THE EFFECTS OF ELECTROSTATIC, MOLECULAR DRAG AND GRAVITATIONAL FORCESON THE BEHAVIOR OF PARTICLE CLOUDS IN AN RF DISCHARGE

A Gaseous Electronics Conference (GEC) Reference Cell was used to inve stigate the behavior of graphite particles sputtered in an argon plasm a. 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 ob served that particle clouds in specific locations adjacent to the wafe r (refered to as particle traps) were identical in both configurations . Drag forces from the showerhead grounded electrode were sufficient t o prevent particles from accumulating in the traps over the wafer. Dra g forces from flow parallel to the wafer (greater-than-or-equal-to 25 cm/s) easily emptied the traps. Gas flow from a ring source mounted ab ove a downward facing driven electrode were observed to affect the man ner in which particles were removed from the ring-shaped trap surround ing the cathode. In that case, particles were observed to flow down ei ght fluid streamlines toward the eight openings through which the gas flowed from the reaction chamber to the pump. Comparison of results fo r the normal and inverted configurations also permitted estimates of t he number of electrons attached to each particle.