The electrical properties of contamination particle traps in a process plasma.

Abstract

Clouds of contamination particles suspended in process plasma have been observed by several workers. This dissertation reports on the electrical properties of such clouds (referred to as Electrostatic Particle Traps or EPT) in an argon sputter plasma using a silicon wafer placed upon a graphite substrate. Particle traps were illuminated using a specially adopted laser scanning technique. A tuned Langmuir probe was then inserted into the region of the trap and used to map several parameters including the time-averaged plasma potential. The trap was found to be as much as 5 volts higher in plasma potential than the surrounding plasma. Elementary electrostatics dictates that the trap is a region of net positive charge with an electric field being directed outward from the trap. Thus, negatively charged particles will flow into the trap. It was also found that the electrical properties of contamination particle traps are highly dependent upon the topography of the target and the materials used, with different results being obtained for each material combination. The Langmuir probe was also found to be an effective tool for mapping the interface between the plasma and the sheath to within 0.5 mm; the interface follows the topography on the wafer electrode.