Deep silicon etching in an inductively coupled plasma (ICP) reactor offers a high etch-rate (7 mu m/min), heavily vertical profile with simple oxide masking.. Test structures with patterns of different sizes (from a few microns to over 100 mu m) and shapes (square and circular holes and trenches of variable width/length) have been etched to depths up to 500 mu m. Long narrow features are etched falter than wide short features, indicating the three-dimensional nature of the reactive ion etching lag. Experiments have been done for many different etch times in order to understand aspect ratio dependence of deep etching. Simple flow conductance model explains most of the observed aspect ratio and feature size dependence.