Wave propagation and power deposition in magnetically enhanced inductively coupled and helicon plasma sources

Magnetically enhanced inductively coupled plasma (MEICP) and helicon sources for materials processing are of interest because of their ability to deposit power within the volume of the plasma beyond the classical skin depth. The location and manner of power deposition can vary substantially depending on the mode of operation and reactor conditions. The coupling of electromagnetic fields to the plasma typically occurs through two channels; a weakly damped heliconlike wave that penetrates into the bulk plasma and an electrostatic wave. The electrostatic wave can often be suppressed resulting in the helicon component being responsible for the majority of the power deposition. A computational investigation was conducted to quantify this heating and determine the conditions for which power can be deposited in the downstream region of MEICP devices. For typical process conditions (10 mTorr, 1 kW ICP) and magnetic fields above 40 G, radial and axial electric fields exhibit nodal structure consistent with helicon behavior. As the magnetic fields are increased, axial standing wave patterns occur with substantial power deposition downstream. The ability to deposit power downstream with increasing B field is ultimately limited by the increasing wavelength. For example, if the plasma is significantly electronegative in the low power–high magnetic field regime, power deposition resembles conventional ICP due to the helicon wavelength exceeding the reactor.