ENHANCED RELIABILITY OF W-ENCAPSULATED ALSI-INTERCONNECTIONS USING SELECTIVE W-CVD

Reliability of Al based interconnections can be enhanced by using refr actory 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 vacu um load lock. The deposition process was based on the silane reduction of WF6, which takes place preferentially on electron donating materia ls (metals, silicides, TiN, etc.). Different pretreatments were examin ed with respect to their influence on selectivity and nucleation behav iour at the surface (top and sidewall) of AISI lines. Optimized pretre atment combinations (HF dip; HF dip + H-2 plasma) resulted in selectiv e growth of tungsten and uniform encapsulation of the lines. Electromi gration tests were performed on 2 mu m and 3 mu m wide NIST structures . The determined mean times to failure (MTF) are up to 7 times larger compared to conventional AISI interconnections depending on W thicknes s. The evolution of resistance during an EM test shows that void forma tion takes place in the Al alloy. In this case current flow can be mai ntained by the encapsulation layer due to the high electromigration re sistance of tungsten.