Electrical control of the spatial uniformity of reactive species in plasmas

We report a new method for controlling the spatial distribution of reactive chemical species in a parallel-plate plasma reactor, by means of a variabl e-impedance load placed between the unpowered electrode and ground. The tec hnique was demonstrated in 89% CF4/11% O-2 and 51% C2F6/49% O-2 chamber-cle aning plasmas at 13.3-133 Pa (0.1-1.0 Torr) in a Gaseous Electronics Confer ence Reference Cell. The rf current and voltage at both electrodes were mea sured, and plasma spatial characteristics were observed using two-dimension al (2D) planar laser-induced fluorescence of the CF2 radical and 2D broadba nd optical emission measurements. By adjusting the load impedance to cancel the impedance of stray capacitances in parallel with the load, or the shea th capacitance in series with the load, the rf current at the load electrod e could be made higher or lower than the current received when the electrod e is grounded. When the rf current at the load electrode was minimized, reg ions 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 el ectrode was maximized, regions of intense optical emission and high CF2 den sity shifted radially inward, and the distribution of CF2 across the electr ode surfaces became more uniform. These results suggest that variable loads could be used to direct reactive species in chamber-cleaning plasmas to th e surfaces most in need of cleaning, or to increase the radial uniformity o f reactive species in etching plasmas. [S0734-2101(99)00706-X].