VARIATION OF RADIAL PLASMA-DENSITY PROFILE WITH THE EXCITATION-FREQUENCY IN A MAGNETRON-TYPE PLASMA

The variation of the radial profile of plasma density with the excitat ion frequency was investigated using a magnetron-type plasma source. T he plasma source is comprised of a cylindrical shaped rf electrode and two annular magnets attached to the outer surface of the rf electrode . As the excitation frequency of the plasma, 13.56 MHz. 40 MHz were us ed. The radial profile of plasma density was observed in the downstrea m at 12 cm below the axial center of the rf electrode. Measurements we re carried out for Ar plasma for pressures < 10 mTorr. The applied rf power varied from 200W to 1500W. The radial profile of the plasma dens ity at a given distance from the axial center of the rf electrode chan ges with the change of excitation frequency. When the excitation frequ ency is 40 MHz, a radially uniform plasma could be obtained 12 cm belo w the axial center of the rf electrode. The nonuniformity of this plas ma over an area of 300 mm diameter is < +/-5% regardless of the applie d rf power. A change of pressure slightly affects the radial uniformit y of the plasma. Plasmas produced by two other frequencies show a high er nonuniformity at the same distance from the axial center.