Langmuir probe measurements in a low pressure inductively coupled plasma used for diamond deposition

Abstract

The characterization of 13.56 MHz low pressure inductively coupled plasmas used for diamond deposition has been performed with a Langmuir probe. The plasma potential (Vp), electron temperature (Te), electron density (Ne), ion density (Ni), and electron energy distribution function (EEDF) were measured in a CH4/H2 plasma with 10–50 mTorr of the gas pressure at 1 kW of the plasma power, and were compared with those of an Ar plasma. We found that the Vp, Ne, and Ni have a similar radial distribution, which has a peak at the center axis and decreases outward in the radial direction, while the Te is almost constant within the radius of 20 mm and slightly decreases toward the chamber wall. It was also found that with increasing pressure the Vp and the Te decrease, whereas the Ne increases, except for a CH4/H2 plasma at 50 mTorr. The transition from a Maxwellian distribution to a Druyveysten distribution was observed at 10 mTorr in the EEDFs of Ar plasmas, while it occurred at 20 mTorr in CH4/H2 plasmas. The EEDF of a CH4/H2 plasma at 50 mTorr has a hump at ∼6 eV corresponding to the resonant peak of the vibrational excitation cross section of CH4 molecule.