A method of suppressing the etch rate for exposed silicon-and-oxygen-containing material on patterned heterogeneous structures is described and includes a two stage remote plasma etch. Examples of materials whose selectivity is increased using this technique include silicon nitride and silicon. The first stage of the remote plasma etch reacts plasma effluents with the patterned heterogeneous structures to form protective solid by-product on the silicon-and-oxygen-containing material. The plasma effluents of the first stage are formed from a remote plasma of a combination of precursors, including a nitrogen-containing precursor and a hydrogen-containing precursor. The second stage of the remote plasma etch also reacts plasma effluents with the patterned heterogeneous structures to selectively remove material which lacks the protective solid by-product. The plasma effluents of the second stage are formed from a remote plasma of a fluorine-containing precursor.