Diffusion Of Metals In Polymers And During Metal/Polymer Interface Formation

Abstract

The increasing application of polymers in microelectronics, particularly their potential use as low dielectric constant materials in IC applications has aroused much interest in diffusion of metals in polymers. Various investigations, involving surface spectroscopy, electron microscopy, ion scattering, direct radiotracer measurements, and Monte Carlo simulations have contributed considerably to our understanding of the diffusion behavior. Metal diffusion in polymers proved to differ markedly from ordinary diffusion because of the sharply contrasting properties of the materials. A strong correlation has been established between the reactivity of a metal and its diffusion behavior. Reactive metals like Cr and Ti form relatively sharp interfaces with polymers. Metals of lower reactivity, on the other hand, diffuse into polymers at elevated temperatures but have a very strong tendency to be immobilized by aggregation. Diffusion into the polymer increases strongly at low deposition rates, where a large fraction of isolated metal atoms is able to diffuse into the polymer before being trapped by other atoms at or near the surface. The extent of diffusion appears to be determined by the early stage of the deposition process. Here, condensation coefficients may deviate markedly from unity, thus keeping the initial metal concentration at the surface very low. No significant diffusion is expected from a continuous metal film as a consequence of the high cohesive energy of the metal, unless chemical interaction involves the formation of metal ions. Metal ions are highly mobile and do not aggregate due to electrostatic repulsion. The model emerging from these observations allows us to predict the salient features of diffusion and interface formation between metals and polymers in general, and particularly with respect to the new low dielectric constant polymers.