Diffusion of Copper in Nanoporous Dielectric Films

Abstract

Water adsorption is undesirable in microelectronics processing because it increases the dielectric constant of the insulator, may lead to the corrosion of metal lines, and can act as a source for the generation of copper ions that can lead to copper drift in a dielectric. We have shown by chemical modification of the surface that the degree of hydrophobicity is a function of the chemical nature of porous low-k dielectric films. Chemical surface modification of nanoporous silica films markedly reduces the moisture uptake and reveals the importance of chemically bound or adsorbed water species in the dielectric and how it triggers metal diffusion. That is, when the organic groups left by the surface modifier are removed by high-temperature sintering, the hydrophobicity of the dielectric film is markedly reduced, more copper ions are generated, and copper drift in the dielectric is increased significantly. We propose that water-related traps in the dielectric films have two effects on metal diffusion: (a) they ionize the metal to form a nonstoichiometric oxide, which acts as the source of metal ions for diffusion; and (b) water-related traps in the dielectric are generated by the action of the external electric field and once generated they create space-charged regions where the local electric field exceeds the external applied field by a substantial amount; this enhances metal charge injection. The relationship between moisture uptake and porosity in nanoporous dielectrics is presented and correlated to metal charge injection. A quantitative analysis by capacitance-voltage measurements is presented of Cu drift in dense and nanoporous low-k dielectric films that reveals the role of Cu ions in the degradation and breakdown of a dielectric. The mechanism for metal diffusion and charge injection, and its dependence on porosity, pore size, surface area, and surface chemistry of the dielectric, are discussed. A physically based mathematical model of diffusion through dense and nanoporous solids has been developed considering bulk and surface diffusion and different concentrations of water-related traps. This model is used to interpret some of the experimental results obtained and confirms that diffusion barriers markedly reduce the injection of copper into dielectrics.