The effects of electrostatic, molecular drag and gravitational forces on the behavior of particle clouds in an RF discharge

A Gaseous Electronics Conference (GEC) Reference Cell was used to investigate the behavior of graphite particles sputtered in an argon plasma. By configuring the reactor in a normal (wafer facing upward) and in an inverted (wafer facing downward) configuration, and by varying the direction and flow of the argon, the relative importance of gravity, molecular drag and electrostatic trapping could be observed. It was observed that particle clouds in specific locations adjacent to the wafer (referred to as particle traps) were identical in both configurations. Drag forces from the showerhead grounded electrode were sufficient to prevent particles from accumulating in the traps over the wafer. Drag forces from flow parallel to the wafer (⩾25 cm/s) easily emptied the traps. Gas flow from a ring source mounted above a downward facing driven electrode were observed to affect the manner in which particles were removed from the ring-shaped trap surrounding the cathode. In that case, particles were observed to flow down eight fluid streamlines toward the eight openings through which the gas flowed from the reaction chamber to the pump. Comparison of results for the normal and inverted configurations also permitted estimates of the number of electrons attached to each particle