The accuracy of Langmuir probe ion density measurements in low-frequency RF discharges

The ion densities of low-frequency inductively coupled discharges are estimated from Langmuir probe ion saturation currents over a wide range ( ) of plasma densities. The ion densities are compared to the corresponding electron densities obtained from microwave interferometry. For low plasma densities, the cylindrical probes tend to overestimate the ion densities by factors of up to three. These discrepancies correlate well with RF probe currents and can be reduced with RF compensation or with a symmetrical double probe. For high plasma densities, the ion densities from simple probes without RF compensation agree within 50% with the electron densities from the interferometer.