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In situ observation of Cu electrodeposition and dissolution behavior on Au(111) by high speed AFM
Electrochimica Acta 302 (2019) 422e427
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
In situ observation of Cu electrodeposition and dissolution behavior on Au(111) by high speed AFM Taiki Yoshioka, Hisayoshi Matsushima*, Mikito Ueda Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Sapporo, Hokkaido, 060-8628, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 September 2018 Received in revised form 30 January 2019 Accepted 10 February 2019 Available online 13 February 2019
We studied the early stages of Cu electro-deposition and -dissolution on Au(111) over a wide overpotential regime by using high speed atomic force microscopy (HS-AFM) with a high temporal resolution. The HS-AFM images revealed an instantaneous nucleation mechanism where epitaxial nuclei were created at a low cathode overpotential. Nuclei with straight edges were caused by the influence of the Au(111) lattice, whereas arched and uneven edges were associated with rapid structural fluctuations of Cu adatoms. For deposits formed at a high overpotential, many fine column-like deposits covered the whole surface plane and the number of Cu nuclei increased with the cathode overpotential. These features were related to the masstransportation. Furthermore, we observed small clusters, a few nm in size, which remained on the substrate after the rapid dissolution process. © 2019 Elsevier Ltd. All rights reserved.
Keywords: High speed AFM Cu Nucleation Deposition Dissolution
1. Introduction The importance of nanotechnology is attracting much attention. Fine Cu wires are required for damascene processing [1], and Cu microfabrication is widely applied in actuators [2]. Plating techniques are used to produce such superfine Cu structures. This technique is simple to use under normal conditions and allows the deposition of metal without any hydrogen bubble generation. However, technologies that allow manipulation of sub-Angstrom areas, equal to the size of individual nucleation events, have yet to be established [3]. Many researchers have investigated Cu nucleation during electrodeposition. Fukunaka et al. examined the relationship between the Cu2þ concentration on the electrode surface and the number of nuclei in microgravity environment [4,5]. In microgravity, where natural convection can be ignored, the number of nucleus sites was lower than that under gravity owing to fluctuation of the surface concentration. Others have reported that the crystal orientation of the substrate is an important factor for changing the nucleation and growth process, because the geometric and electronic properties influence the surface potential [6e10]. Di et al. studied the influence of the electrode potential on nucleation [6]. They confirmed
* Corresponding author. E-mail address: [email protected] (H. Matsushima). https://doi.org/10.1016/j.electacta.2019.02.044 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
that the number of the nucleus sites increased exponentially at an overpotential. Although many factors such as diffusion, discharge, and crystallization processes affect Cu nucleation, systematic and quantitative investigations have yet to be performed [11e14]. Nucleation occurs over a small region and a short time period making it is difficult to discuss the mechanism of this phenomena. Experiments with various ideas have been conducted to address the above problems. Magnussen et al. improved scanning tunnel microscopy (STM) for in situ observations of electrodeposition at a video frame rate of faster than 1 s [15e18]. These findings clearly demonstrated dynamic nucleation on an atomic scale. In an acidic solution containing Cl ions, Cu deposited and grew along the step edge of a Cu single crystal substrate [18]. Radisic et al. captured Cu nucleation and reported the quantitative analysis of the nuclei growth based on transmission electron microscope (TEM) observations [19,20]. However, in the above reports, it was necessary to consider whether the fine nano-sized clusters were affected by electrical factors. We have focused on HS-AFM, which is breakthrough technique for observing the motion of the cell and proteins [21,22]. HS-AFM is highly suited to studying soft and brittle objects because it can minimize electronic influence; however, applications of this technique to electrochemistry have been limited [23]. In a previous report, HS-AFM was applied to electrodeposit Cu on a Au (100) substrate [24]. In that study Cu islands of several atomic layers
T. Yoshioka et al. / Electrochimica Acta 302 (2019) 422e427
formed and spread on the substrate within a few seconds. Recnetly there are several paper reproting Cu deposition on Au(111) surface. Rost et al. reported STM observation of a Cu underpotential deposition (UPD) and a Cu overpotential deposition (OPD) on a Au(111) single crystal surface [25,26]. In addition, Kolb et al. investigated very comprehensively the adsorption of sulfate and phosphate species during Cu electrodeposition on Au(111) by using in situ STM [27,28]. In the present experiment, we performed Cu electro-deposition and -dessolution on an Au (111) substrate at various overpotentials. The effects of both the electrode potential and substrate are discussed in terms of the dynamic behavior observed by HS-AFM. 2. Experimental Experiments were conducted in a three-electrode cell (Fig. 1). The working electrode was an Au(111) single crystal (f 3 mm, MaTeck GmbH), the counter electrode was a platinum wire (f 3 mm, Nilaco Cor.), and the reference electrode was a copper wire (f 3 mm, Nilaco Cor.). The counter and the reference electrodes were ultrasonically cleaned in acetone, washed with ultrapure water (18.2 MU, Merck), and then installed in an electrolytic cell made of PEEK. The working electrode was washed with persulfuric acid solution and then ultrapure water. Before the measurements, the single crystal was annealed with a butane burner for a few seconds and then allowed to cool in the atmosphere. The working electrode was fixed at the base of the AFM scanner and held with the electrode surface facing downward. The reaction area and the electrolyte were contacted through a meniscus. The electrolyte was a mixture of 3 mM copper sulfate solution with 50 mM sulfuric acid. All reagents (Sigma-Aldrich) were ultrahigh pure grade. Experiments were conducted with an electrochemical measurement system (HZ-7000, Hokuto Denko Cor.). A constant potential was applied at room temperature. To avoid Cu deposition before the observation, the initial potential was set to a noble potential against the equilibrium potential of Cu2þ/Cu. The potential was then switched to the deposition potential, by a step potential method. The HS-AFM (Research Institute of Biomolecule Metrology) was used for in situ observations. The HS-AFM operating in a dynamic mode can measure surfaces at frame rates up to 10 frame s1. In the present experiment, the scan area range was in the range of 375e1000 nm2 and the scanning speed was 1 frame s1.
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of 0.025 V s1. The CV measurements were performed in 3 mM CuSO4 solution, with Cu electrodeposition occurring at 0.04 V, as shown in Fig. 2. During the sequence of negative potential scanning, the Cu2þ diffusion was limited to 0.11 V and the cathode current decreased. Since there was no steep increase of the cathodic current, the hydrogen evolution was not confirmed. That is, the Cu growth mode is governed purely by the Cu2þ diffusion process in the present case. Cu was continuously deposited even after reversing the potential scan at 0.30 V. The dissolution commenced at 0.02 V, as confirmed by the large anodic peak. The Cu film was clearly removed at 0.12 V. The substrate surface after annealing had a roughness of 0.6 nm or less [Fig. 3 (a)]. Some straight lines, from the upper right to the lower left direction, appeared as black/white contrast. These features corresponded to the steps of the substrate. When the experiment was started at a low cathode potential (0.08 V), five Cu nuclei (I-V) appeared within 1 s [Fig. 3 (b)]. The individual sizes of these nuclei ranged from 3 to 80 nm and the height was 0.84 nm (i.e., 4 Cu atomic layers). Nucleus IV soon disappeared in the next frame [Fig. 3 (c)], because it was absorbed by larger neighboring nuclei through Ostwald growth or because the nucleus size was too small to remain stable. During deposition, the remaining four nuclei grew and no additional nucleation occurred. This result was consistent with the instantaneous nucleation mechanism. At 3 s after starting the deposition [Fig. 3 (d)], there were differences in the growth manner among individual nucleus. For example, nuclei IeIII grew predominantly in the lateral direction and their size became larger than 100 nm at 10 s [Fig. 3 (e)]; however, nucleus V grew in the perpendicular direction and became the tallest particle with a height of about 20 nm. A characteristic morphology was observed. In the nuclei I-III, the straight end was formed at the lateral deposition process. The shape appeared clearly as the deposition time proceeded. At 10 s, the cross point of the two edge lines was 60 [as indicated in Fig. 3 (e)], suggesting that the nuclei consist of a single crystal and that certain step directions are preferred. The steps of the nuclei parallel to each other indicates that deposit grows epitaxially and aligned to the substrate lattice, but that the Cu is not the hollow sites of the substrate. Once the straight edge-face formed, this part was very stable. The left side of nucleus III was arched shape. Detailed observations of the video images showed very fine projections moving in this part. The diffusing Cu adatoms were actively adsorbing and desorbing.
3. Results and discussion 3.1. Cu deposition The sample was investigated by performing cyclic voltammetry (CV) where the potential was swept from 0.2 to 0.3 V at a scan rate
Fig. 1. Schematic electrochemistry.
illustration
of
HS-AFM
experimental
setup
applied
for
Fig. 2. Voltammogram of Cu electrodeposition and dissolution on Au(111) in 3 mM CuSO4 with 50 mM H2SO4 solution (sweep rate 25 mV/s).
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Fig. 3. Series of HS-AFM images of Cu electrodeposition at 0.08 V after starting experiment at (a) 0 s, (b) 1 s, (c) 2 s, (d) 3 s (e) 10 s, and (f) 20 s (375 500 nm2, 1 Hz image acquisition rate).
To investigate the nuclei in detail, we analyzed the crosssectional profile for the nuclei III at 20 s [dotted line in Fig. 3 (f)]. The deposit size was approximately 150 nm laterally with a height of approximately 9 nm (Fig. 4). The deposit grew epitaxially from left to right to create three bunched layers. The thicknesses of the bottom, middle, and top layers were 5.8 nm (27 Cu atomic layers), 1.8 nm (9 Cu atomic layers), and 1.4 nm (7 Cu atomic layers). There was a gentle slope between the bottom and the middle layer. This result indicates that there were many steps in a staircase pattern within a width of 10 nm. Similar to the results obtained on Au (100) [24], epitaxial growth of the nuclei did not occur as one atomic layer but rather as multiple layers simultaneously. Rost et al. in-situ observed the two-dimensional growth of Cu deposits with STM in detail [25]. There, the epitaxial growth with mono-atomic layer was not confirmed at the initial stage of electrodeposition, but a stable unit called “building block” was found. The size of the minimum unit was about a few nm. That is, unlike Bi electrodeposition where a single adatom is taken into the kink and step edge [15], it is suggested that Cu clusters are attributed to the deposition process.
Fig. 4. Surface height profile of the cross-sectional image of nucleus III in Fig. 3 (f).
We note that the thickness between the middle and top layers enlarged. The white contrast of the left end portion increased for all nuclei in Fig. 3. We attribute this result to active nucleation sites or mechanical factors. That is, because the probe scanned from left to right and hit the edge of the deposits there is a chance that tip damping caused the height to be overestimated. The growth rate in the in plane direction was considered for the case of nucleus III. Supposing the deposit shape is circular, the approximate diameter can be calculated from the projected area. Nuclei with a size of approximately 30 nm were created within 1 s and grew to approximately 75 nm at 3 s [Fig. 5 (A)]. When the nuclei reached a certain size, the growth rate slowed. Fletcher et al. expressed the relationship between the deposition time, t, and the diameter, R, using a hemispherical diffusion model [29],
RðtÞ ¼ ð2DCb Vm tÞ1=2
(1)
where D is the diffusion coefficient, Cb is the bulk concentration of Cu2þ, and Vm is the molar volume. With the transition of time, the growth rate consisted of two regions where both the diameter and time were converted in the exponential values [Fig. 5 (B)]. Here, the slope of the approximate line was defined as the growth index a. Initially, the value of a was 0.54. Because CbVm was much less than 1 in the present conditions, R(t) can be simplified as R(t)ft1/2. The present results suggest that the early growth rate followed the hemisphere diffusion model. After 5 s, the nuclei grew slowly with a ¼ 0.35. The reason for the change in the growth index later in the deposition might be attributed to overlap of the diffusion layer. That is, the individual layers surrounding the respective nuclei thickened and extended over another layer. This caused a shift from a hemispherical to a planner diffusion model [5]. The later growth leads to a Cottrell type behavior when the growth is diffusion limited. It is considered that the amount of charge is proportional to time t1/2. From the Faraday's law, the charge associated with a hemispherical shape of the diffusion layer is proportional to radius R3. Therefore, the time dependence of the radius for 1D diffusion-limited growth is expected to be R(t)ft1/6 [19]. However, the present deviation of the value is considered as the unideal shape of the electrodeposit or the
T. Yoshioka et al. / Electrochimica Acta 302 (2019) 422e427
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Fig. 5. Growth curve of the nucleus III diameter in (A) a linear and (B) a logarithm plots.
interaction between the electrodeposits. Fig. 6 shows HS-AFM images obtained when the electrodeposition was performed in the diffusion-controlled region (0.16 V). Some small nuclei partially appeared as soon as deposition started [Fig. 6 (b)]. Deposits approximately 80 nm in size covered the entire substrate within 2 s [Fig. 6 (c)]. Similar to the measurements at 0.08 V (Fig. 3), the number of nuclei did not increase and the black/white contrast of the images became clearer as the deposition progressed [Fig. 6(c)e(f)]. The nucleus grew predominantly in the lateral direction at 0.08 V, but at 0.16 V growth perpendicular to the substrate dominated. The average height of the nucleus was initially approximately 2 nm and reached approximately 15 nm at 20 s [Fig. 6 (f)]. A comparison of the images between Fig. 6 (c) and (f), showed little change in the shape of the deposits. Fig. 7 shows the cross-section profile of the deposit VI, as shown in Fig. 6 (f). The nuclei were approximately 80 nm in width and 12 nm in height. There was no terrace on the upper part and the shape was convex. Under mass transfer limitations, a thick diffusion layer formed. Therefore, the Cu2þ supply decreased in the vicinity of the substrate as the deposition proceeded. This effect might have contributed to the Cu2þ shortage at the lower part of the deposit.
Fig. 7. Surface height profile of the cross-sectional image of nucleus VI in Fig. 6 (f).
Fig. 6. Series of HS-AFM images of Cu electrodeposition at 0.16 V after starting experiment at (a) 0 s, (b) 1 s, (c) 2 s, (d) 3 s (e) 10 s, and (f) 20 s (750 1000 nm2, 1 Hz image acquisition rate).
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T. Yoshioka et al. / Electrochimica Acta 302 (2019) 422e427
Conversely, the mass supply reached the upper part more easily. Moreover, the electric force line was focused in a convex shape, which likely promoted mass transfer and nucleation. The present dynamic observations showed that the morphological variation shifted to columnar (dendrite) type at the initial stage of deposition. The lateral size of nuclei VI is shown in Fig. 8 (A). Initially, the nucleus size was approximately 30 nm, which was similar to the results shown in Fig. 5. After 3 s of electrodeposition, the deposit size remained at an almost constant value of approximately 60 nm. On a logarithmic scale [Fig. 8 (B)], the relation between the size and time are considered by Eq. (1). The slope a was 0.42 at the initial deposition, when a spherical diffusion layer formed close to the nucleus. However, the deviation of the slope value became less than 0.5 after a few seconds. Because the Cu2þ flux is limited at high cathode potentials, the diffusion layer rapidly thickened. Additionally, overlap of the diffusion layers occurred easily owing to many nucleation sites. These factors might contribute to the disturbance of the growth index. The electrodeposition was investigated in the range of 0.06 to 0.16 V. Because instantaneous nucleation occurred at whole potentials, we counted nucleation sites immediately after starting the electrodeposition. When the number of Cu nuclei was logarithmically represented, the nucleation rate was consisted of two regions (Fig. 9). The reduction reaction was dominated by electron transfer or the combined electron and mass transfer of copper ions, which depended on the overpotential. Milchev et al. reported that Cu nucleation took place through two reactions, I) Cu2þ þ ee / Cuþ and II) Cuþ þ ee / Cu. Since the reaction (I) is fast, the concentration of Cuþ is accumulated near the electrode surface with increasing in the cathode potential [30]. That is, the supersaturation degree of Cuþ rises just before the occurrence of Cu nucleation, which might be contributed to form the number of Cu cluster (or “building block”) at low overpotenital region. In the diffusion control at high cathode potential, as the diffusion layer is thickened to some extent, it is overlapped each other. This would suppress the nucleation rate. 3.2. Cu dissolution The dissolution process was investigated. Immediately after conducting constant potential electrolysis at 0.08 V for 20 s [Fig. 10 (a)], the dissolution process was observed by switching 0.05 V. The set value was close to the onset potential where the oxidation current was detected (Fig. 2), otherwise HS-AFM could not be used to follow the rapid dynamic dissolution. The dissolution
Fig. 9. Dependence of the number of Cu nuclei on the cathode potential.
was much faster than deposition and the oxidation current increased sharply as the anodic potential increased. All the deposits became small when the potential was changed [Fig. 10 (b)]. In the deposition, the edge part of the nucleus consisted of an irregular structure or a line configuration along the substrate orientation. The dissolution rate was independent of the edge type. After 3 s [Fig. 10 (c)], the effects of dissolution became clear and the step lines of the nuclei became uneven. Interestingly, as dissolution progressed beyond 5 s, some clusters [indicated by arrows in Fig. 10 (d)] remained on the substrate briefly. This behavior had the appearance of small water droplets drying on a flat surface. The average size of the clusters was approximately 8 nm. After the large nuclei dissolved, these small clusters gradually dissolved [Fig. 10(e) and (f)]. 4. Conclusions We used HS-AFM to study the initial stage of Cu electrodeposition and dissolution on Au(111) surfaces in sulfuric acid solution. In situ dynamic observations revealed instantaneous nucleation whereby five Cu nuclei, less than 100 nm in size, appeared on Au terraces. The nuclei grew in the lateral direction at low cathode potentials (0.08 V). Epitaxial growth proceeded in one direction and was accompanied by deposition of multiple layers. The nuclei formed column shapes at high cathode potentials (0.16 V) owing to limited Cu2þ mass transfer. This result suggests that the seeds of dendrite growth were caused by Cu2þ shortages in
Fig. 8. Growth curve of the nucleus VI diameter in (A) a linear and (B) a logarithm plots.
T. Yoshioka et al. / Electrochimica Acta 302 (2019) 422e427
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Fig. 10. Series of HS-AFM images of Cu dissolution under potential of 0.05 V after starting experiment at (a) 0 s, (b) 1 s, (c) 3 s, (d) 5 s, (e) 7 s and (f) 10 s (375 500 nm2, 1 Hz image acquisition rate).
the vicinity of the electrode surface. Similar to the results at 0.08 V, the growth exponent, a, decreased from 0.6 to 0.3 as the deposition time elapsed. We attribute this result to variation in the shape of the diffusion layer from hemispherical to planar. Dissolution was much faster than the deposition. The dissolution occurred heterogeneously at all points along the edges of nuclei. Some clusters of a few nm in size remained on the substrate momentarily. Acknowledgment The authors appreciate financial support from the Ministry of Education, Culture, Sports, Science and Technology (Project No. 16K14429, 17H03528). One of the authors, H. Matsushima, gratefully acknowledges financial support from the Iketani Science and Technology Foundation, Futaba Electronics Memorial Foundation and Hitachi Metals Materials Science Foundation in Japan. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.02.044. References [1] T.P. Moffat, M. Walker, P.J. Chen, J.E. Bonevich, W.F. Egelhoff, L. Richter, C. Witt, T. Aaltonen, M. Ritala, M. Leskel€ a, D. Josell, J. Electrochem. Soc. 153 (2006) C37. [2] Z.R. Yi, Y. Lei, X.Y. Zhang, Y.N. Chen, J.J. Guo, G.J. Xu, M.F. Yu, P. Cui, Nanoscale 9 (2017) 12524. [3] D.M. Kolb, R. Ullmann, T. Will, Science 275 (1997) 1097. [4] K. Nishikawa, Y. Fukunaka, E. Chassaing, M. Rosso, Electrochim. Acta 100
(2013) 342. [5] M. Morisue, Y. Fukunaka, K. Kusaka, R. Ishii, K. Kuribayashi, J. Electroanal. Chem. 559 (2003) 155. [6] N. Di, A. Damian, F. Maroun, P. Allongue, J. Electrochem. Soc. 163 (2016) D3062. [7] L. Guo, S.L. Zhang, P. Searson, Phys. Rev. E 79 (2009), 051601. [8] T. Zapryanova, N. Jordanov, A. Milchev, J. Electroanal. Chem. 612 (2008) 47. [9] M. Kleinert, H.F. Waibel, G.E. Engelmann, H. Martin, D.M. Kolb, Electrochim. Acta 46 (2001) 3129. [10] T. Homma, K. Naito, M. Takai, T. Osaka, Y. Yamazaki, T. Namikawa, J. Electrochem. Soc. 138 (1991) 1269. [11] B. Madry, K. Wandelt, M. Nowicki, Electrochim. Acta 217 (2016) 249. [12] Y.I. Yanson, M.J. Rost, Angew. Chem. Int. Ed. 52 (2013) 2454. [13] Y.F. Liu, K. Krug, P.C. Lin, Y.D. Chiu, W.P. Dow, S.L. Yau, Y.L. Lee, J. Electrochem. Soc. 159 (2012) D84. [14] M. Kang, M.E. Gross, A.A. Gewirth, J. Electrochem. Soc. 150 (2003) C292. [15] H. Matsushima, S.W. Lin, S. Morin, O.M. Magnussen, Faraday Discuss 193 (2016) 171. [16] H. Matsushima, A. Taranovskyy, C. Haak, Y. Grunder, O.M. Magnussen, J. Am. Chem. Soc. 131 (2009) 10362. [17] W. Polewska, R.J. Behm, O.M. Magnussen, Electrochim. Acta 48 (2003) 2915. [18] O.M. Magnussen, L. Zitzler, B. Gleich, M.R. Vogt, R.J. Behm, Electrochim. Acta 46 (2003) 3725. [19] A. Radisic, F.M. Ross, P.C. Searson, J. Phys. Chem. B 110 (2006) 7862. [20] A. Radisic, P.M. Vereecken, J.B. Hannon, P.C. Searson, F.M. Ross, Nano Lett. 6 (2006) 238. [21] Y. Ruan, A. Miyagi, X.Y. Wang, M. Chami, O. Boudker, S. Scheuring, Proc. Natl. Acad. Sci. U.S.A. 114 (2017) 1584. [22] T. Ando, Curr. Opin. Struct. Biol. 28 (2014) 63. [23] N. Hoshi, M. Nakamura, C. Yoshida, Y. Yamada, M. Kameyama, Y. Mizumoto, Electrochem. Commun. 72 (2016) 5. [24] T. Yoshioka, H. Matsushima, M. Ueda, Electrochem. Commun. 92 (2018) 29. [25] Y. Yanson, M.J. Rost, Angew. Chem. Int. Ed. 52 (2013) 2454. [26] Y. Yanson, J. Frenken, M.J. Rost, Phys. Chem. Chem. Phys. 13 (2011) 16095. [27] M. Petri, D.M. Kolb, U. Memmert, H. Meyer, Electrochim. Acta 49 (2003) 175. [28] A. Cuesta, M. Kleinert, D.M. Kolb, Phys. Chem. Chem. Phys. 2 (2000) 5684. [29] S. Fletcher, J. Chem. Soc. Faraday. Trans. 1 (79) (1983) 467. [30] A. Milchev, T. Zapryanova, Electrochim. Acta 51 (2006) 2926.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
In situ observation of Cu electrodeposition and dissolution behavior on Au(111) by high speed AFM Taiki Yoshioka, Hisayoshi Matsushima*, Mikito Ueda Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Sapporo, Hokkaido, 060-8628, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 September 2018 Received in revised form 30 January 2019 Accepted 10 February 2019 Available online 13 February 2019
We studied the early stages of Cu electro-deposition and -dissolution on Au(111) over a wide overpotential regime by using high speed atomic force microscopy (HS-AFM) with a high temporal resolution. The HS-AFM images revealed an instantaneous nucleation mechanism where epitaxial nuclei were created at a low cathode overpotential. Nuclei with straight edges were caused by the influence of the Au(111) lattice, whereas arched and uneven edges were associated with rapid structural fluctuations of Cu adatoms. For deposits formed at a high overpotential, many fine column-like deposits covered the whole surface plane and the number of Cu nuclei increased with the cathode overpotential. These features were related to the masstransportation. Furthermore, we observed small clusters, a few nm in size, which remained on the substrate after the rapid dissolution process. © 2019 Elsevier Ltd. All rights reserved.
Keywords: High speed AFM Cu Nucleation Deposition Dissolution
1. Introduction The importance of nanotechnology is attracting much attention. Fine Cu wires are required for damascene processing [1], and Cu microfabrication is widely applied in actuators [2]. Plating techniques are used to produce such superfine Cu structures. This technique is simple to use under normal conditions and allows the deposition of metal without any hydrogen bubble generation. However, technologies that allow manipulation of sub-Angstrom areas, equal to the size of individual nucleation events, have yet to be established [3]. Many researchers have investigated Cu nucleation during electrodeposition. Fukunaka et al. examined the relationship between the Cu2þ concentration on the electrode surface and the number of nuclei in microgravity environment [4,5]. In microgravity, where natural convection can be ignored, the number of nucleus sites was lower than that under gravity owing to fluctuation of the surface concentration. Others have reported that the crystal orientation of the substrate is an important factor for changing the nucleation and growth process, because the geometric and electronic properties influence the surface potential [6e10]. Di et al. studied the influence of the electrode potential on nucleation [6]. They confirmed
* Corresponding author. E-mail address: [email protected] (H. Matsushima). https://doi.org/10.1016/j.electacta.2019.02.044 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
that the number of the nucleus sites increased exponentially at an overpotential. Although many factors such as diffusion, discharge, and crystallization processes affect Cu nucleation, systematic and quantitative investigations have yet to be performed [11e14]. Nucleation occurs over a small region and a short time period making it is difficult to discuss the mechanism of this phenomena. Experiments with various ideas have been conducted to address the above problems. Magnussen et al. improved scanning tunnel microscopy (STM) for in situ observations of electrodeposition at a video frame rate of faster than 1 s [15e18]. These findings clearly demonstrated dynamic nucleation on an atomic scale. In an acidic solution containing Cl ions, Cu deposited and grew along the step edge of a Cu single crystal substrate [18]. Radisic et al. captured Cu nucleation and reported the quantitative analysis of the nuclei growth based on transmission electron microscope (TEM) observations [19,20]. However, in the above reports, it was necessary to consider whether the fine nano-sized clusters were affected by electrical factors. We have focused on HS-AFM, which is breakthrough technique for observing the motion of the cell and proteins [21,22]. HS-AFM is highly suited to studying soft and brittle objects because it can minimize electronic influence; however, applications of this technique to electrochemistry have been limited [23]. In a previous report, HS-AFM was applied to electrodeposit Cu on a Au (100) substrate [24]. In that study Cu islands of several atomic layers
T. Yoshioka et al. / Electrochimica Acta 302 (2019) 422e427
formed and spread on the substrate within a few seconds. Recnetly there are several paper reproting Cu deposition on Au(111) surface. Rost et al. reported STM observation of a Cu underpotential deposition (UPD) and a Cu overpotential deposition (OPD) on a Au(111) single crystal surface [25,26]. In addition, Kolb et al. investigated very comprehensively the adsorption of sulfate and phosphate species during Cu electrodeposition on Au(111) by using in situ STM [27,28]. In the present experiment, we performed Cu electro-deposition and -dessolution on an Au (111) substrate at various overpotentials. The effects of both the electrode potential and substrate are discussed in terms of the dynamic behavior observed by HS-AFM. 2. Experimental Experiments were conducted in a three-electrode cell (Fig. 1). The working electrode was an Au(111) single crystal (f 3 mm, MaTeck GmbH), the counter electrode was a platinum wire (f 3 mm, Nilaco Cor.), and the reference electrode was a copper wire (f 3 mm, Nilaco Cor.). The counter and the reference electrodes were ultrasonically cleaned in acetone, washed with ultrapure water (18.2 MU, Merck), and then installed in an electrolytic cell made of PEEK. The working electrode was washed with persulfuric acid solution and then ultrapure water. Before the measurements, the single crystal was annealed with a butane burner for a few seconds and then allowed to cool in the atmosphere. The working electrode was fixed at the base of the AFM scanner and held with the electrode surface facing downward. The reaction area and the electrolyte were contacted through a meniscus. The electrolyte was a mixture of 3 mM copper sulfate solution with 50 mM sulfuric acid. All reagents (Sigma-Aldrich) were ultrahigh pure grade. Experiments were conducted with an electrochemical measurement system (HZ-7000, Hokuto Denko Cor.). A constant potential was applied at room temperature. To avoid Cu deposition before the observation, the initial potential was set to a noble potential against the equilibrium potential of Cu2þ/Cu. The potential was then switched to the deposition potential, by a step potential method. The HS-AFM (Research Institute of Biomolecule Metrology) was used for in situ observations. The HS-AFM operating in a dynamic mode can measure surfaces at frame rates up to 10 frame s1. In the present experiment, the scan area range was in the range of 375e1000 nm2 and the scanning speed was 1 frame s1.
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of 0.025 V s1. The CV measurements were performed in 3 mM CuSO4 solution, with Cu electrodeposition occurring at 0.04 V, as shown in Fig. 2. During the sequence of negative potential scanning, the Cu2þ diffusion was limited to 0.11 V and the cathode current decreased. Since there was no steep increase of the cathodic current, the hydrogen evolution was not confirmed. That is, the Cu growth mode is governed purely by the Cu2þ diffusion process in the present case. Cu was continuously deposited even after reversing the potential scan at 0.30 V. The dissolution commenced at 0.02 V, as confirmed by the large anodic peak. The Cu film was clearly removed at 0.12 V. The substrate surface after annealing had a roughness of 0.6 nm or less [Fig. 3 (a)]. Some straight lines, from the upper right to the lower left direction, appeared as black/white contrast. These features corresponded to the steps of the substrate. When the experiment was started at a low cathode potential (0.08 V), five Cu nuclei (I-V) appeared within 1 s [Fig. 3 (b)]. The individual sizes of these nuclei ranged from 3 to 80 nm and the height was 0.84 nm (i.e., 4 Cu atomic layers). Nucleus IV soon disappeared in the next frame [Fig. 3 (c)], because it was absorbed by larger neighboring nuclei through Ostwald growth or because the nucleus size was too small to remain stable. During deposition, the remaining four nuclei grew and no additional nucleation occurred. This result was consistent with the instantaneous nucleation mechanism. At 3 s after starting the deposition [Fig. 3 (d)], there were differences in the growth manner among individual nucleus. For example, nuclei IeIII grew predominantly in the lateral direction and their size became larger than 100 nm at 10 s [Fig. 3 (e)]; however, nucleus V grew in the perpendicular direction and became the tallest particle with a height of about 20 nm. A characteristic morphology was observed. In the nuclei I-III, the straight end was formed at the lateral deposition process. The shape appeared clearly as the deposition time proceeded. At 10 s, the cross point of the two edge lines was 60 [as indicated in Fig. 3 (e)], suggesting that the nuclei consist of a single crystal and that certain step directions are preferred. The steps of the nuclei parallel to each other indicates that deposit grows epitaxially and aligned to the substrate lattice, but that the Cu is not the hollow sites of the substrate. Once the straight edge-face formed, this part was very stable. The left side of nucleus III was arched shape. Detailed observations of the video images showed very fine projections moving in this part. The diffusing Cu adatoms were actively adsorbing and desorbing.
3. Results and discussion 3.1. Cu deposition The sample was investigated by performing cyclic voltammetry (CV) where the potential was swept from 0.2 to 0.3 V at a scan rate
Fig. 1. Schematic electrochemistry.
illustration
of
HS-AFM
experimental
setup
applied
for
Fig. 2. Voltammogram of Cu electrodeposition and dissolution on Au(111) in 3 mM CuSO4 with 50 mM H2SO4 solution (sweep rate 25 mV/s).
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Fig. 3. Series of HS-AFM images of Cu electrodeposition at 0.08 V after starting experiment at (a) 0 s, (b) 1 s, (c) 2 s, (d) 3 s (e) 10 s, and (f) 20 s (375 500 nm2, 1 Hz image acquisition rate).
To investigate the nuclei in detail, we analyzed the crosssectional profile for the nuclei III at 20 s [dotted line in Fig. 3 (f)]. The deposit size was approximately 150 nm laterally with a height of approximately 9 nm (Fig. 4). The deposit grew epitaxially from left to right to create three bunched layers. The thicknesses of the bottom, middle, and top layers were 5.8 nm (27 Cu atomic layers), 1.8 nm (9 Cu atomic layers), and 1.4 nm (7 Cu atomic layers). There was a gentle slope between the bottom and the middle layer. This result indicates that there were many steps in a staircase pattern within a width of 10 nm. Similar to the results obtained on Au (100) [24], epitaxial growth of the nuclei did not occur as one atomic layer but rather as multiple layers simultaneously. Rost et al. in-situ observed the two-dimensional growth of Cu deposits with STM in detail [25]. There, the epitaxial growth with mono-atomic layer was not confirmed at the initial stage of electrodeposition, but a stable unit called “building block” was found. The size of the minimum unit was about a few nm. That is, unlike Bi electrodeposition where a single adatom is taken into the kink and step edge [15], it is suggested that Cu clusters are attributed to the deposition process.
Fig. 4. Surface height profile of the cross-sectional image of nucleus III in Fig. 3 (f).
We note that the thickness between the middle and top layers enlarged. The white contrast of the left end portion increased for all nuclei in Fig. 3. We attribute this result to active nucleation sites or mechanical factors. That is, because the probe scanned from left to right and hit the edge of the deposits there is a chance that tip damping caused the height to be overestimated. The growth rate in the in plane direction was considered for the case of nucleus III. Supposing the deposit shape is circular, the approximate diameter can be calculated from the projected area. Nuclei with a size of approximately 30 nm were created within 1 s and grew to approximately 75 nm at 3 s [Fig. 5 (A)]. When the nuclei reached a certain size, the growth rate slowed. Fletcher et al. expressed the relationship between the deposition time, t, and the diameter, R, using a hemispherical diffusion model [29],
RðtÞ ¼ ð2DCb Vm tÞ1=2
(1)
where D is the diffusion coefficient, Cb is the bulk concentration of Cu2þ, and Vm is the molar volume. With the transition of time, the growth rate consisted of two regions where both the diameter and time were converted in the exponential values [Fig. 5 (B)]. Here, the slope of the approximate line was defined as the growth index a. Initially, the value of a was 0.54. Because CbVm was much less than 1 in the present conditions, R(t) can be simplified as R(t)ft1/2. The present results suggest that the early growth rate followed the hemisphere diffusion model. After 5 s, the nuclei grew slowly with a ¼ 0.35. The reason for the change in the growth index later in the deposition might be attributed to overlap of the diffusion layer. That is, the individual layers surrounding the respective nuclei thickened and extended over another layer. This caused a shift from a hemispherical to a planner diffusion model [5]. The later growth leads to a Cottrell type behavior when the growth is diffusion limited. It is considered that the amount of charge is proportional to time t1/2. From the Faraday's law, the charge associated with a hemispherical shape of the diffusion layer is proportional to radius R3. Therefore, the time dependence of the radius for 1D diffusion-limited growth is expected to be R(t)ft1/6 [19]. However, the present deviation of the value is considered as the unideal shape of the electrodeposit or the
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Fig. 5. Growth curve of the nucleus III diameter in (A) a linear and (B) a logarithm plots.
interaction between the electrodeposits. Fig. 6 shows HS-AFM images obtained when the electrodeposition was performed in the diffusion-controlled region (0.16 V). Some small nuclei partially appeared as soon as deposition started [Fig. 6 (b)]. Deposits approximately 80 nm in size covered the entire substrate within 2 s [Fig. 6 (c)]. Similar to the measurements at 0.08 V (Fig. 3), the number of nuclei did not increase and the black/white contrast of the images became clearer as the deposition progressed [Fig. 6(c)e(f)]. The nucleus grew predominantly in the lateral direction at 0.08 V, but at 0.16 V growth perpendicular to the substrate dominated. The average height of the nucleus was initially approximately 2 nm and reached approximately 15 nm at 20 s [Fig. 6 (f)]. A comparison of the images between Fig. 6 (c) and (f), showed little change in the shape of the deposits. Fig. 7 shows the cross-section profile of the deposit VI, as shown in Fig. 6 (f). The nuclei were approximately 80 nm in width and 12 nm in height. There was no terrace on the upper part and the shape was convex. Under mass transfer limitations, a thick diffusion layer formed. Therefore, the Cu2þ supply decreased in the vicinity of the substrate as the deposition proceeded. This effect might have contributed to the Cu2þ shortage at the lower part of the deposit.
Fig. 7. Surface height profile of the cross-sectional image of nucleus VI in Fig. 6 (f).
Fig. 6. Series of HS-AFM images of Cu electrodeposition at 0.16 V after starting experiment at (a) 0 s, (b) 1 s, (c) 2 s, (d) 3 s (e) 10 s, and (f) 20 s (750 1000 nm2, 1 Hz image acquisition rate).
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Conversely, the mass supply reached the upper part more easily. Moreover, the electric force line was focused in a convex shape, which likely promoted mass transfer and nucleation. The present dynamic observations showed that the morphological variation shifted to columnar (dendrite) type at the initial stage of deposition. The lateral size of nuclei VI is shown in Fig. 8 (A). Initially, the nucleus size was approximately 30 nm, which was similar to the results shown in Fig. 5. After 3 s of electrodeposition, the deposit size remained at an almost constant value of approximately 60 nm. On a logarithmic scale [Fig. 8 (B)], the relation between the size and time are considered by Eq. (1). The slope a was 0.42 at the initial deposition, when a spherical diffusion layer formed close to the nucleus. However, the deviation of the slope value became less than 0.5 after a few seconds. Because the Cu2þ flux is limited at high cathode potentials, the diffusion layer rapidly thickened. Additionally, overlap of the diffusion layers occurred easily owing to many nucleation sites. These factors might contribute to the disturbance of the growth index. The electrodeposition was investigated in the range of 0.06 to 0.16 V. Because instantaneous nucleation occurred at whole potentials, we counted nucleation sites immediately after starting the electrodeposition. When the number of Cu nuclei was logarithmically represented, the nucleation rate was consisted of two regions (Fig. 9). The reduction reaction was dominated by electron transfer or the combined electron and mass transfer of copper ions, which depended on the overpotential. Milchev et al. reported that Cu nucleation took place through two reactions, I) Cu2þ þ ee / Cuþ and II) Cuþ þ ee / Cu. Since the reaction (I) is fast, the concentration of Cuþ is accumulated near the electrode surface with increasing in the cathode potential [30]. That is, the supersaturation degree of Cuþ rises just before the occurrence of Cu nucleation, which might be contributed to form the number of Cu cluster (or “building block”) at low overpotenital region. In the diffusion control at high cathode potential, as the diffusion layer is thickened to some extent, it is overlapped each other. This would suppress the nucleation rate. 3.2. Cu dissolution The dissolution process was investigated. Immediately after conducting constant potential electrolysis at 0.08 V for 20 s [Fig. 10 (a)], the dissolution process was observed by switching 0.05 V. The set value was close to the onset potential where the oxidation current was detected (Fig. 2), otherwise HS-AFM could not be used to follow the rapid dynamic dissolution. The dissolution
Fig. 9. Dependence of the number of Cu nuclei on the cathode potential.
was much faster than deposition and the oxidation current increased sharply as the anodic potential increased. All the deposits became small when the potential was changed [Fig. 10 (b)]. In the deposition, the edge part of the nucleus consisted of an irregular structure or a line configuration along the substrate orientation. The dissolution rate was independent of the edge type. After 3 s [Fig. 10 (c)], the effects of dissolution became clear and the step lines of the nuclei became uneven. Interestingly, as dissolution progressed beyond 5 s, some clusters [indicated by arrows in Fig. 10 (d)] remained on the substrate briefly. This behavior had the appearance of small water droplets drying on a flat surface. The average size of the clusters was approximately 8 nm. After the large nuclei dissolved, these small clusters gradually dissolved [Fig. 10(e) and (f)]. 4. Conclusions We used HS-AFM to study the initial stage of Cu electrodeposition and dissolution on Au(111) surfaces in sulfuric acid solution. In situ dynamic observations revealed instantaneous nucleation whereby five Cu nuclei, less than 100 nm in size, appeared on Au terraces. The nuclei grew in the lateral direction at low cathode potentials (0.08 V). Epitaxial growth proceeded in one direction and was accompanied by deposition of multiple layers. The nuclei formed column shapes at high cathode potentials (0.16 V) owing to limited Cu2þ mass transfer. This result suggests that the seeds of dendrite growth were caused by Cu2þ shortages in
Fig. 8. Growth curve of the nucleus VI diameter in (A) a linear and (B) a logarithm plots.
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Fig. 10. Series of HS-AFM images of Cu dissolution under potential of 0.05 V after starting experiment at (a) 0 s, (b) 1 s, (c) 3 s, (d) 5 s, (e) 7 s and (f) 10 s (375 500 nm2, 1 Hz image acquisition rate).
the vicinity of the electrode surface. Similar to the results at 0.08 V, the growth exponent, a, decreased from 0.6 to 0.3 as the deposition time elapsed. We attribute this result to variation in the shape of the diffusion layer from hemispherical to planar. Dissolution was much faster than the deposition. The dissolution occurred heterogeneously at all points along the edges of nuclei. Some clusters of a few nm in size remained on the substrate momentarily. Acknowledgment The authors appreciate financial support from the Ministry of Education, Culture, Sports, Science and Technology (Project No. 16K14429, 17H03528). One of the authors, H. Matsushima, gratefully acknowledges financial support from the Iketani Science and Technology Foundation, Futaba Electronics Memorial Foundation and Hitachi Metals Materials Science Foundation in Japan. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.02.044. References [1] T.P. Moffat, M. Walker, P.J. Chen, J.E. Bonevich, W.F. Egelhoff, L. Richter, C. Witt, T. Aaltonen, M. Ritala, M. Leskel€ a, D. Josell, J. Electrochem. Soc. 153 (2006) C37. [2] Z.R. Yi, Y. Lei, X.Y. Zhang, Y.N. Chen, J.J. Guo, G.J. Xu, M.F. Yu, P. Cui, Nanoscale 9 (2017) 12524. [3] D.M. Kolb, R. Ullmann, T. Will, Science 275 (1997) 1097. [4] K. Nishikawa, Y. Fukunaka, E. Chassaing, M. Rosso, Electrochim. Acta 100
(2013) 342. [5] M. Morisue, Y. Fukunaka, K. Kusaka, R. Ishii, K. Kuribayashi, J. Electroanal. Chem. 559 (2003) 155. [6] N. Di, A. Damian, F. Maroun, P. Allongue, J. Electrochem. Soc. 163 (2016) D3062. [7] L. Guo, S.L. Zhang, P. Searson, Phys. Rev. E 79 (2009), 051601. [8] T. Zapryanova, N. Jordanov, A. Milchev, J. Electroanal. Chem. 612 (2008) 47. [9] M. Kleinert, H.F. Waibel, G.E. Engelmann, H. Martin, D.M. Kolb, Electrochim. Acta 46 (2001) 3129. [10] T. Homma, K. Naito, M. Takai, T. Osaka, Y. Yamazaki, T. Namikawa, J. Electrochem. Soc. 138 (1991) 1269. [11] B. Madry, K. Wandelt, M. Nowicki, Electrochim. Acta 217 (2016) 249. [12] Y.I. Yanson, M.J. Rost, Angew. Chem. Int. Ed. 52 (2013) 2454. [13] Y.F. Liu, K. Krug, P.C. Lin, Y.D. Chiu, W.P. Dow, S.L. Yau, Y.L. Lee, J. Electrochem. Soc. 159 (2012) D84. [14] M. Kang, M.E. Gross, A.A. Gewirth, J. Electrochem. Soc. 150 (2003) C292. [15] H. Matsushima, S.W. Lin, S. Morin, O.M. Magnussen, Faraday Discuss 193 (2016) 171. [16] H. Matsushima, A. Taranovskyy, C. Haak, Y. Grunder, O.M. Magnussen, J. Am. Chem. Soc. 131 (2009) 10362. [17] W. Polewska, R.J. Behm, O.M. Magnussen, Electrochim. Acta 48 (2003) 2915. [18] O.M. Magnussen, L. Zitzler, B. Gleich, M.R. Vogt, R.J. Behm, Electrochim. Acta 46 (2003) 3725. [19] A. Radisic, F.M. Ross, P.C. Searson, J. Phys. Chem. B 110 (2006) 7862. [20] A. Radisic, P.M. Vereecken, J.B. Hannon, P.C. Searson, F.M. Ross, Nano Lett. 6 (2006) 238. [21] Y. Ruan, A. Miyagi, X.Y. Wang, M. Chami, O. Boudker, S. Scheuring, Proc. Natl. Acad. Sci. U.S.A. 114 (2017) 1584. [22] T. Ando, Curr. Opin. Struct. Biol. 28 (2014) 63. [23] N. Hoshi, M. Nakamura, C. Yoshida, Y. Yamada, M. Kameyama, Y. Mizumoto, Electrochem. Commun. 72 (2016) 5. [24] T. Yoshioka, H. Matsushima, M. Ueda, Electrochem. Commun. 92 (2018) 29. [25] Y. Yanson, M.J. Rost, Angew. Chem. Int. Ed. 52 (2013) 2454. [26] Y. Yanson, J. Frenken, M.J. Rost, Phys. Chem. Chem. Phys. 13 (2011) 16095. [27] M. Petri, D.M. Kolb, U. Memmert, H. Meyer, Electrochim. Acta 49 (2003) 175. [28] A. Cuesta, M. Kleinert, D.M. Kolb, Phys. Chem. Chem. Phys. 2 (2000) 5684. [29] S. Fletcher, J. Chem. Soc. Faraday. Trans. 1 (79) (1983) 467. [30] A. Milchev, T. Zapryanova, Electrochim. Acta 51 (2006) 2926.