Langmuir Probe Perturbation in Inductively Coupled Plasmas

Abstract

For radial plasma density profile measurements, Langmuir probes are usually used. In this paper, a Langmuir probe was located at 4 cm below a dielectric window and the measurement was performed at 25 mTorr of argon at various rf powers (13.56 MHz). We found that the radial plasma density distributions became asymmetric as the rf power increases. To investigate the cause of this asymmetric density distribution, we installed a floating probe that can measure plasma densities and the electron temperatures in real time on the chamber wall. At high rf powers, as the probe body went in, the plasma density measured by the floating probe decreased. This indicates that the entire plasma density is affected by the probe intrusion. It appears that the Langmuir probe passes through the skin layer and the probe body impedes the electron heating process in the skin layer. As a solution, a probe body shape to avoid this perturbation is presented.