Maximum etch rates of similar to 400 Angstrom min-1 were obtained for Ni0.8Fe0.2 and Ni0.8Fe0.13Co0.07 thin films in CO/NH3 inductively coupled plasmas (ICP). There is a small chemical contribution to the etch mechanism (i.e., formation of metal carbonyls) as determined by a comparison with Ar and Nz physical sputtering. The etch rates are a strong function of ion flux, ion energy, pressure, substrate temperature, and discharge composition. The discharge should be NH3-rich to achieve the highest etch rates. Several different mask materials were investigated, including photoresist, thermal oxide, and deposited oxide. Photoresist etches very rapid ly in CO/NH3 and use of a hard mask is necessary to achieve pattern transfer. Due to its physically dominated nature, the CO/NH3 chemistry appears suited to shallow etch depth (less than or equal to 0.5 mu m) applications, but mask erosion leads to sloped feature sidewalls for deeper features.