H. Sugai

In order to improve the process repeatability of silicon dioxide etching, surface control of reactor wall based on ion bombardment is proposed. In this new technique, a reactor wall is almost fully covered with two floating bias walls, and rf bias is applied between the two walls with transformer coupling at 400 kHz. The biased walls are alternately bombarded by energetic ions in the negative phase of the rf bias. This alternating ion bombardment (AIB) technique enables us not only to suppress the polymer deposition during dielectric etching but also to clean the walls deposited with polymer during run-to-run etching. The AIB has little influence on electron density and plasma potential during etching. The radical diagnostics revealed that, during the AIB, mainly CF and CF2 densities increase due to ion-enhanced desorption from the wall at the rate estimated from the removal rate of the fluorocarbon polymer layer.

The effects of the plasma profile on the global energy confinement have been studied in Heliotron E with special regard to differences between heating methods (ECH, NBI and NBI + ECH). With high power NBI, peaked T_i and peaked n_e profiles (T_i(0)/T_i 2.7, n_e(0)/n_e 4.5) were simultaneously achieved under low recycling conditions. A peaked n_e profile (n_e(0)/n_e 2.5 ) could lead to the high T_i mode where the ion heat transport in the central region is substantially reduced. By changing the ECH launching condition (on-axis, off-axis and toroidally oblique injection), the peakedness of the T_e profile could be controlled in the range 1.3 T_e(0)/T_e 4.5. A peaked T_e profile and a flat n_e profile (3.5 T_e(0)/T_e, n_e(0)/n_e 1.8) were brought about by the well focused on-axis ECH. The ECH plasma with a peaked T_e profile has higher stored energy than that with a moderately peaked T_e profile for the same injected ECH power and the same density region. The global energy confinement time normalized by the LHD scaling, τ_E ^G/τ_E ^LHD, showed n_e(0)/n_e dependence for the low T_i mode NBI plasmas. For the high T_i mode, the n_e(0)/n_e dependence of τ_E ^G/τ_E ^LHD was weak. These findings suggest that the LHD scaling should be modified to scale the global energy confinement of the helical plasmas in a wide range of n_e(0)/n_e.

In a methane/argon discharge used for deposition of hydrogenated amorphous carbon (a‐C: H), energetic ion bombardment yields radicals on solid surfaces by two mechanisms: (i)fragmentation of hydrocarbon ions at their impact on the surface and (ii)sputtering of the already deposited a‐C:H film. To discriminate between these two mechanisms, the emission intensity of CH(A‐X) in the vicinity of a negatively biased electrode was measured as a function of the ion impact energy. The threshold energy for yielding the excited CH radical was found to be ∼2 eV for fragmentation and ∼80 eV for sputtering. The fragmentation yield is much larger on a metal surface than on a‐C:H layer. The sputtering yield dominates over the fragmentation yield for the high impact energy(>150 eV).

Hydrogenated amorphous boron films have been produced by a toroidal glow discharge of B2H6/He mixtures at low pressures (10−2 Torr). Chemically stable pure films with a low hydrogen content can be formed at a moderate substrate temperature (∼200 °C) when the diborane flow rate is raised above 10 sccm with the discharge power lowered below 100 W. The controllability of H content is potentially interesting for applications in the coating of nuclear fusion devices, as well as for use in electronic devices.

Hydrogenated amorphous silicon films are formed in a low‐pressure dc toroidal discharge. The good plasma confinement by a toroidal magnetic field ensures considerable growth rate of the films even at a low pressure (0.1–10 mTorr). The contributions of ions and radicals to a‐Si:H film growth are clearly separated by the control of substrate bias and gas pressure. The films formed mainly by ion flux indicate large values of optical band gap and hydrogen content, in contrast with films formed by neutral radicals only. In addition, the spectrum of dominant IR absorption is observed to vary with the ratio of ion to radical contributions.