Electrical control of the spatial uniformity of reactive species in plasmas

Abstract

We report a new method for controlling the spatial distribution of reactive chemical species in a parallel-plate plasma reactor, by means of a variable-impedance load placed between the unpowered electrode and ground. The technique was demonstrated in 89% CF4/11% O2 and 51% C2F6/49% O2 chamber-cleaning plasmas at 13.3–133 Pa (0.1–1.0 Torr) in a Gaseous Electronics Conference Reference Cell. The rf current and voltage at both electrodes were measured, and plasma spatial characteristics were observed using two-dimensional (2D) planar laser-induced fluorescence of the CF2 radical and 2D broadband optical emission measurements. By adjusting the load impedance to cancel the impedance of stray capacitances in parallel with the load, or the sheath capacitance in series with the load, the rf current at the load electrode could be made higher or lower than the current received when the electrode is grounded. When the rf current at the load electrode was minimized, regions of intense optical emission and high CF2 density were shifted radially outward from the center of the reactor. When the rf current at the load electrode was maximized, regions of intense optical emission and high CF2 density shifted radially inward, and the distribution of CF2 across the electrode surfaces became more uniform. These results suggest that variable loads could be used to direct reactive species in chamber-cleaning plasmas to the surfaces most in need of cleaning, or to increase the radial uniformity of reactive species in etching plasmas.