The evolution of an etch profile of an open-area half trench is modeled. The etching of silicon dioxide is by a hydrofluorocarbon plasma, CHF3/CF4/Ar, in a magnetically enhanced reactive ion etching reactor. Two noticeable characteristics of the scanning electron microscope micrographs, the constant slope sidewall and microtrenching, are included in the model. The constant slope sidewall is due to sidewall passivation and the equation for the sidewall arises from a nonlinear evolution equation. Microtrenching can arise from the bombarding anisotropic ions that reach the bottom corner of the half trench to a greater degree than the deposition neutrals but may be enhanced by reflection of ions from the half trench and photoresist sidewalls. The etch rate expression due to the reflected ions is derived as an approximate analytical expression for small grazing angles and an energy threshold. The reflected ions do not contribute significantly to microtrenching for our etch profiles.