Experimental study of transition from electron beam to rf-power-controlled plasma in DFCCP in argon with additional ionization by an electron beam

Abstract

Radio-frequency (rf) plasma with additional ionization by an electron beam (EB) is considered as a possible method for the independent control of plasma density, mean electron energy and mean ion energy. In this study, spatial transition from EB to rf-power-controlled dual-frequency capacitively coupled plasma (DFCCP) was studied using the following movable diagnostics: Langmuir and hairpin probes, a retarding field energy analyzer and optical emission spectroscopy. The beam (1.1–1.4 keV) is generated by a runaway EB module placed near the plasma chamber wall, while the plasma transition is caused by EB degradation with the distance from the EB module. The study was conducted in Ar at 200 and 400 mTorr gas pressures in 81 and 12 MHz DFCCP. When the EB is on, a significant decrease in the mean electron energy is observed, from 6 eV in the rf plasma down to 0.2–0.8 eV in the EB plasma. The EB also changes the shape of the electron energy probability function, from Druyvesteyn-like in rf plasma to Maxwellian-like. When both EB and rf power are applied, the mean electron energy increase and the electron density decrease, with the distance from the EB module, are observed due to the beam degradation. The ion energy distribution at the bottom electrode in rf plasma peaks at 25–30 eV and shifts down to a few eV in EB plasma. As in conventional DFCCP, the ion energy distribution can be fine-tuned by the application of a low-frequency rf bias. However, the use of an EB allows us to reduce the range of ion energies down to a few eV, which cannot be achieved in conventional rf discharges.