rf and dc discharge characteristics for opposed-targets sputtering
An rf opposed‐targets sputtering (OTS) system, which can also be used as a dc OTS one, has been constructed and its discharge and plasma characteristics have been investigated. The rf OTS system has discharge and plasma characteristics similar to those of rf magnetron sputtering. Ionization of sputtering gases using high‐energy γ electrons is the main mechanism for producing high‐density plasma in both rf and dc OTS, though the ionization of gases by the rf excitation of electrons in the rf electric field becomes prominent at low plasma density in rf OTS. The absolute value of the self‐bias potential (‖Vsb‖) of targets in rf OTS during sputtering is much higher than that of the applied target voltage (‖Va‖) in dc OTS. This is mainly caused by the fact that the trapping efficiency of γ electrons is much higher in dc OTS than in rf OTS, since the γ electrons are lost at the target surface in rf OTS but not in dc OTS. It was found that the bombardment of ions to the substrate surface is markedly heavier in rf OTS than in dc OTS, though it can be made lighter by sputtering at higher gas pressure and plasma densities.