H. Sugai

H. Toyoda

K. "Nakamura

T." Kuroda

National Institute for Fusion Science (NIFS-Repository)

§8. Electrostatic Coupling in an Inductive RF Discharge Nakamura, K., Toyoda, H., Sugai, H. (Faculty of Eng., Nagoya Univ.) Kuroda, T. Recently, much attention has been attracted to an inductively coupled RF plasma as an innovative plasma source for neutral beam production [ 1] as well as material processing. Even in an inductive discharge, a capacitive coupling from a RF antenna has been observed to be distributed along the antenna conductor[2] and leads to the sputtering of insulating wall materials covering the antenna [3]. In this study, electrostatic shielding of a RF coupler is attempted, and the shielding effects are investigated. The electrostatic shield of an antenna is formed as follows. The 20-cm-diam. one-turn insulated antenna is wrapped by grounded metal sheet which has many slits of 2 mm in width at intervals of ,..., 2 em in the azimuthal direction to prevent a secondary RF current from flowing over the shielding metal sheet. Figure 1 shows the RF voltage <1>RF induced capacitively on ·a static probe as a function of the distance &- from the surface of the antenna in the absence of plasmas. The electrostatic coupling from the shielded antenna is drastically reduced compared with the unshielded antenna. Futhermore, a magnitude of the electrostatic coupling is evaluated by monitoring thtbias voltage V DC on a metal segment which is attached on the surface of the antenna immersed in a plasma. As shown in Fig. 2, the value of V DC for the shielded antenna becomes positive and close to the floating potential given by a Langmuit:_ probe. Therefore, antenna shielding leads to the decrease in the potential difference between plasma and antenna surface, suggesting the possibility of extending the operation lifetime of RF antenna by electrostatic shielding since the thermal load by ion bombardment is reduced. On the other hand, the antenna shielding makes the electron temperature not changed but the plasma density halved, probably caused by the eddy current loss in the shielding metal sheet and partly by the lack of power input due to the capacitive (electrostatic) coupling. 78 8 7 6 > 5 LL. 4 a: --e.. 3 2 1 with Shield 0 -2 0 2 4 6 8 D.r (em) Fig. 1. L1r v. s. RF voltage <1>RF for the unshielded and shielded antenna. 30 ~ 20 Plasma Potential 0 0 > 10 ~ (1) Ol ro ....... 0 0 > en ·ca -10 en I -(1) -20 (/) -30 0 200 400 600 800 1000 Power(W) Fig. 2. Self-bias voltage V DC v. s. RF power for the unshielded and shielded antenna References 1) Y. Takeiri, T. Takahashi, 0. Kaneko et al. : Proc. 5th Int. Toki Conf. on Plasma Physics and Controlled Nuclear Fusion, Toki, Japan, 1993. 2)H.Sugai, K. Nakamura and K. Suzuki: Jpn. J. Appl. Phys., 33 (1994) 2189. 3) Y. Hikosaka, M. Nakamura and H.Sugai: Jpn. J. Appl. Phys., 33 (1994) 2157. 

Electrostatic Coupling in an Inductive RF Discharge

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