Transition from E to H mode discharge in pulse-modulated inductively coupled plasmas

Abstract

Time-resolved measurements of pulse-modulated inductively coupled plasmas were carried out by using a Langmuir probe. It was found that under a certain set of conditions (a mixture of 20% O2 and 80% Ar, a pressure of 2.67 Pa, an rf power of 200 W, a pulse frequency of 500 Hz, and an rf-off time of 100 μs), a plasma transits from E mode (capacitive coupling mode) to H mode (inductive coupling mode) after the rf-off time. With a shorter rf-off time of 50 μs, the plasma returned to H mode without passing through E mode. Whether H mode or E mode appears after the rf-off time depends on the electron density at the end of the afterglow. Namely, the restoration to H mode after the rf-off time occurs if the plasma has an adequate electron density, and E mode occurs if electron density is not enough to sustain H mode. It was also found that electron temperature decreases and plasma potential increases gradually during E mode because of the change of the electron-energy distribution. The change of the plasma reduces the impedance mismatching gradually and increases the electron density until the transition to H mode occurs. Such mode transition behavior strongly depends on the basic characteristics of the plasma processing apparatus during continuous discharge and also on the condition of the chamber wall.