Transition From E to H Mode Discharge in Pulse-Modulated Inductively Coupled Plasmas
M Edamura, Eric C. Benck
Time-resolved measurements of pulse-modulated inductively coupled plasmas were carried out by using a Langmuir probe. It was found that under a certain condition (a mixture of 20 % O2 and 80 % Ar, a pressure of 2.67 Pa, a radio-frequency power of 200 W, a pulse frequency of 500 Hz, and an RF-off time of 100 ms), a plasma transits from E mode (capacitive coupling mode) to H mode (inductive coupling mode) after the RF-off time. In a shorter RF-off time of 50 ms, 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.