The mechanisms of the oxide etching process are studied using the overhang test structures etched on the Applied Materials inductively coupled plasma oxide etcher. The results from the overhang test structures clearly show many details of the surface interactions that etched trenches would not show. Together with results obtained with standard trench structures etched with the same process, a new model for oxide etching is developed to explain the observed profiles and trends. The two neutral, simultaneous deposition and etching model contains several key features. Components controlling the deposition behavior are : (1) polymer chemical vapor deposition, (2) ion enhanced deposition, and (3) angle dependent polymer sputtering. The etch rate is simulated with a saturation model, where the rate can be ion or neutral flux limited. The model is incorporated into the profile simulator SPEEDIE and the simulations are run with one set of parameters. The simulation results show good agreement with the scanning electron microscopy experiment results for both profile and lag trends.