Characterization of transformer coupled oxygen plasmas by trace rare gases-optical emission spectroscopy and Langmuir probe analysis

Abstract

Trace rare gases-optical emission spectroscopy (TRG-OES) and Langmuir probe analysis have been used to measure the electron temperature, Te, in a high-density inductively (transformer) coupled (TCP) 10 mTorr oxygen plasma as a function of the 13.56 MHz radio frequency (rf) power. The oxygen atomic densities were estimated by O-atom optical emission (8446 Å), and rare gas actinometry (Ar, 7504 Å). In the H-(inductive)-mode, Te increases from 2.6 to 3.4 eV for the low-energy electrons sampled by the Langmuir probe and from ~3.5 to 6.0 eV for the high-energy electrons sensed by TRG-OES as the rf power is increased from 120 to 1046 W. In the E-(capacitive)-mode, below 50 W, Te measured by TRG-OES increases with rf power from ~4 eV at very low power (~7 W) to ~6.1 eV at 45 W. Between the highest E-mode power (~50 W) and lowest H-mode power (~120 W), the Te measured by TRG-OES drops from 6.1 to 3.5 eV, while Te derived from Langmuir probe measurements drops only slightly from 3.0 to 2.6 eV. In the H-mode, the electron energy distribution function (EEDF) is bi-Maxwellian from ~120 to 1046 W. In the E-mode, the EEDF changes from nearly Maxwellian (possibly Druyvesteyn) at low rf powers (~7 W) to bi-Maxwellian at the higher E-mode powers (~45 W). O2 dissociation is low (~2%) at the maximum rf power density of 5.7 W cm-2 (1046 W), and this low value is attributed to the high rate of O-atom recombination on the mostly stainless-steel walls. A detailed accounting of the sources of O (8446 Å) emission revealed significant contributions from electron impact excitation from O(1S) and dissociative excitation of O2.