High resolution, reproducible dry etching of Al/Si can be achieved using and mixtures in a hexagonal cathode reactive ion etching system. The etch directionality is a function of the composition, the number of wafers in the reactor, and the d‐c self‐bias voltage. Increases in the concentration of etching species produced by raising the percentage of in and by decreasing the wafer load increases the total etch rate while enhancing the isotropic etch rate relative to the anisotropic etch rate. The d‐c self‐bias voltage (or applied rf power) appears to partition the active etching species between isotropic etching and anisotropic etching without affecting the total etch rate.