The present invention relates to methods of improving the fabrication of interconnect structures of the single or dual damascene type, in which there is no problem of hard mask retention or of conductivity between the metal lines after fabrication. The methods of the present invention include at least steps of chemical mechanical polishing and UV exposure or chemical repair treatment which steps improve the reliability of the interconnect structure formed. The present invention also relates to an interconnect structure which include a porous ultra low k dielectric of the SiCOH type in which the surface layer thereof has been modified so as to form a gradient layer that has both a density gradient and a C content gradient.