Impact of post via clean solution on Self Aligned Barrier integrity: Corrosion and Cu/Co galvanic coupling

This study aims at understanding the behavior of cobalt based Self Aligned Barriers (SAB) during various acidic clean treatments using diluted aqueous solutions of organic acid (OA) and hydrofluoric acid (HF) for post etch clean. The partial dissolution of cobalt based alloys in diluted OA and in OA/HF mixture is demonstrated with SEM analysis on patterned structures. An exhaustive electrochemical study of pure cobalt and copper is performed in both diluted OA and HF, as well as in their mixture. This dissolution is quantified and the contributions of spontaneous corrosion and cobalt/copper galvanic coupling are discriminated. Considering via geometry used in the 32 nm technology, it is demonstrated that the galvanic coupling is negligible compared to the spontaneous corrosion of cobalt in OA and in OA/HF, but is considerable in diluted HF. Two additives are tested to prevent Co and Cu from corrosion in the OA/HF mixture. Add2 amplifies Cu and Co corrosion, whereas Add1 is showing a smooth etching action of copper and a clear passivating action regarding Co.