S x SO x 0.4 mm. Substrate temperature was carefully controlled within 3°C. Film thickness was measured by a thickness monitor with a quartz crystal oscillator. In addition, the film thickness was measured afterwards in air by a surface profilometer to make a precise calibration. During thin film formation, internal stress was measured from the change in curvature induced in the substrate by using an optically levered laser method. The light source was a He-Ne laser with a power of 1.S mW. The laser beam was focused on the detector by the cylindrical les with 1000 mm focal length. The resulting translation of the reflected beam was measured by a position sensitive detector (PSD, Hamamatsu S3931) with a sensitive area of I x 6 mm2• As deposition rate was increased, tensile stress i~ the bor~n thin films increased, which suggested that the mteratomIC distance of the boron films increased with the deposition rate. This result agreed with the results of radial distribution 2) h '1 function of boron thin films measured by TEM . T e tensl e stress for the samples in this experiment is about 1 GPa. Figure 1 shows TDS spectra for the cases of carbon concentration of 0.4 % (a) and O.OS%(b). For the low carbon 220 4------~-----r-----.----_. (a) 1 ~keV H3+~: C - O . ~ % . . ......... , ................. . • .............. OL-----~-----L----~ ____ ~ 100 200 300 400 500 Temperature (C) 4------~------~-----.-----. OL------L--____ ~ ____ ~ ____ ~ 100 200 300 400 500 Temperature (C) Fig.1 TDS spectra from boron thin films irra~iat~d by 1 keV H3 + ion beam. Carbon concentration IS 0.4 % for (a) and 0.05 % for (b). concentration case (b), desorption peak was observed around 200°C and 3S0 °C and most of hydrogen was des orbed below the temperature of about 400°C. On the other hand, for the high carbon concentration case (a), new desorption peak appeared around SOO°C, in addition to the peak observed in the low C case. In the previous works done by Hino et aI.3) and Tsuzuki et aI.4>, hydrogen desorption behavior from boron thin films formed by glow discharge using diborane or decaborane gas showed the highest peak around 3S0°C and two other small peaks around 200°C and SOO°C. The peak positions of our experimental result are similar to these results, though the highest peak appeared around 200°C in our experiment. Since the desorption peak around SOO"C appeared for the high C concentration case, this peak could be related to carbon impurity. In the future, the dependence on fluence, C concentration, irradiation angle, and formation condition of boron thin films will be investigated. References 1) T. Shimada, et aI., Rev. Sci. Instrum. 73 (2002) 1741. 2) H. Kemi et aI., J. Nuci. Mater. 266-269 (1999) 1108. 3) T. Hino et aI., Thin Solid Films 2S3 (1994) S18. 4) K. Tsuzuki et aI., J. NucI. Mater. 2S6 (1998) 166. 

Retention Characteristics of Boron Thin Films under High Flux Ion Irradiation

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