A method for fabricating an interconnect structure by a dual damascene process is described, in which a first low dielectric constant material is formed on a substrate, followed by forming a gradient silicon oxy-nitride layer on the first low dielectric constant. A second low dielectric constant layer is further formed on the gradient silicon oxy-nitride layer. A trench line is then formed in the second low dielectric constant material using the gradient silicon oxy-nitride layer as an etch-stop, followed by forming a via under the trench line.