Study of negative-ion kinetics in processing plasmas

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The electron and negative ion densities in a RF SF_6 plasma have been measured by using laser photodetachment and microwave cavity resonance techniques ; the positive ion density has also been measured with a Langmuir probe. The negative-ion/electron density ratio for a low-pressure (-100 mTorr), low-power (-10 W) plasma was found to be a few hundred, which increased up to a few thousand when the pressure was increased to -1 Torr. The major contribution for the photodetachment signal came from F^-, and the photodetachment efficiency for the dominant negative-ion species (supposed to be SF_5^-) was suggested to be small.In mass spectrometric measurements of negative ion species, F^-, SF_5^-, SF_6^-, SF_3^-, and F_2^-, were detected, with F^- giving the largest signal and SF_5^- the second largest, However, the relative sensitivity of the measurement system for different negative ion species is not known and hence it is difficult to specify from the result what is the major negative ion species in the plasma. The current decay waveform in the afterglow for each negative ion species was very different from that of the positive ion saturation current to the Langmuir probe, indicating the complicated nature of the negative ion sampling process from the plasma. The current decay waveforms were also sensitive to the material of the sampling tip as well as to the tip voltage biasing, suggesting the effect of possible charging up of the sampling tip due to negative ions.The laser photodetachment and microwave resonance measurements were also carried out for a RF NF_3 plasma. It was found that NF_3 attaches electrons in the plasma more rapidly than SF_6 at the same pressure. Due to the large attachment rate, the response speed of the measurement system was not sufficient for determining the negative ion density accurately.