Grain boundary self-diffusion in Cu polycrystals of different purity

The temperature dependence of grain boundary (GB) self-diffusion in Cu polycrystals was systematically investigated using the Cu-64 radiotracer and the serial sectioning technique. Two different high-purity Cu materials were used: 99.9998% Cu and 99.999% Cu. The difference in impurity content was checked by time-of-flight secondary-ion mass spectrometry (SIMS). The two materials were studied in the temperature ranges 1066-720 K (99.9998% Cu) and 973-784K (99.999% Cu). A significant dependence of the GB self-diffusion on the material purity was observed, resulting in a different Arrhenius behaviour of the diffusivity delta D-GB where delta is the GB width and D-GB is the GB diffusion coefficient. The Arrhenius parameters are 3.89 x 10(-16) m(3)/s and 72.47 kJ/mol for the highest purity material, and 1.16 x 10(-15) m(3)/s and 84.75 kJ/mol for the high purity material. The differences in GB diffusion coefficients and activation enthalpies are explained in terms of strong impurity-Cu atom bonds in the boundary. In the present investigation sulphur is the dominant impurity. Remarks are made relating the impurity segregation and the reduction of the GB diffusivity to the lifetimes of thin film metallic interconnects determined by electromigration effects in GBs. Using a semi-empirical relation of Borisov ct al. the GB energy, gamma(GB), of large angle boundaries and its temperature dependence were determined from the self-diffusion data of Cu. The resulting values of gamma(GB) (e.g. 650 mJ/m(2) at 1000 K) are in good agreement with GB energy measurements and model calculations for Cu. (C) 1997 Acta Metallurgica Inc.