A method of forming a damascene structure comprises preparing a film stack on the substrate, wherein the film stack comprises a SiCOH-containing layer formed on the substrate, a silicon oxide (SiOx) layer formed on the SiCOH-containing layer, and a first mask layer formed on the silicon oxide layer. A trench pattern is created in the first mask layer. The trench pattern in the first mask layer is transferred to the silicon oxide layer, and then the first mask layer is removed. A second mask layer is formed on the silicon oxide layer. A via pattern is formed in the second mask layer. The via pattern is transferred to the SiCOH-containing layer using a first etching process, and then the second mask layer is removed. The trench pattern is transferred to the SiCOH-containing layer using a second etching process with plasma formed from a process composition comprising NF3.