It was noted that Pt electrodes did not oxidize, but they permitted rapid O diffusion through Pt films while a dielectric was deposited and annealed. This then caused unwanted oxidation below the Pt film. By using first-principles electronic structure calculations, the O diffusion mechanism in polycrystalline Pt was determined. It was found that O diffused as interstitial O along the grain boundary. The calculated barrier was consistent with the experimental estimate. Various elements were tested for their potential to retard O diffusion when added to Pt in small amounts. It was found that Be was the most promising candidate. It segregated to Pt grain boundaries, at interstitial and substitutional sites. As a grain-boundary interstitial, Be diffused at a rate which was comparable to that of O, and repelled O. This led to stuffing of the grain boundary. As an absorbed substituent, Be offered a high diffusion barrier and formed strong bonds with O. The O was therefore trapped in the grain boundary. Experimental results confirmed these theoretical predictions. The general question of dopant-retardation of interdiffusion along the grain boundaries of close-packed metals was considered. It was concluded that the first requirement was that the dopant had to segregate to the grain boundary. The relative atomic sizes of the dopant and host were important in determining the segregation energy and the absorption site. Smaller elements segregated strongly to grain-boundary interstitial sites. If they were not too small, leading to fast diffusion, they could stuff the grain boundary and reduce interdiffusion. If the dopants were more evenly matched in size to the host, they preferred interstitial sites at the grain boundary; leading to slow dopant diffusion. These dopants then interacted differently with the interdiffusing species than with the host, and interdiffusion was hindered by site-blocking or by trapping. In all cases, there was an optimum concentration of the dopant at the grain boundary. In order to predict real segregation coefficients or diffusion rates, it would be necessary to perform free-energy calculations and include vibrational and configurational entropies.

Retardation of O Diffusion through Polycrystalline Pt by Be Doping. R.Stumpf, C.L.Liu, C.Tracy: Physical Review B, 1999, 59[24], 16047-52