Comparison of plasma densities measured by using electrical probes to those measured by using microwave probes in inductively coupled plasmas

The plasma density in an argon inductively coupled plasma is measured at a point by using two electrical probes (a single Langmuir probe and a double Langmuir probe) and two microwave probes (a cut-off probe and a hairpin probe). In the case of the single Langmuir probe, to avoid the effect of RF fluctuations, we adapt an RF compensated probe and obtain the electron density or the plasma density from the electron energy distribution function (EEDF) measurement through a proper integral. In the case of the double probe, the plasma density is derived from the ion current. Of the microwave probes, the cut-off probe appears to be the most reliable because it works regardless of sheath formation. The plasma density obtained by using the double probe is found to be larger than those obtained by using the other probes, and the plasma densities obtained by using the cut-off probe and the single probe are in good agreement, within 5%, over a wide range. The plasma density obtained by using the hairpin probe is observed to be 30% larger than that obtained by using the cut-off probe. This discrepancy seems to be due to the effect of the sheath on the hairpin probe.