First-principles quantum chemical calculations have been performed to reveal the reaction mechanisms of HF molecules with metal impurities (Al or Cu) on Si surfaces. The lowest energy paths of this HF attack reaction have been obtained in two situations when a metal atom adheres to a Si surface with and without an O atom incorporated between the metal atom and the surface. The potential energy changes along these reaction paths were evaluated. It was revealed that a HF molecule has the ability to etch an Al monoxide on the Si surface. On the other hand, HF is not effective in the etching of a metal atom (Al or Cu) which has directly bonded to a Si surface.