Capacitively coupled discharge and inductively coupled discharge are usually driven by an RF power supply with a frequency range from tens of kHz to less than 100MHz. When frequency reaches the GHz range, a different plasma excitation mechanism applies. Microwave plasma usually works in one or both of the following two modes:
- Surface-wave mode. Microwaves can only penetrate into plasma at a depth that corresponds to skin depth. Fortunately, plasma is highly conductive. Input microwave can create an evanescent wave that propagates along the plasma surface. Thus, electrons will be accelerated along the plasma surface and ionize more neutral particles. (http://en.wikipedia.org/wiki/Evanescent_wave)
- Resonator mode. If the plasma density does not exceed a critical threshold, a standing wave defined by the resonator cavity can penetrate plasma and create plasma with density varying with wave magnitude.