Stability and electric structure of an electronegative inductively coupled plasma
Inductively coupled plasmas are widely for machining nano-scaled devices. These plasmas often contain a large amount of negative ions and are subject to two kinds of instabilities. The first one is described as relaxation oscillations between two power-coupling modes: the capacitive (E) and inductive (H) modes. Time-resolved measurement of the plasma parameters during these oscillations are similar to published results. The second instability is related to the charged particles' transport and has only been observed when the plasma can diffuse. Space and time resolved measurements of the plasma parameters show that the observed fluctuations are linked to the periodic formation and propagation of a medium double-layer. This double-layer is an internal sheath between a high electron density, high electron temperature electropositive plasma in the source and a low electron density, low electron temperature electronegative plasma in the diffusion chamber. The propagating double-layer appears when the negative ion fraction is above a critical threshold. Below the threshold, the double-layer is static and eventually disappears for very low negative ion fractions. The transitions between these regimes are subject to strong ion acoustic activity. Adding a static magnetic field lead to a new power coupling mode, through the absorption of an helicon wave. Relaxation oscillations between the inductive and helicon modes have been observed for a very narrow range of parameters. The second consequence is the modification of the charged particles' confinement: the core plasma is free of negative ions, while the outer plasma is electron free; an ion-ion plasma has been created.