Design and achievement of a complete bottom-up electroless copper filling for sub-micrometer trenches

The bottom-up filling of electroless copper usually depends on inhibiting the Cu deposition at the surface or accelerating the Cu deposition in the bottom of trenches to achieve a relative high deposition rate of electroless copper in the bottom of trenches. In this paper, a complete bottom-up filling of electroless copper, in which the deposition rates of the electroless copper were not only inhibited at the surface of the substrate, but also were accelerated in the bottom of the trenched, was designed and achieved in the plating bath with an addition of bis (3-sulfopropyl) disulfide (SPS) and polyethylene glycol (PEG). The cross-sectional SEM observation of trenches indicated that all trenches with different widths ranging from 100 to 290 nm were filled completely by electroless copper, and no void was found. This was attributed to three factors, the electroless Cu deposition was accelerated markedly by an addition SPS only; but it was suppressed sharply by a combination addition SPS and PEG-4000; the large molecular weight and lower diffusion coefficient of PEG-4000 resulted in concentration gradient of PEG-4000 in the trenches. The effects of PEG-4000 and SPS on the polarization behaviors of the electroless copper plating bath were investigated by the linear sweep voltammetry method and mixed potential theory.