A composition of matter comprising atoms of at least Si, C, H and N, said N atoms are non-uniformly distributed within the composition, and said composition having a dielectric constant of less than 3.5, being located on a substrate, and having a lower region of dielectric material of a first phase comprised of at least atoms of Si, C and H and an upper region of said dielectric material that has a density that is greater than that of the lower region, said upper region including a nitrided upper surface layer containing said N atoms.