Low temperature interdiffusion in Cu/Ni thin films

Interdiffusion in Cu/Ni thin films was studied by means of Auger electron spectroscopy in conjunction with Ar⁺ ion sputter profiling. The experimental conditions used aimed at simulating those of typical chip-packaging fabrication processes. The Cu/Ni couple (from 10 μm to 60 nm thick) was produced by sequential vapor deposition on fused-silica substrates at 360, 280 and 25 °C in 10^-6 Torr vacuum. Diffusion anneals were performed between 280 and 405 °C for times up to 20 min. Such conditions define grain boundary diffusion in the regimes of B- and C-type kinetics. The data were analyzed according to the Whipple-Suzuoka model. Some deviations from the assumptions of this model, as occurred in the present study, are discussed but cannot fully account for the typical data scatter. The grain boundary diffusion coefficients were determined (for nickel through copper, Q_b = 33.7 kcal mol^-1 (1.46 eV), D_b⁰ = 4.2 × 10^-2 cm²s^-1; for copper through nickel, Q_b = 30.2 kcal mol^-1 (1.3 eV), D_b⁰ = 7.6 × 10^-5 cm²s^-1) allowing calculation of respective permeation distances.