Characterization and Purification of Commercial SPS and MPS by Ion Chromatography and Mass Spectrometry

SPS (bis-(3-sulfopropyl) disulfide) is an essential electrolyte additive used in the fabrication of copper interconnects by electrodeposition. 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(I) 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 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, mass fraction). IC fractionation was used to purify and isolate SPS for surface and electroanalytical studies. Stability of SPS, MPS, and PDS in the presence of and Cu(II) was also examined. No degradation of SPS or PDS in aqueous solution was observed over a period. Solutions of MPS were metastable to saturation, but the addition of Cu(II) resulted in formation of SPS by dimerization as well as parasitic PDS generation.