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The electrodeposition of copper–tin alloy thin films was studied as a potential application for on-chip interconnection, where Cu–Sn alloys may be less susceptible to failure caused by electro- and stress-migration than pure copper. Alloys with variable Sn content were plated from acidic copper sulfate electrolytes by polarizing copper deposition into the region where Sn deposition became possible. The effective polarization was demonstrated by means of several halogen–polyether pairs, with Br–EPE (poly(ethylene glycol)- block -poly(propylene glycol)- block -poly(ethylene glycol)) pair exhibiting the strongest polarization. Alloy content of Sn between 0 and 7 at% was obtained above the reduction potential of Sn 2+ , which can be attributed to UPD mechanism. Higher Sn content of up to 20 at% was achieved when Cu deposition was suppressed below the reduction potential of Sn 2+ by the combination of Br − and EPE. A positive correlation between Sn content and concentration of Sn 2+ in the electrolyte was observed. At low rotation speeds of a disk electrode ( i.e. 25 rpm versus 100 rpm) the tin content in the alloy was higher, possibly due to stronger suppression of copper deposition at low rotation speeds. The relationship between Sn content and the applied current density was specific to the employed additive chemistry.

A chromatographic method for the detection of bis-(3-sulfopropyl) disulfide (SPS), a common additive in acidic copper plating baths, and its breakdown products is demonstrated. The detection scheme involves a combination of solid-phase extraction for sample pre-treatment, C(18) reversed-phase high-performance liquid chromatography column for separation, and electrochemical sensor for detection of all non-fully oxidized sulfur-containing compounds. We were able to achieve an effective separation and accurately assign chromatographic peaks to all detectable species. Owing to a high sensitivity of the utilized electrochemical detector, detection in low parts per billion range was possible. This can prove crucial for plating bath control, since minute amounts of certain by-products significantly affect the bath performance.

The impact of the ferric/ferrous (Fe 3+ /Fe 2+ ) redox couple on the behavior of polyethylene glycol (PEG) and bis-(3-sulfopropyl) disulfide (SPS) during copper electrodeposition is investigated. The Fe 3+ /Fe 2+ couple is used in plating of copper onto printed circuit boards and may have advantages when implemented for on-chip metallization of copper. Experiments show that in the presence of Fe 3+ /Fe 2+ the suppressing behavior of PEG does not change; in contrast, the accelerating activity of SPS increases when Fe 3+ /Fe 2+ is present in a copper-plating bath. Furthermore, potentiostatic experiments suggest that SPS interacts with Fe 2+ ions in the bulk electrolyte, probably to produce 3-mercapto-1-propane sulfonic acid (MPS) at low ppb levels.

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