A new technique is described for high‐spatial‐resolution (<2‐&mgr;m linewidth) etching of Al thin films. The process is based upon moderate local heating by a tightly focused Ar+laser beam to activate an etching reaction in mixtures of phosphoric acid, nitric acid, and potassium dichromate. By chemically biasing the reaction near its passive/active transition, the laser can enhance the reaction rate by more than six orders of magnitude. The etching mechanism has been studied by etch‐rate measurements, ellipsometry, and Auger spectroscopy, and is ascribed to a competition between the formation of soluble aluminum phosphates and insoluble aluminum oxides.