Methods of etching silicon nitride faster than silicon oxide are described. Exposed portions of silicon nitride and silicon oxide may both be present on a patterned substrate. A self-assembled monolayer (SAM) is selectively deposited over the silicon oxide but not on the exposed silicon nitride. Molecules of the self-assembled monolayer include a head moiety and a tail moiety, the head moiety forming a bond with the OH group on the exposed silicon oxide portion and the tail moiety extending away from the patterned substrate. A subsequent gas-phase etch using anhydrous vapor-phase HF may then be used to selectively remove silicon nitride much faster than silicon oxide because the SAM has been found to delay the etch and reduce the etch rate.