In Situ STM Study of Cu Electrodeposition on TBPS-Modified Au(111) Electrodes

Abstract

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

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.…”