Chemically assisted ion beam etching, using Ar+ ions and Cl2 as the reactive gas, has been investigated for the etching of common materials used in silicon-based microfabrication : SiO2, Si3N4, single-crystal Si, and polycrystalline Si. Two possible etch masks for etching the Si substrate have been characterized : SiO2 and TiW Etch rates for all these materials and Si etch profiles have been studied as a function of ion energies from 300 to 1000 eV, current densities between 0.05 and 0.15 mA/cm2, and reactive Cl2 flow rates at substrate temperatures of 30 and 85-degrees-C. Trenches etched into Si using TiW as a mask showed steeper profiles and less enhanced etching in the bottom corners than those with an SiO2 mask. These etch effects are attributed to a deposited layer on the trench sidewalls, when using TiW as a mask. This Si etch process has important applications in shallow trench isolation for deep submicron complementary metal oxide semiconductor integrated circuits.