A method for etching a target material in the presence of a structural material with improved selectivity uses a vapor phase etchant and a co-etchant. Embodiments of the method exhibit improved selectivities of from at least about 2-times to at least about 100-times compared with a similar etching process not using a co-etchant. In some embodiments, the target material comprises a metal etchable by the vapor phase etchant. Embodiments of the method are particularly useful in the manufacture of MEMS devices, for example, interferometric modulators. In some embodiments, the target material comprises a metal etchable by the vapor phase etchant, for example, molybdenum and the structural material comprises a dielectric, for example silicon dioxide.