T. E. Sheridan* and R. W. Boswell
Plasma Research Laboratory, Australian National University, Canberra, ACT 0200
Terry.Sheridan@anu.edu.au

We examine the effects of neutral pressure (SF6 and Ar) and magnetic field configuration (B = 50 G) on plasma density profiles in a helicon reactor. At low fill pressures (P < 5 mtorr), the axial density profile is convex, while at high neutral pressures it is concave, irrespective of rf power. These results are explained by noting that at low pressures the mean free path for electron impact ionization is longer than the distance from the source to the substrate. Consequently, plasma creation is approximately uniform in the source chamber (on B-field lines connected to the antenna). At high pressures, the mean free path decreases, and plasma creation is localized within the source chamber. Plasma then diffuses out of the source chamber, radially and axially, with a concomitant decrease in density. Experimental results are modeled qualitatively by assuming that the number of energetic electrons coming from the source decreases exponentially with distance (i.e. a constant mean free path), while plasma losses (governed by ion transport) are diffusive. At low pressures, the density profiles are found to have roughly the same shape irrespective of discharge mode (capacitive or inductive) and magnetic field profile. The highest density is attained when the magnetic field is greatest in the diffusion chamber and least in the source. That is, for this case, plasma creation in the diffusion chamber is greatest-high energy electrons are more effectively guided into the diffusion chamber and confined there. For other field configurations, more electrons are either lost in the source (to the walls), or are not as effectively confined in the diffusion chamber, leading to lower plasma densities.