Kinetics Model for the Self-Encapsulation of Ag/Al Bilayers
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Kinetics model for the self-encapsulation of Ag/Al bilayers Y. Wang, T. L. Alford, and J. W. Mayer Department of Chemical, Bio, and Materials Engineering NSF Center for Low Power Electronics Arizona State University, Tempe, AZ 85287-6006, USA Abstract A model is proposed to describe the temperature dependence of the aluminum oxynitride (AlxOyNz) diffusion barrier formation during a silver self-encapsulation process. These barrier layers form in the temperature range of 500-725 °C during anneals of the Ag/Al bilayers on oxidized Si substrates in an ammonia ambient. Experimental results show that temperature has a significant effect on the kinetics of this process. In this investigation, the diffusion of Al atoms through the Ag layers during self-encapsulation process is modeled using an analytical solution to a modified diffusion equation. This model shows that higher anneal temperatures will minimize the retardation effect by i) reducing the chemical affinity between Al and Ag atoms, and ii) allowing more Al atoms to surmount the interfacial energy barrier between the metal layer (Ag) and the newly formed AlxOyNz diffusion barriers. The theoretical predictions on the amount of segregated Al atom correlate well with experimental results from Rutherford backscattering spectrometry. This model in addition confirms the self-passivation characteristics of AlxOyNz diffusion barriers formed by Ag/Al bilayers annealed between 500∼725 °C. I.
INTRODUCTION
In the future, ULSI technologies will employ thinner and narrower multiple metal layers in order to achieve higher device speeds and higher component density1. This tendency will make the RC delay (inherent with the metallization schemes) much more significant than before. This is why Ag with the lowest resistivity among all the metals has been explored as an alternative for future metallization schemes2-4. At the same time, this scaling tendency will also require more robust diffusion barriers and better thermal management. The previous study on selfencapsulation of Ag/Al bilayer provided a good approach for the above questions since this process has achieved the lower resistivity (~ 1.75 µΩ-cm) of the as-processed Ag layer and formed a thin AlxOyNz diffusion barrier with higher thermal stability than TiN barrier4. Our experimental results showed that higher temperatures increased the speed of Al diffusion through the Ag layer and helped to deplete Al atoms from the Ag layer by accelerating both the diffusion and the reaction of Al with NH3 or O2 at the surface. Since most of the research on the encapsulation process2-5 was only focused on the experimental issues; it is hence worthy to investigate this process theoretically and to focus especially on the kinetics aspect in order to optimize this self-encapsulation process further. In this article, we quantitatively confirmed our previous explanation and obtained a better understanding of the self-encapsulation process. II.
EXPERIMENTS
Ag(100nm)/Al(8nm) and Ag(200nm)/Al(8nm) bilayers were deposited sequentially by electro
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