Enhanced reliability of W-encapsulated AlSi-interconnections using selective W-CVD

Abstract

Reliability of Al based interconnections can be enhanced by using refractory metal base and top layers. Therefore encapsulation of Al alloy lines by selective W-CVD was examined with respect to electromigration stability. The selective W-CVD was performed in a commercial cluster tool with a specially equipped selective W-CVD chamber and a high vacuum load lock. The deposition process was based on the silane reduction of WF 6, which takes place preferentially on electron donating materials (metals, silicides, TiN, etc.). Different pretreatments were examined with respect to their influence on selectivity and nucleation behaviour at the surface (top and sidewall) of AlSi lines. Optimized pretreatment combinations (HF dip; HF dip + H 2 plasma) resulted in selective growth of tungsten and uniform encapsulation of the lines. Electromigration tests were performed on 2 μm and 3 μm wide NIST structures. The determined mean times to failure (MTF) are up to 7 times larger compared to conventional AlSi interconnections depending on W thickness. The evolution of resistance during an EM test shows that void formation takes place in the Al alloy. In this case current flow can be maintained by the encapsulation layer due to the high electromigration resistance of tungsten.