Diffusion barrier properties of single- and multilayered quasi-amorphous tantalum nitride thin films against copper penetration

摘要

Tantalum-related thin films containing different amounts of nitrogen are sputter deposited at different argon-to-nitrogen flow rate ratios on (100) silicon substrates. Using x-ray diffractometry, transmission electron microscopy, composition and resistivity analyses, and bending-beam stress measurement technique, this work examines the impact of varying the nitrogen flow rate, particularly on the crystal structure, composition, resistivity, and residual intrinsic stress of the deposited Ta_(2)N thin films. With an adequate amount of controlled, reactive nitrogen in the sputtering gas, thin films of the tantalum nitride of nominal formula Ta_(2)N are predominantly amorphous and can exist over a range of nitrogen concentrations slightly deviated from stoichiometry. The single-layered quasi-amorphous Ta_(2)N (a-Ta_(2)N) thin films yield intrinsic compressive stresses in the range 3-5 GPa. In addition, the use of the 40-nm-thick a-Ta_(2)N thin films with different nitrogen atomic concentrations (33 and 36) and layering designs as diffusion barriers between silicon and copper are also evaluated. When subjected to high-temperature annealing, the single-layered a-Ta_(2)N barrier layers degrade primarily by an amorphous-to-crystalline transition of the barrier layers. Crystallization of the single-layered stoichiometric a-Ta_(2)N (Ta_(67)N_(33)) diffusion barriers occurs at temperatures as low as 450℃. Doing so allows copper to preferentially penetrate through the grain boundaries or thermal-induced microcracks of the crystallized barriers and react with silicon, sequentially forming {111}-facetted pyramidal Cu_(3)Si precipitates and TaSi_(2) Overdoping nitrogen into the amorphous matrix can dramatically increase the crystallization temperature to 600℃. This temperature increase slows down the inward diffusion of copper and delays the formation of both silicides. The nitrogen overdoped Ta_(2)N (Ta_(64)N_(36)) diffusion barriers can thus be significantly enhanced so as to yield a failure temperature 100℃ greater than that of the Ta_(67)N_(33) diffusion barriers. Moreover, multilayered films, formed by alternately stacking the Ta_(67)N_(33) and Ta_(64)N_(36) layers with an optimized bilayer thickness (λ) of 10 nm, can dramatically reduce the intrinsic compressive stress to only 0.7 GPa and undergo high-temperature annealing without crystallization. Therefore, the Ta_(67)N_(33)/Ta_(64)N_(36) multilayered films exhibit a much better barrier performance than the highly crystallization-resistant Ta_(64)N_(36) single-layered films.