A semiconductor device is disclosed, which comprises a capacitor structure formed over a device region of a substrate, and a buffer layer. The capacitor structure comprises a lower electrode having a U-shaped profile that opens away from the substrate, the U-shaped profile defines an interior surface and an opposing exterior surface; a dielectric liner extending into the U-shaped profile and conformally covering the interior surface of the lower electrode; and an upper electrode formed over the dielectric liner, extending into and filling the U-shaped profile, the upper electrode) includes a top conductive layer. The buffer layer formed on the top conductive layer of the upper electrode, wherein the lattice constant of the buffer layer is greater than that of the top conductive layer.