The adsorption and desorption of bis-(3-sulfopropyl) disulfide (SPS) on Cu and Au electrodes and its electrochemical effect on Cu deposition and dissolution were examined using cyclic voltammetry stripping (CVS), field-emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS). SPS dissociates into 3-mercapto-1-propanesulfonate when it is contacted with Au and Cu electrodes, producing Cu(I)- and Au(I)-thiolate species. These thiolates couple with chloride ions and promote not only the reduction of Cu(2+) in Cu deposition but also the oxidation of Cu(0) to Cu(+) in Cu stripping. During Cu electrodeposition on the SPS-modified Au electrode, thiolates transfer from Au onto the Cu underpotential deposition (UPD) layer. The Cu UPD layer stabilizes a large part of the transferred thiolates which subsequently is buried by the Cu overpotential deposition (OPD) layer. The buried thiolates reappear on the Au electrode after the copper deposit is electrochemically stripped off. A much smaller part of thiolates transfers to the top of the Cu OPD layer. In contrast, when SPS preadsorbs on a Cu-coated Au electrode, almost all of the adsorbed SPS leaves the Cu surface during Cu electrochemical stripping and does not return to the uncovered Au surface. A reaction mechanism is proposed to explain these results.

An accelerator is an indispensable organic additive for the bottom-up filling of copper electroplating in nano- or micro-scale features. However, its effective concentration is too low to be easily determined and controlled. Herein, a new electrochemical analysis method based on self-assembly monolayers of thiol molecules on a gold electrode was developed to accurately determine a trace amount of accelerator. The accelerator employed in copper plating solutions is bis-(3-sulfopropyl) disulfide (SPS), which is the most common accelerator for the filling of vias and trenches of interconnects. The SPS concentration in copper plating solutions ranged from 0.3 to 9.0 ppm. Following selective chemisorption of SPS onto the gold electrode, the SPS-modified gold electrode was transferred into a specific electrolyte composed of CuSO4, H2SO4, polyethylene glycol and chloride ions to run cyclic voltammetry (CV) for copper deposition and stripping. A specific peak current of copper reduction formed in the CV, and its peak area depended on the SPS concentration in the copper plating solution. A good linear calibration line was obtained by using this electrochemical analysis method, which can determine a trace amount of SPS in a concentration range of 0.3-1.0 ppm, which is a significant challenge for traditional instruments

Self-assembled monolayers (SAMs) of 3,3′-thiobis(1-propanesulfonic acid, sodium salt) (TBPS) on Au(111) electrodes have been characterized by scanning tunneling microscopy and cyclic voltammetry in aqueous perchloric acid solutions. TBPS exhibits an adsorption behavior typically observed for dialkyl sulfides including intact adsorption and low coverage phases with molecules predominantly lying flat on the surface. On the other hand, an untypical chemical bond and well-ordered domains were determined which resemble the characteristics of alkenethiol SAMs. When the adlayer was prepared at its open circuit potential (OCP), a (6 × 3√3) TBPS adlayer phase was observed at potentials E > 0.7 VRHE in TBPS-free electrolyte. At more cathodic potentials, the adlayer transforms irreversible to a disordered phase. In contrast, in situ STM studies in TBPS-containing electrolyte reveal a very complex, potential-dependent adsorption behavior. With increasing electrode potential, the structure of the adlayer transforms in sequence from a disordered phase σ, to a low coverage stripe phase α, to a high coverage stripe phase β, and finally to a disordered aggregate phase σa. The reverse cathodic sweep shows transitions from σa to β, back to σa, and to an ordered adlayer phase γ. All of these phases significantly differ from the (6 × 3√3) phase and are not transient phases at OCP. This behavior is attributed to the influence of the electrode potential on intermolecular and molecule−substrate interactions as well as on the TBPS coverage. Furthermore, the cathodic deposition of Au−TBPS complexes results in the formation of Au islands with fractal morphology.

The impact of 3,3'-thiobis(1-propanesulfonic acid, sodium salt) (TBPS) on Cu/Au(111) electrodeposition has been investigated by electrochemical methods and scanning tunneling microscopy (STM). Cyclic voltammetry and galvanostatic experiments indicate that Cu growth on Au(111) - which is known to be strongly kinetically hindered in additive-free, aqueous perchloric acid solutions - proceeds significantly faster in the presence of TBPS. The TBPS molecules either "float" on top of the growing film or become incorporated into the deposit. Complementary in situ STM studies show that Cu underpotential deposition (UPD) proceeds via two distinct mechanisms. One-dimensional growth of Cu stripes was observed between 0.05 and 0.35 V-RHE for TBPS-modified Au(111) electrodes. Each stripe is composed of two or three parallel rows of Cu atoms oriented along the main crystallographic directions of the Au(111) substrate. An increase of the TBPS concentration near the solid/liquid interface restricts the Cu stripe growth to a narrow potential regime between 0.3 and 0.35 V-RHE and two-dimensional Cu island growth becomes the favored growth mechanism. The latter fully dominates in TBPS-containing electrolyte. Cu growth in the overpotential deposition (OPD) regime results in a smooth Cu film with low surface roughness, in contrast to defect-mediated 3D island growth in additive-free electrolytes. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.057202jes] All rights reserved

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