P. Chabert, J. Perrin# and R. W. Boswell*

Lab. PRIAM, UMR 9927 CNRS-ONERA, F-91761 Palaiseau Cedex
chabert@shiva.thomson-lcr.fr
#Balzers Process Systems S.A. F-91124 Palaiseau Cedex
*Space Plasma Group, RSPhysSE,ANU, Canberra ACT 0200, Australia

High density plasma sources are now widely used in etching processes since they can provide high positive ions fluxes with a scalable bombardment energy [1]. Although these sources operate at rather low pressure, a significant proportion of negative ions may exist when a highly electronegative gas such as SF6 is used [2]. A qualitative description of a helicon plasma with SF6 gas have been previously proposed [3] and we now present results obtained in the diffusion chamber concerning the relative variations of negative ion to electron density ratio (a = n-/ne) when scanning the pressure and the radiofrequency power. a is estimated from positive to negative saturation currents ratio (Isat-/Isat+) of planar electrostatic probes. Results obtained with this simple technique are in fairly good agreement with those published by St-Onge et al. [2 ] and show that a significant amount of negative ions (a > 1) seems to be present when P > 2 mTorr.

[1] M.A.Lieberman and A.J.Lichtenberg, Principles of Plasma Discharges and Materials Processing, John Wiley and Sons Inc., New York, 1994
[2] L. St-Onge and J. Margot and M. Chaker, Appl. Phys. Lett. 72 (3), 19 January 1998
[3] P. Chabert, R.W. Boswell and C. Davis, J. Vac. Sci. Technol. A 16(1), Jan/Feb 1998 p. 78