Capactiviely coupled discharge and inductively coupled discharge are usually driven by RF power supply with frequency range from tens of KHz to less than 100MHz. When frequency reaches GHz range, a different plasma excitation mechanism applies. Microwave plasma usually works in one or both of the following two modes:
- Surface-wave mode. Microwave can only penetrates into plasma at a depth corresponds to skin depth. Fortunately, plasma is highly conductive. Input microwave can create an evanescent wave propagate along plasma surface. Thus, electrons will be accelerated along 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 resonator cavity can penetrate plasma and create plasma with density varying with wave magnitude.