Properties and Barrier Material Interactions of Electroless Copper used for Seed Enhancement

Abstract

AbstractThe conventionally used sequence for copper damascene metallization consists of barrier deposition, physical vapor deposition (PVD) Cu seed and electroplated copper. Due to the limited step coverage of PVD copper, the extendibility of this sequence to feature dimensions below 90 nm is at risk. To reduce the risk of pinch-off of very small features, the PVD layer thickness will be reduced well below 100 nm, the drawback being poor seed coverage at the bottom of the features. Void free fill by electroplating is hence at risk by both pinch-off and discontinuous seed coverage (3-5). In this paper, the use of a conformal metal deposition method, electroless copper, to enhance PVD seed layers as thin as 10 nm is presented. It is demonstrated that sparse, discontinuous copper films provide a catalytic surface for electroless copper deposition. With electroless copper, void-free copper fill of 12.5 aspect ratio (AR) trenches (70 nm width) and 8.3 AR vias is achieved. Furthermore, 6 nm thin electroless copper films were integrated in a dual damascene process and electrically characterized. A yield of approximately 85% was achieved on via chains (360000 links, 0.25 by 1.1 μm vias), with 10 nm PVD seed. This was comparable to the yield when using 100 nm PVD seed. Hydrogen, generated as a byproduct during the electroless copper ion reduction, was found in the copper deposits as well as in the barrier films underneath. In some cases, spontaneous blistering in the plated copper film was observed, and is believed to be due to hydrogen incorporation. The interaction of electroless copper films with various barrier materials (PVD Ta, PVD TaN, CVD TiN(Si) and combinations) is discussed. Electromigration test results presented in this paper indicate that the failure mechanism is not qualitatively different from reference samples with the conventional PVD seed.