In process development for submicron and sub-half-micron metal etch, etching rates have been observed to depend on aspect ratio of microstructures. Three major mechanisms, including neutral transport, ion transport, and inhibitor deposition into high aspect ratio features and on open areas, were modeled to interpret the aspect ratio dependent etching (ARDE) phenomena and to explain the experimental results. It is found that the ARDE is mainly controlled by the ion to neutral flux ratio and ion to inhibitor flux ratio. The increase of ion to neutral flux ratio will exaggerate the normal reactive ion etching (RIE) lag. The decrease of ion to inhibitor flux ratio tends to make a reverse RIE lag. Both model and experiment indicate that the appropriate ion to neutral flux ratio and ion to inhibitor flux ratio are the key in minimizing the ARDE and reducing the microscopic nonuniformity.