Controlled potential plasma source

Abstract

The occurrence of internally-formed contaminants or negatively-charged particulates within a plasma is minimized by preventing such from becoming trapped in the plasma. The plasma is formed in a plasma chamber having control electrodes and reference electrodes. The control electrodes are biased with a negative potential. The plasma assumes a potential more positive than the control electrodes. The reference electrodes are then biased to be more positive than the plasma. Hence, negative ions or negatively-charged particulates in the plasma are attracted to the more positive reference electrodes, and thus escape the plasma without being trapped therein, and are not available to serve as nucleation or agglomeration points for contaminants. A pair of Helmholtz coils produce a magnetic field having magnetic field lines that run longitudinally between the control electrodes. The magnitude of the magnetic field is sufficiently strong to confine electron current only along the magnetic field lines, yet sufficiently weak to allow negative ion current and negatively-charged particulates to cross the magnetic field lines. Because the plasma current density is dominated by electron current as opposed to ion current (due to the higher thermal velocity of electrons compared to ions), and because electron current is controlled only through the control electrodes, the plasma is effectively controlled by the potential applied to the control electrodes.