Disclosed is a method for improving the selectivity of dielectric layers to photoresist layers and base layers. The method is performed in a plasma processing chamber, and the photoresist layer is coated over the dielectric layer. The method includes introducing an etchant source gas into the plasma processing chamber, which consists essentially of a CxFy gas and an N.sub.2 gas. The method further includes striking a plasma in the plasma processing chamber from the etchant source gas. The method additionally includes etching at least a portion of the dielectric layer with the plasma through to a base layer that underlies the dielectric layer. The method is also well suited for anisotropically etching an oxide layer with very high selectivities to Si, Si.sub.3 N.sub.4, TiN, and metal silicides.