Interactions Between Nanoporous Silica and Copper

Porous materials have been proposed to replace SiO2 as the interlayer dielectric in future microelectronic devices. The interaction of porous materials with contiguous layers is very important for the long-term stability of devices. To investigate the interaction of silica xerogels with Cu, thin Cu layers were deposited on xerogel films and subsequently annealed. When Cu films were directly exposed to an oxidizing environment, they underwent morphological changes, and these changes may lead to the erroneous interpretation of the Rutherford backscattering spectra as depicting metal diffusion. When the samples are capped with an Si 3N4 layer, which precludes oxidation, Cu does not show diffusion into the xerogel when annealed up to 650°C. Bias-temperature stressing ~BTS! was conducted to assess Cu drift through the xerogel in the presence of an electric field. Contrary to what is normally observed with other dielectrics, the leakage current and capacitance-voltage ~C-V! curve shifts were larger with an Al electrode than with a Cu electrode. When the organic capping groups are removed from the xerogel by high temperature annealing before metal deposition, C-V tests indicate Cu ion diffusion occurs even as low as 100°C. Surface modification of the xerogel by trimethylsilyl groups may contribute to the smaller charge injection from the Cu/xerogel interface or to the inhibition of Cu diffusion, thus offering the possibility of designing future monolayer diffusion barriers for porous materials. A model of diffusion through porous solids has been developed. Two possible paths of mass transfer in porous solids are identified: bulk and surface. Three driving forces are also considered: concentration gradient, electric field, and curvature gradient. The upper limit of Cu diffusivity in xerogel was estimated, and the model simulated the current observed during BTS testing reasonably well. A new, better insulator is needed to replace SiO2 in future generations of microelectronic devices. The dielectric constants of porous materials, such as silica xerogels, can be tailored by adjusting the porosity so that their use can be extended to more than one generation of devices. This is a distinct advantage for silica which, together with aluminum, has been used for years for interconnects. In the structure of interconnects, the dielectric is in contact with another material, such as a metal ~copper!, diffusion barrier ~e.g. ,T a or Ta nitride!, or hard mask ~Si nitride!. Any interactions between these different materials must be minimized to achieve long-term device stability. These interactions, such as interdiffusion or interfacial reactions, should be limited to a few nanometers beneath the interface. Some minimal interaction is necessary to provide bonding and good adhesion between layers. In this paper, we focus on Cu diffusion through porous dielectrics at elevated temperatures ~thermal diffusion!, and diffusion in the presence of electric fields ~drift!. Copper, the material of choice for integrated circuit metallization, diffuses quickly through both SiO2 and Si and does not adhere to most dielectric materials, including SiO 2 . 1 This indicates the need for both diffusion barriers and adhesion promoters in systems with copper metallization. However, Cu diffuses particularly fast through silicon and dielectric materials, including SiO 2 , when subjected simultaneously to slightly elevated temperatures ~100-300°C! and an electrical bias. These conditions are encountered during actual operation of a semiconductor device. Therefore, it is very important to evaluate the drift of metals in the presence of an electric field. Cu drift through many low-k materials, such as benzocyclobutene, polyarylene ether, Parylene-F, and SiLK, in the presence of an electric field of around 1 MV/cm is slower than through SiO 2 , yet higher than through plasma-enhanced chemical vapor deposited ~PECVD! oxynitride. 2 Highly cross-linked polymers may reduce Cu diffusion by reducing the available free space for Cu motion, and the mobility of Cu ions may be enhanced by polar groups in the dielectric, which behave as centers of strong partial charge that can attract Cu 1 electrostatically. 2