Molecular. dynamics simulations of silicon (Si) and silicon dioxide (SiO2) etching by energetic halogen (fluorine or chlorine) atoms in the energy range of 50-150 eV are performed using new sets of interatomic potentials for Si-O-F and Si-O-Cl systems. Etch rates and selectivities obtained from numerical simulations are compared with available experimental data. Etching mechanisms in the atomic scale, especially the difference between chlorine and fluorine direct ion etching characteristics, are discussed on the basis of the simulation results.