Pressure and frequency dependence of ion bombardment energy distributions from rf discharges

Experimental measurements of the energy distribution and flux of ions bombarding an electrode of a parallel-plate rf reactor were performed in argon, neon, and a 95% helium-5% argon mixture using a hemispherical retarding grid energy analyzer. Quadrupole mass analysis was performed to identify the ions which were bombarding the electrode. Frequencies from 2.5 to 20 MHz and pressures from 0.025 to 1.6 Torr were studied. For a given amplitude of the applied rf potential, the ratio of the frequency of the applied rf potential to the reactor gas pressure was found to be the critical parameter in determining the shape of the measured ion bombardment energy distributions. The total ion flux to the electrode surface was found to be proportional to the square of the frequency and insensitive to the reactor gas pressure, for a fixed amplitude of the applied rf potential. These results are discussed with reference to the physics of rf gas discharges.