Scanning Tunneling Microscopy and Cyclic Voltammetry Study of Self-Assembled 3,3′-Thiobis(1-propanesulfonic acid, sodium salt) Monolayers on Au(111) Electrodes

Abstract

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.

Accelerator Screening by Cyclic Voltammetry for Microvia Filling by Copper Electroplating

“…21,25,38,[57][58][59][60] STM studies have confirmed that the thioether group (i.e., the C-S−C bond) of TBPS does not chemically cleave but that TBPS still can form a SAM on an Au(111) surface after TBPS is adsorbed onto the Au(111) surface, even facilitating copper deposition on the Au(111) surface. 45,46 This result implies that the thioether group of TBPS can chemically adsorb onto gold and copper surfaces, similar to the thiol group of MPS and the disulfide group of SPS; however, it also implies that the adsorptive bonding strength between the thioether group and copper is weaker than the adsorptive bonding strength of Cu-thiolate. Therefore, its accelerating effect on copper electrodeposition is weaker than the effects of SPS and MPS, as confirmed by the CV analysis shown in Fig.…”

“…31,38 TBPS is a new additive that is able to accelerate copper electrodeposition. 45,46 However, an effective plating formula for copper superfilling using TBPS as an accelerator has not yet been developed. Figure 2 shows the CV curves of these organosulfide-modified Au electrodes in an electrolyte containing 0.88 M CuSO 4 , 0.54 M H 2 SO 4 , 200 ppm PEG, and 70 ppm Cl − .…”

“…Figure 4a shows that not only MPS and SPS but also TBPS can chemically adsorb onto a copper surface (i.e., a cathode) via the sulfur moiety. 45,46 The sulfonic end group can trap Cu 2+ ions due to electrostatic attraction, thereby causing these Cu 2+ ions to be dehydrated. 34,53 When chloride ions are simultaneously present on the cathodic surface, the sulfonatetrapped Cu 2+ ions can be transferred to the chloride ions, as illustrated in Fig.…”

“…45,46 However, both of these compounds contain a sulfonic acid group but no a disulfide or thiol group. Therefore, the identification of the molecular structure and functional groups is necessary for understanding the accelerating mechanism of copper deposition.…”

“…The basic concept of the method is based on a self-assembly monolayer (SAM) of disulfide or thiol molecules on an Au electrode. 25,38,39,[45][46][47][48][49][50][51] If an organic additive can interact with chloride ions to accelerate copper nucleation and deposition, then the additive can adsorb onto a Cu or Au surface and can float on the copper deposit during electroplating. 31,38,39 The floatability of the organic additive determines its accelerating ability.…”