Langmuir probes in RF plasma: surprising validity of OML theory

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-motion-limited (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.