A semiconductor device includes a semiconductor substrate, a porous low-k dielectric layer, a copper interconnect structure in the porous low-k dielectric layer, a diffusion barrier layer disposed between the copper interconnect structure and the porous low-k dielectric layer, and a silicon nitride layer disposed between the diffusion barrier layer and the porous low-k dielectric layer The low-k dielectric layer has a smooth and dense surface that increases the adhesion strength between the low-k dielectric layer and the diffusion barrier layer to improve reliability and yield of the semiconductor device.