Low-temperature, partially ionized plasmas are commonly used in industry for materials processing, and many of these are created by radiofrequency (RF) power. For the characterization of plasma sources, electrostatic probes are the easiest to use, but interpretation of the current-voltage (I -V) curves is not straightforward. The presence of strong RF pickup and neutral collisions further complicates the problem. Langmuir's orbital-motionlimited (OML) theory provides a simple formula for ion current, but this is not expected to be valid in high-density plasmas. With carefully designed probes, it is found experimentally that OML can be used successfully under adverse conditions. Careful examination of the OML theory shows that its validity is fortuitous but can give correct results in commonly encountered conditions. The probe design, results, caveats, and methodology are given for use of probes in RF plasmas.

Helicon discharges have extensively been studied for over 25 years both because of their intriguing physics and because of their utility in producing high plasma densities for industrial applications. Almost all measurements so far have been made away from the antenna region in the plasma ejected into a chamber where there may be a strong magnetic field (B-field) but where the radiofrequency (RF) field is much weaker than under the antenna. Inside the source region, the RF field distorts the current-voltage (I -V ) characteristic of the probe unless it is specially designed with strong RF compensation. For this purpose, a thin probe was designed and used to show the effect of inadequate compensation on electron temperature (T e ) measurements. The subtraction of ion current from the I -V curve is essential; and, surprisingly, Langmuir's orbital motion limited theory for ion current can be used well beyond its intended regime.