Abstract: Microstructures are ion milled in an existing layer by a process including forming a metal mask then milling in the presence of a reactive gas which combines with the metal to form a surface that mills at a lower rate than the metal alone. The metal mask is formed through wet etching, with a wet etch stop film preventing the existing layer from being attacked by the wet etchant. In a preferred embodiment, cobalt-zirconium-tantalum (CZT) is milled to form top poles for tape write heads. A gold. (Au) wet etch stop film is first applied to the CZT layer. Next, a titanium (Ti) layer is deposited. A photoresist mask is formed and the Ti is wet etched, producing a metal mask. Milling with nitrogen (N.sub.2) in the presence of argon (Ar) ions causes the exposed layer of Ti to form titanium nitride (TiN), which mills at a slower rate than Ti.