High density fluorocarbon etching of silicon in an inductively coupled plasma: Mechanism of etching through a thick steady state fluorocarbon layer

Abstract

For various fluorocarbon processing chemistries in an inductively coupled plasma reactor, we have observed relatively thick (2–7 nm) fluorocarbon layers that exist on the surface during steady state etching of silicon. In steady state, the etch rate and the surface modifications of silicon do not change as a function of time. The surface modifications were characterized by in situ ellipsometry and x-ray photoelectron spectroscopy. The contribution of direct ion impact on the silicon substrate to the etching mechanism is reduced with increasing fluorocarbon layer thickness. Therefore, we consider that the silicon etch rate is controlled by a neutral etchant flux through the layer. Our experimental data show, however, that ions play an import role in the transport of silicon etching precursors through the layer. A model is developed that describes the etch kinetics through a fluorocarbon layer based on a fluorine diffusion transport mechanism. The model is consistent with the data when one or two of the following roles of the ions on the etching process are assumed. The first role is an enhancement in the diffusivity of fluorine atoms through the fluorocarbon layer and an enhancement in the reaction probability of fluorine in the fluorocarbon layer. In this case the fluorine is assumed to originate from the gas phase. The second role includes ion fragmentation and dissociation of the fluorocarbon surface molecules.