A semiconductor device is disclosed. The device includes a substrate, a first porous SiCOH dielectric layer, a second porous SiCOH dielectric layer, and an oxide layer. The first porous SiCOH dielectric layer overlies the substrate. The second porous SiCOH dielectric layer overlies the first porous SiCOH dielectric layer. The oxide layer overlies the second porous SiCOH dielectric layer. The atomic percentage of carbon in the second porous SiCOH dielectric layer is between 16% and 22% of that in the first porous SiCOH dielectric layer.