Brightener breakdown at the insoluble anode by active chlorine species during Cu electrodeposition

https://doi.org/10.1016/j.jiec.2021.10.027Get rights and content

Highlights

  • Brightener breakdown at insoluble anode by active chlorine species is examined.

  • Active chlorine species formed from trace Cl ions accelerate brightener oxidation.

  • Sacrificial perfluorinated polymer coating on anode lowers brightener consumption.

  • Suppression of Cl2 evolution at the anode could reduce brightener consumption.

Abstract

It is highly important to design and develop appropriate insoluble anodes for industrial Cu electroplating to lower the amount of organic additives that this process consumes. Conventionally, this rapid consumption of additives (e.g., brighteners, suppressors, or levelers) is known to be due to the active radical species (e.g., radical dotOH) formed during oxygen evolution. In this study, we found that trace amounts of chloride ions present in the electroplating bath are the source of the active chlorine species that accelerate the breakdown at the insoluble anode. A sacrificial, perfluorinated polymer coating effectively decreased the emission of active chlorine species from the electrode, thereby lowering the consumption of the brightener. This study reveals that the suppression of chlorine evolution at the anode can be an effective approach for decreasing brightener consumption during Cu electrodeposition.

Introduction

Organic additives, which are commonly categorized as brighteners, suppressors, or levelers according to their function, play crucial roles in the electrodeposition of Cu. Their competitive adsorption onto the substrate surface controls the local reduction rate of Cu2+, enabling void-free filling of patterned substrates in the metallization of various electronic devices [1], [2], [3], [4]. In addition, their incorporation into the deposited film modifies the grain boundary structure, thus affecting the electrical and mechanical properties of the film [5], [6], [7]. Representative additives include bis-(sulfopropyl)-disulfide (SPS) used as a brightener, polyethylene glycol (PEG) used as a suppressor, and Janus green b (JGB) used as a leveler. Recently, bromide ion, diketopyrrolopyrrole-based species, and quaternary ammonium surfactants were also suggested as leveler, which enabled high-speed filling of vias [8], [9], [10] and conformal deposition [11], [12], [13].

Most organic additives are unstable and are gradually oxidized during electrodeposition. It has been recognized that the breakdown of the additives is generally initiated by reactive chemical species formed during plating or direct electron transfer at the anode. For instance, it has been reported that the radical dotOH radical formed by a Fenton-like reaction between Cu+ and O2 attacks the –S–S– linkage of the brightener and the –C–O– link of the suppressor, thereby causing bond dissociation [14], [15]. As a consequence, the active additives were degraded, and the corresponding breakdown products (3-mercapto-1-propane sulfonate (MPS), 1,3-propane disulfonate (PDS), and low-molecular-weight PEG, respectively) gradually accumulated in the bath. These breakdown products usually have a negative impact on the deposited metal films, thereby resulting in various negative effects, such as voids in the pattern [16] and at the solder joint [17], inconsistent or poor filling in printed circuit board [18], [19], [20], and increase in the resistivity of the deposited film [21].

While Cu2+ ions are reduced on the cathode surface, the coupled oxidation reaction occurs at the anode. Conventionally, anodes are categorized into soluble and insoluble types depending on their electrochemical reactions during Cu electrodeposition. For a soluble anode (either Cu or phosphorized Cu), the dissolution of Cu0 as Cu2+ occurs as shown in Eq. (1).Cu0Cu2++2e-

The drawbacks of the soluble electrodes include their long-term maintenance [22], difficulties in positioning at a large scale, horizontal-type plants [23], and rapid formation of Cu+ ions, resulting in bath degradation [24], [25], [26]. To overcome these issues, insoluble anodes such as IrO2/Ti, RuO2-IrO2/Ti, and IrO2-Ta2O5/Ti have been widely adopted. The dominant anodic reaction with these electrodes is the oxygen evolution owing to water splitting, as shown in Eq. (2).H2O2H++1/2O2+2e-

Despite several advantages, rapid additive consumption in the case of an insoluble electrode in comparison with that observed with a soluble-type anode has been identified as a prominent drawback. According to previous reports, the additive consumption is associated with the catalytic activity of the insoluble electrode in the direct oxidation of the additive and/or formation of active radicals (e.g., radical dotOH) during the oxygen evolution reaction (OER). In this perspective, various methods have been suggested to lower the consumption of additives, which include the modification of the anode to minimize the approach of additive to the electrode surface [27], separation of anode using ion exchange membrane [28], [29] and the addition of organic species into the electrolyte for sacrificial oxidation [30], [31].

Hitherto, most of the previous studies have rarely considered the contribution of the chloride ion to the additive consumption. Chloride ion is an essential component in a sulfate-based Cu bath; it is included in most Cu baths because it facilitates a chemical linkage between the additives and Cu surface, and assists the adsorption of additives. However, electrolysis with an insoluble anode in the presence of chloride ions potentially results in the formation of active chlorine species, as shown in Eqs. (3), (4), (5), (6), (7), (8) [32], [33], [34], [35], [36].2Cl-Cl2+2e-Cl2+H2OHOCl+Cl-+H+HOClOCl-+H+HOCl+OCl-OCl+Cl-+OHOH+OCl-OCl+OH-OCl+OCl-+OH-2Cl-+O2+OH

It has been reported that the active Cl species produced via electrolysis in a NaCl solution degrade organic molecules containing –C–O– or ––S–S– bonds [32], [33], [34], [36], [37], which are crucial functional groups in a suppressor or brightener. Furthermore, Cu2+ ions, which are abundant in the copper plating solution, could accelerate the breakdown of organic molecules during electrolysis in an aqueous halide medium [34]. Although the Cl concentration in the Cu plating bath (<100 ppm) is significantly low, it is worth examining the degree to which the Cl ions influence the breakdown of organic additives at the insoluble anode.

Therefore, in this study, we examined the effects of the chloride ion on the additive breakdown at the insoluble anode during Cu electrodeposition. Specifically, after the electrolysis process in a membrane cell, we monitored the concentrations of the additives as well as the amount of active chlorine species near the anode. The formation of active species in the solution and their influence on the additive breakdown on IrO2-Ta2O5/Ti and perfluorinated polymer-coated IrO2-Ta2O5/Ti anodes were examined comparatively to clarify the influence of these ions.

Section snippets

Materials

CuSO4·5H2O (0.8 M, 99.5%, GR, FUJIFILM Wako Pure Chemical Corporation), H2SO4 (1.0 M, 95.0%, Samchun Chemicals), and HCl (75 ppm, 35.0–37.0%, EP, Samchun Chemicals) were used to prepared the Cu plating bath. Commercial additives (VF100 suppressor, VF100 brightener, and VF100 leveler from MacDermid Enthone) designed for via fill were also added to the bath. A standard N,N-diethyl-p-phenylenediamine (DPD) free chlorine agent (Hach Company) was used to detect free chlorine.

Analysis

An H-type membrane cell,

Results and discussion

In order to examine the effect of Cl on the additive breakdown at the insoluble anode, electrolysis was carried out using Cl-free and Cl-containing electrolytes in an H-cell (Fig. 2). The brightener concentration in the anolyte was monitored by the MLAT-CVS method. During electrolysis in the Cl-free electrolyte, the brightener was consumed at an apparent rate of 9.02 μL/L-min at the current density of 10 mA/cm2. An increase in the current density led to accelerated brightener breakdown

Conclusions

In this study, we examined the brightener breakdown at the insoluble anode by active chlorine species during Cu electrodeposition. The rate of the brightener breakdown increased significantly in the presence of chloride ions. This is because the active chlorine species formed via the chorine evolution reaction at the insoluble anode chemically oxidized the thiol group of the brightener to sulfonate. The brightener consumption by the active chlorine species occurred instantaneously and was

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Technology Development Program funded by the Ministry of SMEs and Startups (MSS, Korea) [grant number S3027289] and the National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT [grant number 2020M3H4A308176212].

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    These authors contributed equally to this work.

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