Investigation of mode transitions in a pulsed ICP system

Summary form only given, as follows. Mode transitions in a low pressure, repetitively pulsed, RF (13.56 MHz) excited, inductively coupled plasma (ICP) source were investigated by using current and voltage probes on the RF coil. In addition, time resolved Langmuir probe measurements of electron temperature, plasma potential, and plasma density were made during these mode transitions. A third diagnostics utilized a fast photomultiplier with narrow band filters to monitor time resolved plasma emission. Argon was used as a representative atomic gas while nitrogen was used as a representative molecular gas. Both pure gases and mixtures were used in the experiment. During a single RF pulse, the discharge was observed to run either capacitively coupled or change from capacitive to inductive mode. Once the transition was made to the inductive mode, that mode was active until the end of the pulse. The observed mode changes were a function of pulse width, RF power, gas pressure, and gas mixture. By comparing the Langmuir probe data from each mode and during a transition, a set of criteria, based on the plasma characteristics, were established to predict the operating mode for this ICP. Results will be presented for plasmas generated under various conditions.