Interactions Between Nanoporous Silica and Copper

Abstract

Porous materials have been proposed to replace 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 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. © 2002 The Electrochemical Society. All rights reserved.