Strong variation of average ion energy in oscillation frequency of sheath potential

In a modern plasma etching device, the plasma sheath potential is usually superposed by an externally driven oscillating voltage to enhance and control the bombarding ion energy. A collisionless particle simulation is used to study the variation of average kinetic energy 〈K〉(v_rf) of bombarding ions as a function of a wide range of sheath oscillation frequency v_rf (0.1 v_pi≤v_rf ≤10v_pi, where v_pi is the ion plasma frequency). It is found that a resonance phenomenon between the ion transit motion and the sheath oscillation can yield a strongly peaked enhancement of 〈K〉 X(v_rf) near v_rf≃0.5v_pi. Ion species with different mass show the peaks at different v_rf. The relative importance of different ion molecules in an ion-enhanced etching process will be sensitive to v_rf. This phenomenon may allow a reduction of the undesirable capacitive coupling by optimizing v_rf to yield an enhanced 〈K〉 of desired ion species at low applied voltages.