Aspect ratio dependent etching (ARDE) has often been observed in various etching processes. During etching of metal films in a high density plasma reactor, this phenomenon is more prominent. With a high amount of ARDE present, the narrow spaces will not be cleared unless there is enough overetch, which removes an excessive amount of the underlying layers. The main focus of this article is on the understanding of the mechanisms behind metal ARDE. The results from an extensive design of experiments on the subject were utilized for this study. SPEEDIE, the Stanford etching and deposition profile simulator, was used to develop and test appropriate models. The Langmuir adsorption model with added surface recombination of the adsorbed species, both etchants and inhibitors, was used to model the phenomenon. The added surface recombination acts as the major species loss mechanism on the feature sidewalls. The simulation results indicate that a process with inhibitors, which are highly adhesive to the metal surface and which do not like to recombine with the etchants to form volatile products, will lead to a low degree of ARDE. This is in agreement with experimental results which showed that the addition of CHF3 to the Al etch process helps to reduce the ARDE in narrow space regions.