Bias-plasma interaction in pulsed plasmas and its relation to damage mechanisms

We report results of a study of the behavior of a chlorine, inductively-coupled pulsed plasma operated with a continuous radio frequency (rf) bias applied to the wafer stage. If applied rf bias power is not too low, its presence when the source power is turned off, causes a drastic change in electron temperature (Te). During the first 20-30 μs of the OFF time Te in the presence of the bias (Tebias) decays as does Te without bias (Tenb). Then, however, after reaching a minimum (~0.5 eV), Tebias starts to increase rapidly and reaches values even higher than the steady state ON time values. Since the electron density (ne) is still quite high when Tebias goes through the minimum, the sheath near the wafer does not collapse and negative ions do not reach the wafer. In a separate study in a similar reactor, the damage was reduced while operating with a pulsed source and a continuous bias. This implies that the discharging of the wafer occurs not by negative ions. We further show that increasing the duration of the OFF time or increasing the bias power can lead to the transition of the plasma into reactive ion etching (RIE) mode. This occurs when the decaying positive ion density (ni+) reaches the level of ion density of pure RIE