A study of vanadium as diffusion barrier between aluminum and gadolinium silicide contacts

Implementation of GdSi₂ as low-barrier (0.4 eV) Schottky contacts in a Si device requires a diffusion barrier between the GdSi ₂ and Al interconnects to prevent Al penetration. In this work, the effectiveness of vanadium films as such a diffusion barrier has been studied by using a set of complimentary experimental techniques: nuclear backscattering, x-ray diffraction, and current-voltage measurement of Schottky barrier height on both n-type and p-type Si substrates. We found that the effectiveness depends strongly on how the thin-film structure of Al/V/GdSi₂/Si was made, and especially on the formation process of GdSi₂. The GdSi ₂, which was formed upon deposition onto a high-temperature substrate, is superior in stability by showing no Al penetration to other processing schemes investigated. These results are explained by structural and compositional changes occurring in the layered films of Al/V/Gd/Si as a function of heat treatment. In order to understand the complex interdiffusion and compound formation in the layered structure, the reactions between neighboring layers have been studied: Al interacts with V at ∼450 °C, forming Al₃V and possibly Al₁₀V; V does not react with Gd, even at 700 °C; and Gd is known to react with Si to form GdSi ₂ at∼350 °C. On the other hand, Al reacts readily with Gd at 250 °C to form a metastable compound of Al₄Gd, which then transforms to Al₃Gd at 400 °C, and the formation of these compounds has been found to retard the reaction between Gd and Si.