Ion bombardment in rf plasmas

Ion bombardment energy and angle distributions have been measured in an argon plasma. The measured ion angle distribution at 10 mTorr shows that 30% of the ions have incident angles greater than 10° from the surface normal. However, ions with large incident angles have much lower energies than those incident perpendicular to the surface. At 500 mTorr a very large fraction of the ions have large incident angles, and the average energies of these ions are relatively independent of incident angle. Monte Carlo simulations of the sheath kinetics predict the trends shown in the experimental data for ion energy and angle distributions. Fine structure in the ion energy distribution was observed below 50 mTorr and is shown to be caused by charge-exchange collisions in the sheath. The average ion energy in a symmetric parallel plate system is linearly related to the voltage applied across the electrodes for measured plasma pressures up to 500 mTorr.