A method of forming an interconnect for a semiconductor device using triple hard layers, comprises: forming a first hard layer serving as an etch stop layer on a metal interconnect-formed dielectric layer; forming a second hard layer on the first hard layer; forming a dielectric layer on the second hard layer; forming a third hard layer on the dielectric layer; forming a hole through the third and second hard layers, the dielectric layer, and the first hard layer; and filling the hole with metal to establish an interconnect. The second and third hard layers are each made of carbon-doped silicon oxide formed from a source gas and a redox gas, while controlling the carbon content in the second hard layer as a function of a flow rate of the redox gas.