A model for the etching of nanoporous silica in C4F8 plasmas based on pore geometry and porosity effects

Plasma etching of nanoporous materials (NPMs) is a complicated phenomenon and depends upon the NPMs parameters, such as the overall porosity, the pore size and structure, and the concentration of organic groups on the surface of the film and inside its pores. Polymerization during fluorocarbon plasma exposure is ubiquitous and suppresses the net etching rate. The model developed here accounts for the polymerization that occurs. In this study, a new plasma etching model is developed that applies in the high-polymerization-rate regime. This new model includes pore structure factors (pore shape and size) as well as mass and volume effects in the form of the film's overall porosity. According to the model, at low porosities the etching rate varies directly with the total porosity. However, as the porosity of the film increases, surface effects become important and the etching rate is affected by both total porosity and pore geometry. Finally, we correlate the corrected etching rate, including the porosity and the average pore size effect, with the etching of solid SiO2 over a wide range of bias voltage. In the fluorocarbon suppression regime, the corrected etching rate expression agrees with the experimental results and collapses all etch rate data onto a single curve. (C) 2005 The Electrochemical Society.