Plasma parameters, including the electron energy distribution function (EEDF), have been measured in 13.56 MHz argon and nitrogen plasmas using a tuned probe. The EEDF measured over a range of pressures in argon are approximately bi-Maxwellian, whereas the nitrogen EEDF becomes highly non-Maxwellian at high gas pressures. The EEDF measurements also demonstrate that the argon radio-frequency (rf) plasma can be sustained entirely by single step ionization, whereas in the case of nitrogen, another source of ionization is necessary to account for the plasma production rate. The low pressure EEDF display an isotropic group of ionizing electrons despite the fact that their mean free paths are greater than the chamber dimension. This persistence of rf plasmas at low pressures is also discussed.
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Scanlan et al. (1992) studied this question.