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

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 $Ta2N$ thin films. With an adequate amount of controlled, reactive nitrogen in the sputtering gas, thin films of the tantalum nitride of nominal formula $Ta2N$ are predominantly amorphous and can exist over a range of nitrogen concentrations slightly deviated from stoichiometry. The single-layered quasi-amorphous $Ta2N$ $(a-Ta2N)$ thin films yield intrinsic compressive stresses in the range 3–5 GPa. In addition, the use of the 40-nm-thick $a-Ta2N$ 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 $a-Ta2N$ barrier layers degrade primarily by an amorphous-to-crystalline transition of the barrier layers. Crystallization of the single-layered stoichiometric $a-Ta2N$ $(Ta67N33)$ diffusion barriers occurs at temperatures as low as $450 °C.$ 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 $Cu3Si$ precipitates and $TaSi2$ Overdoping nitrogen into the amorphous matrix can dramatically increase the crystallization temperature to $600 °C.$ This temperature increase slows down the inward diffusion of copper and delays the formation of both silicides. The nitrogen overdoped $Ta2N$ $(Ta64N36)$ diffusion barriers can thus be significantly enhanced so as to yield a failure temperature $100 °C$ greater than that of the $Ta67N33$ diffusion barriers. Moreover, multilayered films, formed by alternately stacking the $Ta67N33$ and $Ta64N36$ 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 $Ta67N33/Ta64N36$ multilayered films exhibit a much better barrier performance than the highly crystallization-resistant $Ta64N36$ single-layered films.