Electrostatic trapping of contamination particles in a process plasma environment

A number of authors have observed, by using light scattering from a laser beam, contamination particles suspended in an rf process plasma. The region of space occupied by the particles appears finite, e.g., a ring; there is experimental evidence that the particles are negatively charged. We show, by using a tuned Langmuir probe, that the trap is electrostatic in nature. The volume of a trap is as much as 5 V larger in electrostatic potential than the surrounding plasma. This means that the volume of the trap is positively charged with the electric field being directed outward from the trap. Thus, negatively charged particles will flow into it. The electrostatic potential rises so rapidly at a trap boundary that a double layer may exist there. Finally, the plasma-sheath interface is found to follow the topographic contour of the rf electrode surface.