Etching yields of silicon in F-2, Cl-2, Br-2, and HBr high density plasmas have been measured as a function of ion bombardment energy, ion bombardment angle, and plasma composition. This information contributes to a database of experimental values needed for feature profile evolution modeling. For all plasma chemistries, the etching yield increases approximately with the square root of ion energy. Pure Cl-2 and pure HBr plasmas have very similar etching yields. Silicon etching rates are lower in HBr plasmas than in Cl-2 plasmas due to lower ion fluxes, not lower etching yields. The dependence of the etching yield on ion bombardment angle is significantly different for Cl-2 and HBr plasmas. The etching yield in Cl-2 plasmas decreases rapidly for ion angles above 60 degrees (measured from the surface normal), which results in significant ion scattering from the sidewalls, and may cause the sidewall bowing and microtrenching seen when patterning polysilicon with Cl-2 plasmas. The etching yield in HBr plasmas decreases more gradually with the ion angle, resulting in less ion reflection from the feature sidewalls and may explain the much less pronounced sidewall bowing and microtrenching typically seen when patterning polysilicon with HBr plasmas. HBr plasmas have higher etching yields than Br-2 plasmas due to the ability of H atoms to increase the surface coverage and penetrate farther into the silicon lattice. As the temperature of the silicon increases, the etching yield in HBr plasmas decreases, due to the reduced surface coverage by adsorbed Br and H atoms.