Langmuir probe measurements in an inductively coupled plasma: Electron energy distribution functions in polymerizing fluorocarbon gases used for selective etching of SiO2

We are interested in the silicon oxide deep etching by inductively coupled fluorocarbon plasmas for integrated optical applications. The understanding and the improvement of this process requires to know at least the electrical characteristics of the plasma (electron and ion densities, electronic temperature,...). Up to now, very few measurements in these plasmas have been published because of problems encountered when using Langmuir probes in depositing environments. In the present article, we report problems we met and solutions we brought, and then present electron energy distribution function (EEDF) measurements in very polymerizing gases such as CHF3 and CHF3/CH4 mixtures. Experiments have been performed over a wide range of experimental conditions, from 3 to 50 mTorr and from 200 to 2000 W inductive power. The shape of the EEDF and the evolution of the plasma electrical characteristics with experimental conditions are discussed. Finally, we point out the presence of a large negative ion fraction when increasing the pressure, particularly in pure CHF3 plasmas