(A) study on the two-step atomic layer deposition for TaN thin films

Abstract

Multilevel copper interconnections have been used extensively in large-scale integrated circuits. For copper interconnections, Ta and TaN films, which are immiscible with copper and have superior thermal stability, have been widely used as barriers to copper diffusion. They are usually prepared using collimated and ionized physical vapor deposition (PVD) techniques; however, it is held that the PVD technique will eventually meet a technological barrier preventing the formation of conformal thin films on nanoscale contacts and via holes, as feature sizes are scaled below 45 nm, due to its line-of-sight approach. Consequently, atomic layer deposition (ALD) has recently been spotlighted because of its inherent merits in preparing conformal ultrathin films on nanoscaled features. Numerous TaN ALD thin films have been examined using metal-organic (MO) compounds as the Ta precursor, but avoiding carbon impurities, which readily form TaC in the deposited films, is difficult. Therefore, carbon-free Ta halides, such as $TaCl_5$ and $TaBr_5$, have been adopted to make tantalum nitride. In utilizing Ta halides as the Ta precursor, however, it was easy to form the dielectric material of $Ta_3N_5$ because the most Ta halide precursors have an oxidation state of +V. It was reported that a cubic-TaN thin film could be obtained by using a small amount of additional reducing agents, such as zinc, trimethylaluminum, and tert-butylamine to suppress the formation of $Ta_3N_5$. However, the use of reducing agents brought on the incorporation of zinc or carbon resulting in film resistivity higher than 900 μΩ-cm. This paper proposes a novel TaN ALD to avoid the formation of $Ta_3N_5$. The chemical reaction path to TaN is separated into two steps: elementary metal Ta (0 oxidation state) is reduced from $TaF_5$ with a hydrogen plasma pulse, and then the elementary Ta is converted into cubic-TaN with a $NH_3$ pulse. The Ta thin film was successfully deposited using $TaF_5...