SPS (bis-(3-sulfopropyl) disulfide) is an essential electrolyte additive used in the fabrication of copper interconnects by electrodeposition. The process is used over length scales ranging from on-chip Damascene wiring to through-silicon-vias for chip stacking as well as printed circuit board manufacturing. In electroplating baths, the disulfide component of SPS may be cleaved to form the thiol analog, MPS (3-mercaptopropyl sulfonate), by either homogenous interactions with the Cu+ reaction intermediate or by dissociative adsorption onto the copper surface. However, mechanistic studies into the role of these additives in copper electrodeposition are presently constrained by limited knowledge of the purity of commercially-available SPS and MPS. This report details the use of ion chromatography (IC) and electrospray ionization mass spectrometry (MS) to characterize aqueous solutions of commercial SPS and MPS source materials. Sulfate (2.0 %) and propane disulfonic acid (0.9 %) (PDS) were determined to be the principal impurities in SPS (96.3 % estimated purity), while MPS was found to contain approximately 7.0 % SPS and 5.8 % unidentifiable species. Stability of SPS, MPS, and PDS in the presence of O2 and Cu2+ was also examined. No degradation of SPS or PDS in aqueous solution was observed over a three-month period. Solutions of MPS were stable to O2 saturation, but the addition of Cu2+ resulted in rapid and complete degradation of MPS to form PDS and SPS. In practical plating baths, MPS is generated by homogenous reaction of SPS with Cu+ intermediates and is most evident at Cu anodes in conventional, membrane-free electrochemical cells. Subsequent oxidation of the so-formed MPS by Cu2+ leads to either reformation of SPS by dimerization or parasitic PDS generation. IC fractionation was used to purify and isolate SPS for surface and electroanalytical studies.
Citation: International Journal of Electrochemical Science
Pub Type: Journals