The negative ion density profile in a low pressure oxygen rf plasma has been measured by a photodetachment technique. At an rf power of 10 W and a neutral pressure of 10 mTorr, a parabolic negative ion density profile is obtained with a peak density of 8×${10}^{15}$ ${\mathrm{m}}^{\mathrm{-}3}$ and a maximum ratio of negative ion to electron densities ${\mathit{n}}_{\mathrm{-}}$/${\mathit{n}}_{\mathit{e}}$≊18. Under these conditions, the most abundant positive ion, determined by ion mass spectrometry, is ${\mathrm{O}}_2^{+}$ with ${\mathrm{O}}^{+}$ being less than 10% of the positive ion density. The most abundant negative ion is ${\mathrm{O}}^{\mathrm{-}}$ with ${\mathrm{O}}_2^{\mathrm{-}}$ and ${\mathrm{O}}_3^{\mathrm{-}}$ being less than 20% of the total negative charge density. The maximum in the density profile of negative ions shifts closer to the powered rf electrode as the pressure is increased in the asymmetric system. Comparison of the results to theory indicates that the asymmetry follows from an enhancement of the ionization rate near the powered electrode sheath. The parabolic profile is also obtained in ${\mathrm{CCl}}_2$${\mathrm{F}}_2$ at low pressure. Simulations and measurements show a rapid drop in ion density near the sheath that may be related to the recently discussed ‘‘stratification’’ phenomenon in electronegative plasmas.

• Received 10 November 1994

DOI:https://doi.org/10.1103/PhysRevE.51.2436