We have used x-ray photoelectron spectroscopy to determine the chemical elements present on the tops, sidewalls, and bottoms of submicron metal features etched in a high density inductively coupled plasma source using Cl-2/BCl3 gas mixtures. X-ray photoelectron spectroscopy analyses have shown that aluminum oxide is deposited on all the surfaces of the features exposed to the plasma due to erosion of the alumina liner located in the source region. A chlorine rich carbon film is formed on the sidewalls and at the bottom of the aluminum features during the etching process, At the bottom of the features, chlorine species must diffuse through the carbon layer to etch aluminum whereas spontaneous reactions between chlorine and aluminum are blocked on the sides of the features. On the sidewalls of the features, aluminum oxide species coming from the sputtering of the alumina liner are embedded in the carbon rich film as it grows, This sidewall passivation film enhances anisotropic etching by providing a thin protective layer against spontaneous etching reaction of chlorine with aluminum,