Growth and characterization of amorphous ultrathin ruthenium metal films
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Copper interconnect systems in modern microelectronics require the use of one or more liner layers and a capping layer in order to prevent copper diffusion into the other materials of the device. Ruthenium has been suggested as a replacement for the currently-standard Ta/TaN stack used for this purpose due to its low bulk diffusivity of copper and its good adhesion to both substrate materials and copper, but at very low thicknesses the polycrystalline nature of pure Ru allows for diffusion of copper along grain boundaries, resulting in the failure of the barrier. Because amorphous metal alloys do not form grains, amorphous Ru alloys have been examined as a way to eliminate the grain boundary diffusion of copper across the film. Early attempts to produce such films with phosphorus as an alloying element by chemical vapor deposition (CVD) using Ru₃(CO)₁₂ and organic phosphorus precursors such as trimethylphosphine have performed well relative to Ta/TaN as a barrier layer at 5 nm thickness. However, high concentrations of carbon were incorporated into the films during CVD by the P precursors. Carbon increases the resistivity of Ru(P) and adds an unnecessary element to the calculated structure of the amorphous alloy. To reduce resistivity, lower-carbon Ru(P) alloy films are grown at 250 °C using Ru₃(CO)₁₂ and a hydride gas (PH₃) as the P precursor. Diborane (B₂H₆) is used to grow an alternate alloy, Ru(B). Ru(P) and Ru(B) alloys are predicted by first-principles calculations to be amorphous above 20 at.% P for Ru(P) and 10 at.% B for Ru(B). Growth studies revealed amorphous Ru(P) above 17 at.% P and amorphous Ru(B) above 10 at.% B, with polycrystalline films formed at lower concentrations. Both Ru(P) and Ru(B) are found to deposit as smooth, continuous films at the 3 nm thickness. Metal-insulator-semiconductor (MIS) capacitor structures consisting of copper / amorphous alloy / SiO₂ / Si / Al stacks were used to test barrier performance under electrical stress. This testing confirms that the amorphous Ru films perform adequately as Cu diffusion barriers.