A method of etching exposed silicon oxide on patterned heterogeneous structures is described and includes a gas phase etch created from a remote plasma etch. The remote plasma excites a fluorine-containing precursor. Plasma effluents from the remote plasma are flowed into a substrate processing region where the plasma effluents combine with water vapor. Reactants thereby produced etch the patterned heterogeneous structures to remove two separate regions of differing silicon oxide at different etch rates. The methods may be used to remove low density silicon oxide while removing less high density silicon oxide.