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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 m Torr, a parabolic negative ion density profile is obtained with a peak density of 8 x 10' m and a maximum ratio of negative ion to electron densities n /n, -18. Under these conditions, the most abundant positive ion, determined by ion mass spectrometry, is 02+ with 0+ being less than 10% of the positive ion density. The most abundant negative ion is 0 with 02 and 03 being less than 20'Po 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 CC12F2 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.

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Several aspects of negative ions in low pressure discharges are treated. The elementary processes, in which negative ions are produced and destroyed, are summarized. The influence of negative ions on plasma operation is analyzed in terms of transport equations. It is shown that diffusion, electric field, power input, plasma stability and plasma boundary layer are strongly affected by the presence of negative ions. Finally, an overview is given of experimental techniques for the detection of negative ions. Some typical results in low pressure discharges are presented.

Infrared absorption spectroscopy has been used to measure the absolute densities of neutral palticles in various fluorocarbon RF plasmas. The densities of CF2 radicals have been measured using a tunable diode laser. Moreover, broadband absorption spectra obtained using a Fourier transform spectrometer have been used to determine the densities of stable reaction products and to assess the degree of dissociation in the plasma. The results indicate that the rotational temperatures of all species involved are slightly above room temperature. The density of CF2 at high gas pressures increases close to the electrodes, indicating production near or on the electrodes and loss in the gas phase. The dissociation degree in plasmas of gases with a high C/F ratio can be as high as 90%. From an analysis of the flow dependence of the degree of dissociaiion ihe ioiai dissociaiion raie coefficienis of CFA. CF2Ci2, CF&i and C2F& have been calculated.