The use of Langmuir probes and optical emission spectroscopy to measure electron energy distribution functions in RF-generated argon plasmas

Abstract

A single Langmuir probe technique has been used to measure the electron energy distribution function (EEDF) in an RF (13.56 MHz)-generated argon plasma in a reactive ion etcher. This was achieved by applying a driving RF signal to the probe to compensate for the effects of RF fluctuations in the plasma potential and then using the second differential of the probe characteristics to obtain the EEDF. It is observed that the EEDF is not well represented by a Maxwellian, Optical emission spectra (4600-4900 AA) were also recorded. In argon plasmas, generated at a constant power level, the average electron energy deduced from the Langmuir probe increases as the pressure is reduced below 20 mTorr. At 5 mTorr the average energy is 8.5 eV whereas above 20 mTorr the average energy lies in the range 3.5-5.3 eV. This is found to correlate with the variation in the intensity of an emission line (4s'(1/2)10 from 5p(1/2) at 4702.32 AA) in an excited argon atom, which can be accounted for by the presence of an increasing fraction of higher-energy electrons at lower gas pressures.