Time resolved investigations of the E-H transition in a microwave driven ICP source

The transition dynamics from the capacitive to inductive (E-H) coupling in a 2.45 GHz excited inductively coupled plasma (ICP) source is investigated using a set of microwave time resolved records at different frequencies [1]. This method, applied for the first time in the semiconductor physics [2] and for measurements of time resolved cyclotron resonances [3], has been newly adapted for plasma investigations. A number of gases like argon, helium, nitrogen, and oxygen plasmas have been analyzed in the range 20 - 1000 Pa with a constant excitation power in the range of 20-40 W. With a better resolution than $1 \mu$ s one can identify, depending on pressure, the coexistence of the E and H modes, a hybrid EH mode, and in case of oxygen plasma, the transition from negative to positive ions. The E-H transition time increases with pressure. The identification of the E or H modes become possible by analyzing the time evolution of n the resonances in power - frequency diagrams and Smith diagrams. The method requires a resonant structure (always available for microwave excitation) and a vector network analyzer (VNA) capable to meaure in pulsed regime the complex scattering parameterS11. Because of jitter the optical triggering of the VNA is not sufficient for precise determining the ignition time. An additional mathematical procedure has been implemented to precisely detect the ignition time and to make a reliable average of several recorded transients. The time evolution of plasma density is comparable with steady state measurements made at different absorbed powers [4] and described theoretically by global model [5].