Integration of PECVD Tungsten Nitride as a Barrier Layer for Copper Metallization
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*Genus Inc., Sunnyvale, CA 94089 "**New York State Center for Advanced Technology and Physics Department, the University at Albany-SUNY, Albany, NY 12222 ABSTRACT Amorphous tungsten nitride (WN,) is a promising diffusion barrier for extending Cu metallization beyond 0.18 pjm. This study evaluates the barrier performance, adhesion, and step coverage of PECVD WN 05 integrated with a CVD Cu seed layer. The WN 0 5 films exhibit amorphous structure with 33% bottom and side-wall step coverage in 0.14 Pjm wide structures with 9:1 aspect ratio. The potential of 50 A WN 0.5 as an effective Cu barrier is shown by the absence of Secco etch-pits in the Si substrate after a 30 min anneal at 500'C. When deposited on PECVD WN 0 .5 the CVD Cu films exhibit uniform nucleation, and as deposited resistivity of 2.5 jiQ-cm. Step coverage of the CVD Cu is better than 95% in 0.14 [tm structures. Adhesion exceeding epoxy strength of the CVD Cu seed layer even to air-exposed WN0 5 is demonstrated using stud-pull adhesion tests. INTRODUCTION Leading microelectronics manufacturers have begun the implementation of copper and low dielectric constant materials to enable improvement in device performance by lowering the signal propagation time delay [1]. An important aspect of copper metallization is the need for a diffusion barrier to completely encapsulate the interconnect lines. The barrier material and deposition method need to be carefully designed to avoid compromising the resistivity and reliability of the interconnect system. For instance, the effective copper conductance across the
wafer can be maximized by the use of a thin, uniformly deposited barrier layer. As illustrated in Figure 1, the barrier thickness requirements rapidly approach less than 75 A to retain 90% of the conductivity benefit of copper at feature size (FS) of 0.18 pjm and below. Other key integration requirements are low resistivity, good adhesion to copper and low-k materials, low deposition temperature, and thermal stability within the constraints of back-end-of-the-line (BEOL) thermal budget and long-term device operation. Titanium nitride (TiN) is the commonly used liner material with aluminum metallization. However, TiN is not likely to meet the challenges associated with extendibility to sub-0.18 Pjm and copper [2]. Barrier materials naturally superior to TiN are the nitrides of the heavier refractory metals tantalum and tungsten. These two elements are nearest neighbors in last row of transition elements in the periodic table. Thus we would expect similar barrier characteristics, but also some differences in terms of chemical reactivity. For example, an important distinction of tungsten nitride (WN5 ), compared to tantalum nitride (TaN5 ), is easier removal by chemical mechanical polishing (CMP). The removal rate of WN5 matches that of copper, which allows significantly simplified damascene integration with a single CMP step for both copper and barrier [3]. For example, a single step CMP of copper and WNx in an experimental evaluation yielded > 90 % on a comb
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