Naoki Toyoda

Effect of slot antenna structures on production of large-area planar surface wave plasmas excited at 2.45 GHz

The effect of slot antenna structures on plasma production was investigated in the large-area planar surface wave plasmas (SWPs) excited with 2.45 GHz microwave energy. Plasma production characteristics were measured for various types of slot antennae (inclined, transverse, longitudinal and diverging slots) in Ar discharges. In all the slot antennae, we clearly observed the density jumps when the incident power and pressure were varied. These density jumps correspond to the mode jumps between transverse magnetic (TM) eigenmodes in the SWP. In spite of different slot antenna structures, the same optical emission patterns of the TM62 mode were observed at the almost same electron density, except for the diverging slots, in which the TM92 mode was selectively excited under the same operating conditions, where the incident power was 0.8-1.2 kW at a pressure of 100-280 mTorr. At lower pressures, say 10 mTorr, the plasmas entirely broadened over the chamber cross section for all the slot antennae. Among the four slot antennae, the transverse slot antenna was more efficient for plasma production under the same incident microwave power at a fixed pressure.

Local resonant excitation of plasma oscillations in a planar surface-wave plasma device

The paper presents space-resolved microwave intensity measurements in a surface-wave plasma device clearly demonstrating the existence of a local resonance in a resonance layer where the local electron plasma frequency is equal to the surface-wave frequency. It has already been suggested in the literature that Landau damping of electron plasma waves excited in such a resonance layer and/or stochastic electron heating there might contribute to the surface-wave plasma energy balance. Since this does not involve collisions, it may become an important energy channel at low gas pressures. In order to avoid having the resonance layer too close to the plasma boundary, the measurements were performed not in the original surface-wave plasma, but using a weak non-ionizing 2.4 GHz microwave propagating in an inductively coupled plasma created by an internal loop antenna fed by a high-power 13.56 MHz generator. Still the original surface-wave plasma source geometry, including the microwave input port, was preserved. The resonance layer was identified by the microwave intensity peak, which was found to shift on changing the plasma density profile and/or the wave driving frequency in compliance with theoretical expectations. Accompanying space-resolved plasma density measurements confirmed this interpretation.

Characteristics of Ultrahigh-Frequency Surface-Wave Plasmas Excited at 915 MHz

Characteristics of 40-cm-sized planar plasmas excited at 915 MHz using various types of slot antennas in Ar and O2 gases are investigated. Axial profiles of the wave electric field and the electron density clearly reveal that high-density surface-wave plasmas are easily produced even at relatively low powers: the electron density increases linearly from 5×1010 cm-3 (the critical density for pure surface-wave mode excitation) to 6×1011 cm-3 with the net incident power from 100 W to 1 kW at 80 mTorr in Ar. Thus, the 915 MHz discharge enables one to control the plasma, even at low densities where a conventional 2.45 GHz surface-wave discharge suffers from unstable density jumps. In addition, the present results indicate that almost linear plasma production characteristics are obtained in three types of slot antennas, that is, one inclined slot, a pair of transverse slots and a combination of two transverse slots and two longitudinal slots. These advantages of the 915 MHz excitation are due to the critical density being lower than that of the 2.45 GHz excitation by a factor of seven. A large-area uniform plasma with a homogeneity of ±5% is obtained for both Ar and O2 discharges over a horizontal length of about 20 cm at an axial position of z=23 cm away from the top quartz plate just below the slot antennas.

Production of Low-Pressure Planar Non-Magnetized Plasmas Sustained under a Dielectric-Free Metal-Plasma Interface

Large-area Ar and CF4 non-magnetized plasmas were produced in an entirely metal (stainless steel) plasma chamber with a diameter of 220 mm by 2.45 GHz electromagnetic wave launched by slot antennas cut in the top circular metal lid. Dielectric (quartz) was used only for local vacuum sealing over the slot antennas, occupying less than 20% of the top metal area. At low pressure of 10 mTorr, overdense (>1011 cm-3) plasma was produced filling the whole chamber cross-section similarly to the known case of surface-wave plasmas produced below a large dielectric window covering 100% of the top chamber lid. The absence of this large dielectric suggests that this approach can be used for developing large-area non-magnetized plasma source with less impurities for thin-film processing.