Spatially resolved electron temperatures, species concentrations, and electron energy distributions in inductively coupled chlorine plasmas, measured by trace-rare gases optical emission spectroscopy

Abstract

Determining the spatial dependence of charged and neutral species concentrations and energies in inductively coupled plasmas (ICP) is important for understanding basic plasma chemistry and physics, as well as for optimizing the placement of the wafer with respect to the ICP source to maximize properties such as etching rate uniformity, while minimizing charging-induced damage and feature profile anomalies. We have determined the line-integrated electron temperature (Te) and Cl-atom number density (nCl) as a function of the axial distance (z) from the wafer in a chlorine ICP, using trace rare gases optical emission spectroscopy (TRG-OES). By selecting rare gas lines that are either (a) excited mostly from the ground states, or (b) excited mainly from the metastable states we were also able to obtain approximate electron energy distributions functions (EEDFs). The gap between the wafer and the window adjacent to the flat coil inductive source was fixed at 15 cm. The pressure was 2, 10, or 20 mTorr (95% Cl2, 1% each of He, Ne, Ar, Kr, Xe) and the inductive mode source power was 340 or 900 W. Te measured by TRG-OES, mostly characteristic of the high-energy (>10 eV) part of the EEDF, peaked near the source under all conditions except 2 mTorr and 900 W, where a maximum Te of 5.5 eV was observed at midgap. The falloff in this high-electron-energy Te away from the source is mainly due to a preferential loss of high-energy electrons, which can be explained by an increasingly depleted (with increasing energy) EEDF, combined with the nonlocal effect: electrons lose kinetic energy as they approach the higher potential energy regions of lower electron density near the wafer. At 20 mTorr and 340 W, the mean free path for inelastic scattering by high-energy electrons becomes comparable to the reactor dimensions, causing added cooling of the EEDF near the wafer. TRG-OES EEDFs measured at a distance of 3 cm from the wafer and 900 W are in excellent agreement with previous Langmuir probe measurements. nCl increased with power and was highest at 900 W in the region between midgap and the ICP window, reaching a level corresponding to a high degree of dissociation of Cl2.