Conductance considerations in the reactive ion etching of high aspect ratio features

Very simple vacuum conductance arguments indicate that in the reactive ion etching of high aspect ratio features, the conductance is adequate to allow etch products to flow out of the feature without building up a pressure which would allow gas phase collisions to become important. On the other hand, the conductance can be expected to limit the flow of the reactive species to the bottom of the feature where the etching is taking place, thus creating the possibility of an etch rate dependence on the aspect ratio of the etched feature.