Ion velocities in the presheath of electronegative, radio-frequency plasmas measured by low-energy cutoff
Mark A. Sobolewski
Simple kinematic considerations indicate that, under certain conditions in radio-frequency (rf) plasmas, the amplitude of the low-energy peak in ion energy distributions (IEDs) measured at an electrode depends sensitively on ion velocities upstream, at the presheath/sheath boundary. By measuring this amplitude, the velocities at which ions exit the presheath can be determined and long-standing controversies regarding presheath transport can be resolved. Here, IEDs measured in rf-biased, inductively coupled plasmas in CF4 gas determined the presheath exit velocities of all significant positive ions: CF3þ, CF2þ, CFþ, and Fþ. At higher bias voltages, we detected essentially the same velocity for all four ions. For all ions, measured velocities were significantly lower than the Bohm velocity and the electropositive ion sound speed. Neither is an accurate boundary condi- tion for rf sheaths in electronegative gases: under certain low-frequency, high-voltage criteria de- fined here, either yields large errors in predicted IEDs. These results indicate that many widely used sheath models will need to be revised.