Helicon Wave Sources and HDPs
The newest type of High Density Plasma (HDP) is produced by the helicon wave source (HWS). This source requires a magnetic field of 50–1000G and is excited by an RF antenna, as in an ICP. The magnetic field has three functions: a) it increases the skin depth, so that the inductive field penetrates into the entire plasma; b) it helps to confine the electrons for a longer time; and c) it gives the operator extra adjustments to vary the plasma parameters, such as the density uniformity. The antenna launches a wave, called a helicon wave, that propagates along B with a phase velocity comparable to that of a 50–200 eV electron. The wave causes very efficient ionization , so that experiments are often done with densities in the mid-1013 cm-3 range, though such high densities are not usable for semiconductor processing. Nonetheless, HWS densities tend to be an order of magnitude higher than the 1011 cm-3 densities typical of ICPs. Until recently , it was not known why HW sources are so efficient . Initially, it was thought that cool electrons could catch the wave and surf on it up to the wave velocity, thus speeding up to where their ionization cross section was at its peak.
KeywordsIonization Cross Section Skin Depth Blue Mode Helical Antenna Antenna Field
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