The invention provides a conformal sealed dielectric film which is suitable for being used as a dielectric diffusion barrier above a 3D profile. In one embodiment, A dielectric diffusion barrier comprises a dielectric layer such as a metal oxide layer, wherein the dielectric layer can be deposited through atomic layer deposition ALD technology. For a relatively thin continuous sealed diffusion barrier, conformal degree and density of the dielectric layer which is deposited through the atomic layer deposition ALD technology are higher than the conformal degree and the density which can be realized in a traditional silicon-dioxide-based film that is deposited through a PECVD process. In other embodiments, the diffusion barrier is a multilayer film such as a double-layer film, wherein the multilayer film comprises a high-k dielectric layer and a low-k or intermediate-k dielectric layer, thereby reducing the dielectric constant of the diffusion barrier. In other embodiments, silicate (such as metal silicate) in the high-k dielectric layer is formed to reduce the k value of the diffusion barrier through adjusting the silicon content of the silicate while a relatively high conformal degree and a relatively high density of the film are kept.