J. Cent. South Univ. (2013) 20: 2096−2102 DOI: 10.1007/s11771-013-1712-7

Influence of alkylpyridinium ionic liquids on copper electrodeposition from acidic sulfate electrolyte ZHANG Qi-bo(张启波), HUA Yi-xin(华一新), REN Yan-xu(任艳旭), CHEN Li-yuan(陈立源) Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China © Central South University Press and Springer-Verlag Berlin Heidelberg 2013 Abstract: The effect of two alkylpyridinium ionic liquids (py-iLs) including N-butylpyridinium hydrogen sulfate (BpyHSO4) and N-hexylpyridinium hydrogen sulfate (HpyHSO4) on the kinetics of copper electrodeposition from acidic sulfate solution was investigated by cyclic voltammetry and potentiodynamic polarization measurements. Results from cyclic voltammetry indicate that these py-iLs have a pronounced inhibiting effect on Cu2+ electroreduction and there exists a typical nucleation and growth process. Kinetic parameters such as Tafel slope, transfer coefficient and exchange current density obtained from Tafel plots, lead to the conclusion that py-iLs inhibit the charge transfer by slightly changing the copper electrodeposition mechanism through their adsorption on the cathodic surface. In addition, scanning electron microscope (SEM) and X-ray diffraction analyses reveal that the presence of these additives leads to more leveled and fine-grained cathodic deposits without changing the crystal structure of the electrodeposited copper but strongly affects the crystallographic orientation by significantly inhibiting the growth of (111), (200) and (311) planes. Key words: copper electrodeposition; additives; ionic liquids; electrokinetic parameter; adsorption

1 Introduction Electrochemically deposited copper is an important material in the field of nanotechnology, especially for the microfabrication of integrated circuit interconnects due to its high electrical conductivity and excellent electrical migration resistance [1]. In general, in the copper deposition industry, appropriate amounts of certain additives are needed to produce the deposits required. Additives, such as thiourea [2−9], animal glue [9] and gelatine [10−11] are widely used as levelling and brightening agents in copper electrodeposition and electrowinning since they can produce smooth and bright copper deposits, free of voids or porosity. Although advances have been made, in many cases, the use of these additives is still in an empirical way, and there are still many unknown aspects concerning the mechanism of action of additives. For certain conditions, the research for better additives as substitutes for thiourea, the most commonly used addition agent, is continuing, since it has been found that thiourea decomposes and leads to contamination of the cathodic deposit with sulfur [9, 12].

VEREECKEN and WINAND [13] studied the influence of polyacrylamides on the quality of copper deposits from acidic copper sulfate solutions. They found that this compound played its role by the mass transport through forming a viscous film near to the electrode. MURESAN et al [9] reported that horse-chestnut extract (HCE) could increase the polarization of the cathode, leading to an inhibition of copper electrodeposition and hydrogen evolution. BONOU et al [14] showed that polyethylene glycol influenced the electrode kinetics by competing with cuprous ions for the active sites for charge transfer through the adsorption of a monolayer molecule on the cathode. VARVARA et al [12, 15] found that triethyl-benzyl-ammonium chloride (TEBA) and hydroxyethylated-2-butyne-1,4-diol (Ferasine) were efficient levelling agents in copper electrodeposition from acidic sulfate electrolytes. They demonstrated that TEBA acted as an inhibitor of copper electrodeposition process, as a consequence of its adsorption on the electrode surface without changing the reaction pathway, while Ferasine moderately hindered the mass transfer of the copper ions from the bulk solution to the outer limit of the electrode double layer and slightly changed the copper electrodeposition mechanism via its

Foundation item: Projects(51204080, 51274108) supported by the National Natural Science Foundation of China; Project(2011FA009) supported by the Natural Science Foundation of Yunnan Province, China; Project(2011FZ020) supported by the Application Research Foundation of Yunnan Province, China Received date: 2012−05−21; Accepted date: 2012−11−10 Corresponding author: ZHANG Qi-bo, Associate Professor, PhD; Tel: +86−871−5162008; E-mail: [email protected]

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adsorption on the electrode surface. ABDEL-RAHMAN et al [16] found that aromatic and aliphatic amines exhibited a strong influence on the microstructure of the copper deposit without changing the diffusion controlled mechanism of the deposition process. Similar results were also observed by VĂDUVA et al [17]. Moreover, when different mixtures of additives such as polyethylene glycol and chloride ions [14, 18−22], thiourea and gelatine [23], ethoxyacetic alcohol and triethyl-benzyl-ammonium chloride (IT-85) [24], are used, synergetic effects may appear, leading to more leveled and fine-grained cathodic deposits in comparison to the case when each component of the mixture is used alone. In our previous study, 1-butyl-3-methylimidazolium hydrogen sulfate-[BMIM]HSO4, which has been successfully used as leveling agent in zinc electrowinning from acidic sulfate electrolyte [25], was found to be an efficient leveling agent in copper electrodeposition process [26]. It was found that [BMIM]HSO4 increased the cathodic polarization of copper through its adsorption on the cathodic surface and inhibited the kinetics of the Cu2+ reduction process. Recently, pyridinium based ionic liquids were found to be readily adsorbed on the metallic surface similar to that of imidazolium based ionic liquids [27], which may constitute a larger potential group to be researched as novel metal electrodeposition additives. Therefore, it is interesting to investigate their effect on copper electrodeposition from sulfate electrolytes. The goal of the present work is to study the effects of two alkylpyridinium ionic liquids (py-iLs) including N-butylpyridinium hydrogen sulfate (BpyHSO4) and N-hexylpyridinium hydrogen sulfate (HpyHSO4) on the morphology of cathodic deposits and to evaluate the kinetic parameters of the cathodic process. Cyclic voltammetry and potentiodynamic polarization curves were used to characterize the effects of these py-iLs on cathodic polarization and to estimate the kinetic parameters for the copper electrodeposition process. Scanning electron microscopy (SEM) and X-ray diffraction analysis were used to investigate the morphology and the structure of the copper deposits, respectively.

2 Experimental 2.1 Reagents The electrolyte was prepared by using sulfuric acid (analytical grade), commercial hydrated copper sulfate and distilled water. An electrolyte with 50 g/L Cu and 150 g/L H2SO4 was used as a standard electrolyte. The two py-iLs additives including BpyHSO4 and HpyHSO4 were purchased from Lanzhou Greenchem ILS, LICP.

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CAS, China. The highest available quality (purity>99%) and their general structures are shown in Table 1. They have almost the same chemical structure, and the main difference is the carbon chain length of the alkyl connecting with N in pyridinium ring of these compounds. Table 1 Name and molecular structures of alkylpyridinium ionic liquids Name (abbreviation)

Molecular structure

Relative molecule mass

N-butylpyridinium hydrogen sulfate (BpyHSO4)

235

N-hexylpyridinium hydrogen sulfate (HpyHSO4)

263

2.2 Electrochemical measurements Electrochemical studies were based on the analysis of cyclic voltammetry and potentiodynamic polarization measurements. All electrochemical measurements were conducted by using a GAMRY PCl4/300 electrochemical work station and carried out in a conventional three electrodes electrochemical cell at 298 K under atmospheric condition with a platinum wire (1 mm in diameter, and 10 mm in length), counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Before each experiment, the working electrode copper disk (4 mm in diameter) was polished with 1 200 grit silicon carbide paper and 0.5 μm high-purity alumina, rinsed twice with distilled water and finally dried. Cyclic voltametric experiments were carried out at a constant scan rate of 10 mV/s from the open circuit potential of about 0.05 V to the final potential of −0.50 V. The potentiodynamic polarization test was scanned from open circuit potential of about 0.05 V to −0.175 V with a constant scan rate of 5 mV/s. 2.3 Electrolysis Small-scale galvanostatic electrolysis experiment was performed in a 250 cm3 plexiglass cell by chronopotentiometric measurements. A pure copper sheet (>99.95%) and two parallel lead-silver-calciumstrontium alloy (Ag 0.2%, Ca and Sr 0.1%−0.13%) plates with 5 cm2 were used as the cathode and anode, respectively. The interelectrode distance was 2.5 cm. Copper was deposited on both sides of the cathode onto a total area of 4.5 cm2. All the electrodeposition experiments were carried out for 2 h at 298 K and the current density was held at 250 A/m2. 2.4 Deposit examination The surface morphology of the copper deposits was

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examined by scanning electron microscopy (SEM) using a Hitachi S-4800 microscope. The preferential orientation of the crystals was determined by using a Rigaku D/max 2200 X-ray diffractometer and comparing with the standard copper ASTM data.

3 Results and discussion 3.1 Cyclic voltammetry The cyclic voltammograms recorded for copper electrodeposition in the absence and in the presence of different concentrations of py-iLs are depicted in Fig. 1. The voltammograms were started from point ‘A’ (0.05 V vs SCE), scanned in the negative direction down to a potential of −0.50 V versus SCE and reversed in the positive direction. The rapid increase in current observed at point ‘B’, was assigned to the electroreduction of Cu2+ at the cathode. From that, the cathodic current increased sharply to point ‘C’, due to the copper crystallization, followed by a current plateau of about 72 mA/cm2 at point ‘D’, indicating a mass transfer control of the process [14]. Then, the scan was reversed and completed by returning to point ‘A’. By reversing the sweep, a current loop typical of an overpotential-driven nucleation/growth electrodeposition process is seen, which is characteristic of a mechanism of nucleation [28]. This nucleation loop indicates that the electrodeposition of copper on the electrode requires an overpotential in order to initiate the nucleation and subsequent growth of the copper deposit. In the presence of py-iLs, the shape of the voltammogram obtained is slightly modified with regards to the case of additive-free solution. A strong inhibiting effect is noticed because the additives promote a progressive shift of the deposition potential to more negative value. With each addition of 50 mg/L BpyHSO4 and HpyHSO4, the deposition potentials negatively move by 92 and 104 mV, respectively, in comparison to additive-free solution. This inhibiting effect is more pronounced at higher additive concentrations and could be attributed to the surface coverage of the cathode by a strongly adsorbed additive layer, which increases the viscosity at the metal-electrolyte interface and decreases the mass transfer. At higher additive concentration, with the enhancement of the blocking effect on the active sites, a higher driving force for the cathodic electrodeposition is required. The extent of absorption appears to be in the order of BpyHSO4
Fig. 1 Cyclic voltammograms for copper electrodeposition in absence and presence of different concentrations of py-iLs: (a) BpyHSO4; (b) HpyHSO4

complex formation in solution between py-iLs and the copper ions species [14]. 3.2 Polarization studies The polarization curves obtained from solution without and with different amounts of py-iLs are shown in Fig. 2. The kinetic parameters, including Tafel slope, b, cathodic transfer coefficient αc and exchange current densities, J0, for the electrodeposition process, were calculated from their respective cathodic linear sweep polarization curves. The results are given in Table 2, as a function of additive concentration. It is clear that the addition of py-iLs markedly increases the electroreduction potential of Cu2+ ion, denoting an inhibition effect on the kinetics of the copper electrocrystallization process, which is reflected by the decrease of J0, and may be attributed to the electrostatic adsorbate altering the double-layer structure, thereby lowering the rate of electron transfer reaction. In addition, it is found that the cathodic Tafel slopes vary from 122 to 138 mV/(10 a) and the corresponding charge transfer coefficient varies between 0.43 and 0.48. The changes in the cathodic Tafel slope and αc values observed in the presence of additives suggest that the presence of

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additives slightly changes the copper electrodeposition reaction pathway. A possible mechanism of action of these additives may be explained as follows.

Fig. 2 Effect of py-iLs on cathodic polarization for copper electrodeposition with different concentrations: (a) BpyHSO4; (b) HpyHSO4 Table 2 Effects of py-iLs on kinetic parameters during copper electrodeposition from acidic sulphate solution Additives

b/ c/ (mg·L−1) [mV·(10 a)−1]

αc

J0/ (10−2 mA·cm−2)

122

0.48

48.3

10

123

0.48

18.9

25

127

0.47

15.2

50

136

0.43

11.4

100

133

0.44

5.98

10

134

0.44

13.1

25

133

0.44

8.97

50

132

0.45

5.80

100

138

0.43

4.36

Blank

BpyHSO4

HpyHSO

In sulfuric acid, electrochemical deposition of copper takes place via a two-step mechanism [30]:

Cu

2 +



 e  Cu



(1)



Cu  e  Cu (2) It was proposed that the first step in this process occurred slowly and the rate was controlled by Cu+ existing in equilibrium with Cu at the cathode surface. The pyridinium based compounds contain an aromatic ring, which contains π electrons and a nitrogen atom with a free electron pair available to be donated [27]. Therefore, not only can the π electron of the pyridinium based compounds enter unoccupied orbitals of copper, but also the π* orbital can also accept the electrons of d orbitals of copper to form feed back bonds, then produce more than one center of adsorption action [31]. In this way, pyridinium based compounds can adsorb on the metal surface effectively, which is similar to these imidazolium based compounds [26]. At open circuit, py-iLs adsorb at the cathodic surface. When a current is passed, cupric ions first reduce into cuprous ions (Eq. (1)). And then the cuprous ions form a copper-(py-iLs) complex with the adsorbed additive molecules (Eq. (3)) by the equilibria:  Cu +  py-iLs  [Cu  (py-iLs)]ads

(3)

 [Cu  (py-iLs)]ads  e   Cu  py-iLs

(4)

The inhibition effect of py-iLs on the copper electroreduction process is attributed to the adsorption of the complex at active sites, where it may accept an electron from the cathode and discharge copper atoms which are incorporated at the active sites (Eq. (4)). The py-iLs will be released and can then form a complex, however, this process requires high energy and large overpotentials. In particular, this effect is more pronounced with the increase in the carbon chain length of the alkyl connecting with N of pyridinium ring due to their electron-releasing ability [32]. Therefore, compound HpyHSO4 appears stronger adsorbability and complexation, resulting in a higher inhibition effect than that of BpyHSO4. 3.3 Deposit morphology and orientation The copper deposits obtained by small-scale electrolysis from sulfate electrolyte in the absence and presence of py-iLs were examined using scanning electron microscopy and X-ray diffraction to determine the surface morphology and crystallographic orientations. Typical SEM photomicrographs are shown in Fig. 3 and the X-ray diffraction patterns of copper deposits are presented in Fig. 4 and the results are given in Table 3. The investigated additives show marked effect on the surface quality of the copper deposits as compared with that obtained from solution without additive. The copper deposited from additive-free solution is bright but not smooth and consists of relatively large, coarse grains

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Fig. 3 Scanning electron micrographs of copper deposits in absence and presence of different concentrations of py-iLs: (a) Blank; (b) 10 mg/L BpyHSO4; (c) 10 mg/L HpyHSO4; (d) 50 mg/L BpyHSO4; (e) 50 mg/L HpyHSO4

Fig. 4 XRD patterns for copper deposits in absence and presence of different concentrations of py-iLs: (a) BpyHSO4; (b) HpyHSO4 Table 3 Crystallographic orientations of copper deposits in absence and in presence of different concentrations of py-iLs Additives

Concentration/(mg·L−1)

Blank

0 10 50 100 10 50 100

BpyHSO4

HpyHSO4

Peak intensity ratio, (I/Imax)/% (111)

(200)

(220)

(311)

(222)

53 23 12 2 19 8 4

32 14 9 3 9 6 3

100 100 100 100 100 100 100

13 13 8 2 5 5 4

4 2 — — — — —

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(Fig. 3(a)) with (111), (200), (220), (311) and (222) crystal orientations. Introducing the additives decreases the size of the platelets (Figs. 3(b)−(e)) and significantly inhibits the growth in the direction of (111), (200) and (311) planes. This change in crystal plane may be attributed to the adsorption of the additives on these planes during the crystal growth process and this effect is more obvious at higher concentrations (Fig. 4). The results indicate that an inhibition of the electrocrystallization process takes place. The action of py-iLs is one of inhibition of the crystal growth process, so that a relative enhancement of the nucleation process is induced [2]. This results in a fine-grained deposit. It is also noteworthy that the copper deposits morphology remains essentially unchanged with (220) as the most preferred orientation, irrespective of the nature of the additives.

4 Conclusions 1) Both BpyHSO4 and HpyHSO4 are found to be efficient leveling additives in copper electrodeposition, leading to more leveled and fine-grained cathodic deposits. 2) The electrodeposition of copper is associated with a nucleation and growth process for both cases. The addition of py-iLs is found to have a blocking effect on the electrodeposition of copper, leads to an inhibition of the nuclei growth process and induces a relative enhancement of the nucleation process. 3) Both additives increase the cathodic polarization of copper through their adsorption on the cathodic surface and inhibit the kinetics of the Cu2+ reduction process. HpyHSO4 provides a higher inhibition effect than BpyHSO4, which may be attributed to the stronger adsorbability and complexation of HpyHSO4 in comparison to BpyHSO4. 4) The decrease in the cathodic transfer coefficient proves that the copper electrodeposition mechanism is slightly modified in the presence of these two py-iLs. 5) The presence of py-iLs does not change the crystal structure of the electrodeposited copper but strongly affects the crystallographic orientation by significantly inhibiting the growth of (111), (200) and (311) planes with (220) plane as the most preferred plane independent of the nature of the additives.

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