A method of etching a recess in a semiconductor substrate is described. The method may include forming a dielectric liner layer in a trench of the substrate where the liner layer has a first density. The method may also include depositing a second dielectric layer at least partially in the trench on the liner layer. The second dielectric layer may initially be flowable following the deposition, and have a second density that is less than the first density of the liner. The method may further include exposing the substrate to a dry etchant, where the etchant removes a portion of the first liner layer and the second dielectric layer to form a recess, where the dry etchant includes a fluorine-containing compound and molecular hydrogen, and where the etch rate ratio for removing the first dielectric liner layer to removing the second dielectric layer is about 1:1.2 to about 1:1.