32_28nm Beol Cu CMP Development for Ultra Low k Scheme

Abstract

32/28nm BEOL Cu CMP process with ultra low k scheme is investigated in the different aspects. Firstly the barrier metal (BM) slurry selection with proper selectivity is the most critical part to reduce topography. The topography can be minimized by precisely control BM slurry’s selectivity. Secondly, the layout design, such as pattern density, line and space width, has significant impact on WID variation. The test results show more topography correction at more dense Cu line for thinner line where BM polishing is the dominate factor. While for fat line, it shows less topography correction at more dense Cu line, since Cu clearance polishing dominates the final topography. Next k value shift evaluation with different barrier metal slurry, cleaning chemicals and post CMP thermal treatment conditions is also studied. The results indicate that although k value shift occurs during polishing, proper chemicals clean and post thermal treatment can restore parts even the whole of k shift. In addition, the removal of moisture and chemical penetrated into ultra low k film is the key of k value shift and reliabilities improvement.

Effect of Potassium Ions on Tantalum Chemical Mechanical Polishing in H2O2-Based Alkaline Slurries

“…Effect of H 2 O 2 on the tantalum polishing performance.-During the Ta/TaN polishing step of BEOL which is mainly used to remove Ta/TaN, the hard mask and some amount of porous low-κ dielectric, as well as to planarize the whole surface, both tantalum and copper will be polished with the same slurry simultaneously. 32 According to the potential-pH diagram for copper-water system at copper ion activities of 10 −4 and 10 −6 , 36 severe copper corrosion could occur under acidic condition. In order to realize low defectivity of copper including corrosion and pit as well to realize a high tantalum MRR, pH of the tantalum slurry (or denoted by tantalum barrier slurry) should be in alkaline range.…”

“…The only concern of alkaline tantalum slurries is the chemical damage to the underlying porous low-κ dielectric caused by hydroxyl ions. However, Zhao et al 31 and Liu et al 32 reported a part even the whole of the κ value shift could be restored with proper cleaning and post-CMP annealing by removing the moisture and the chemicals penetrated into the low-κ dielectric. Furthermore, Miller et al 33 presented that, for copper CMP, alkaline slurries had advantage for low defectivity including corrosion, pit and organic residue.…”