The surface layer of the aluminum film reactively ion etched in $CF_4$ plasma was ana alyzed by using XPS. $AlF_3$ which is nonvolatile is formed at the aluminum surface. As the analyzed depth increases, the intensity of the $Al_{2p}$ peak of Al - F bonds decreases while that of a aluminum metallic bond increases. The thickness of the $AlF_x$ surface layer is 50~100 $AA$ and the deep penetration of fluorine atoms is attributed to the mixing effect by the bombardment of incident particles. For the aluminum oxide film which is etched in $CF_4$ plasma under the same conditions, oxygen atoms are substituted by fluorine atoms to form $$AIF_x$ surface layer, which is m much thinner than that formed on aluminum surface.