The variation of metal etch rate with spacing between metal lines was measured from scanning electron microscopy micrographs of TiN/Al-0.5%Cu/TiN wafers etched in a high density inductively coupled plasma metal etcher. The metal etch rate was found to depend on the spacing between metal lines, with etch rate significantly decreasing in very narrow spaces for a conventional Cl-2/BCl3 chemistry. The effect of several process parameters on this etch rate dependence was studied. It was found that the dependence could be reduced significantly, and the traditional rolloff of etch rate as spacing decreases could be eliminated, by the choice of process gases. Addition of 15% CHF3 to a BCl3/Cl-2 mixture resulted in a 50% reduction of the effect, and addition of both CHF3 and Ar under certain process conditions resulted in almost complete reduction or even inversion of the effect. A mechanism is proposed for this improvement : sidewall passivants like CHF3 reduce the sticking coefficient of chlorine on aluminum, boosting reactant flux to the bottom of high aspect ratio openings.