Electrochemical Polishing of Copper for Microelectronic Applications

Abstract

Electrochemical polishing (ECP) of copper using hydroxyethylidenediphosphonic acid (HEDP) has been investigated as an alternative to chemical mechanical polishing (CMP) for integration of low-k dialectrics in microelectronic devices. An optimal copper ECP process produces smooth polishing and a fast removal rate. For this investigation copper anodic polarisation curves in HEDP-phosphoric acid solutions were measured and copper ECP effects in various HEDP-phosphoric acid solutions were examined. Copper surface smoothing effects in various HEDP-phosphoric acid solutions were evaluated with atomic force microscopy. It was shown that HEDP and HEDP-phosphoric acid solutions have polarisation curves with a limited current plateau characteristic of ECP. Twenty to forty per cent of HEDP solutions had a maximum limiting current density of ~80 A cm^-2. Adding phosphoric acid to HEDP solutions increased the limiting current to a maximum, beyond which further addition caused a decrease. An optimal ECP effect (removal rate = 1.8 μm min^-1, 80% reduction in mean roughness) was achieved with a solution of 30HEDP + 30H₃PO₄ + 40H₂O (vol.-%). A higher removal rate was achieved by decreasing the concentration of phosphoric acid, but this resulted in a reduction of the smoothing effect.