In an effort to increase throughput, the microelectronics fabrication industry has transitioned to high plasma density etching reactors using large source (>800W) and moderate substrate bias ( > 100 W) powers in which the ion to neutral radical flux is large compared to re active-ion-etching systems. These conditions can lead to microtrenching where etch rates are largest at the base of the sidewalls, Microtrenching has been attributed to specular reflection of high energy particles, usually ions, at, grazing angles on the sidewalls of the mask and trench. These reflections produce a "focusing" of flux to the corners of the trench which results in locally enhanced etching. In this letter, integrated plasma equipment and Monte Carlo feature profile models have been used to examine the processes and conditions which produce focused fluxes and microtrenching, including the degree of specular reflection and sidewall slope of the mask. Quantitative comparisons an made to experimental measurements of etch profiles.