1. A method of etching a dielectric structure comprising: providing a dielectric structure comprising (a) a first dielectric layer of undoped silicon oxide or F-doped silicon oxide; and (b) a second dielectric layer of C,H-doped silicon oxide; and etching said dielectric structure in a plasma-etching step, wherein said plasma-etching step is conducted using a plasma source gas that comprises nitrogen atoms and fluorine atoms, and wherein said second dielectric layer is selectively etched relative to said first dielectric layer in said etching step.