Diffusion barrier formation and characterization for silver metallization

This research project investigated in an integrated manner both the formation of diffusion barriers and their electrical performance with Si devices for Ag metallization. Aluminum oxynitride (Al _xO_yN_z) diffusion barriers were formed in the temperature range of 400–725°C by annealing Ag/Al bilayers on oxidized Si substrates in ammonia (NH ₃) ambient. Rutherford backscattering spectrometry (RBS) showed that the out-diffused Al reacted with NH₃ and O₂ in the ambient and finally encapsulated the Ag films. The resulting Ag resistivity is 1.75 ± 0.35 μΩ-cm, which is close to its bulk value. These diffusion barriers can sustain the interdiffusion between Cu and Ag up to 620°C and 30 minutes at least in both vacuum and forming gas ambient. This temperature is a 200°C improvement over previously reported result.; Theoretical modeling of the above process was based on an analytical solution of a modified diffusion equation, which incorporated the diffusion of Al through the Ag during the encapsulation process. The amount of segregated Al was monitored by both RBS and x-ray diffraction (XRD) measurements, and correlated well with theoretical predictions. These findings showed that the kinetics of self-encapsulation could be significantly affected by both the chemical affinity between Al and Ag atoms and the interfacial energy between Ag layer and the newly formed Al_xO _yN_z barriers. Higher anneal temperatures accelerated the encapsulation process and hence achieved a lower resistivity in the underlying Ag layer.; This project also investigated the electrical properties of Si devices using either Al_xO_yN _z or Ta_xO_yN _z barriers formed by sputtering methods. Comparison of measurements on pn diodes with and without barriers showed that the leakage current density dropped up to 4 orders of magnitude with the as-formed barriers and that the thermal stability of Al_xO _yN_z (higher than 600°C for 30 minutes) was better than that of Ta_xO _yN_z. The failure mechanism was related to the silicides formation by the elemental Ta reacting with Si substrate. The metal-oxide-semiconductor (MOS) capacitors with Ag as the electrodes functioned correctly and the leakage current measurement showed that, with the incorporation of Al_xO_yN _z, the oxide could sustain higher bias voltages after low temperature processing (less than 500°C) without breakdown; but after the high temperature anneals, non-uniformity caused by the interfacial reaction between Al and SiO₂ accelerated the oxide breakdown.