A systematic study of trench profile evolution in a medium-density oxide etch reactor is presented. A Langmuir site balance model is developed in the limit of unity sticking coefficient which exhibits a flat etch front as is frequently required for dual damascene applications. The model indicates that it is desirable to operate in a neutral-limited ion-assisted etch regime. Physical sputtering is also shown to be necessary, but this etch contribution must be kept small with respect to the ion-assisted etch rate. The model also indicates how either microtrenching or bottom rounding may be controlled or avoided altogether. Model predictions are compared with experimental data obtained from a Lam Research 4520XLE medium density reactor. This work includes a study of the trench bottom rounding dependencies upon pressure, etch time, aspect ratio, and process gas flow for a fluorocarbon-based etch process. The model is shown to qualitatively capture experimentally observed process trends. In some regimes, good quantitative agreement with observed measurement is seen. It can thus serve as a useful guide fur trench etch process development.