- Email: [email protected]
Preparation of dendritic tin nanoaggregates by electrodeposition
Journal of Non-Crystalline Solids 321 (2003) 73–80 www.elsevier.com/locate/jnoncrysol
Preparation of dendritic tin nanoaggregates by electrodeposition T. Devers a
a,*
, I. Kante a, L. Allam a, V. Fleury
b
Laboratoire de Physique Electronique de Chartres, Universit e d’Orl eans, IUT de Chartres, 1 place Pierre Mendes France, 28000 Chartres, France b Laboratoire de Physique de la Mati ere Condens ee, CNRS-Ecole Polytechnique, 91128 Palaiseau, France Received 13 February 2002; received in revised form 6 January 2003
Abstract We have used a new galvanic technique to obtain tin nanoaggregates on an insulating surface. This study describes the morphology and the size of these nanoaggregates and the operating parameters. The nanoaggregates show a dendritic structure composed of polycrystalline grains with grain size down to 20 nm. The microstructural characterization has been achieved by scanning and atomic force microscopes. Potential curves allow one to describe the onset of nucleation. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 47.53+n; 68.70+w; 81.15+Lm
1. Introduction The tin coating on insulating substrates plays a very important role in electronics and often requires techniques such as chemical vapour phase deposition, laser ablation technique, or screen printing [1–3]. A new technique recently suggested by one of us [4,5] allows forming metallic coatings on insulators using electrodeposition. While electrodeposition is usually used to coat metallic substrates with very pure metals, this technique allows one to deposit uniform films as well as dendrites
*
Corresponding author. Tel.: +33-2 37 91 83 19/00; fax: +332 37 91 83 19/01. E-mail address: [email protected] (T. Devers).
laying flat on an insulating substrate. It makes possible the fabrication without subsequent treatment of very thin polycrystalline and conducting layers (thickness < 50 nm). Here, we present the influence of the operating parameters on the grain size and the growth speed in a branching regime. The nanoaggregates were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM).
2. Experimental method The electrodeposition cell has been described in part in Ref. [4] (Fig. 1(a)), in the context of copper deposition. Here, the cell is composed of two glass plates (microscope slides) separated by foils of
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(03)00091-7
74
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80 Tin deposit Glass plates Electrolyte
(a)
Copper foil (cathode)
20 Å
Tin foil (anode)
(b)
1000 Å
). Fig. 1. (a) Schematic view of the cell. (b) Glass plates with a golg layer (20 and 1000 A
copper and tin acting respectively as an anode and a cathode. This paper presents the conditions in which such a set-up makes possible a growth that starts from the cathode, and proceeds horizontally towards the anode, in the form of a very thin deposit. The 12.5 lm thickness of the metal foils determines also the electrolyte volume. The thickness of the cell is much bigger than the final thickness of the deposit. The lower glass plate is gold coating at both its ends covered with a 1000 A ) in and covered with a very thin gold coating (20 A gold coating is in the shape the middle. The 20 A of little islands of gold, which do not percolate (Fig. 2). As the experiment described here consists in electrodepositing along a surface, the 1000 A coating is actually necessary to give a better chance to the deposit to start right away on the surface, and remain directly in contact with it. It may happen, however, that growth proceeds on the surface even without this coating. For example, growth may, although rarely, start and continue on the other glass plate, which is the non-coated one. gold coating which is The non-conducting 20 A added all across the glass slide to be coated is used as activation layer to enable the electrolytic deposit to nucleate on the surface, as the deposit progressively invades the glass slide. It also improves the adherence, which allows one to recover the deposit after opening the cell, and rinsing the electrolyte. The gold coatings are obtained by evaporation in a custom evaporator jar. In different operating configurations, the lower plate can gold offer two types of configuration: the 20 A deposit is either uniformly deposited (Fig. 1(a)) or thick contacts in the shape it contains four 1000 A
gold thick layer on glass slide. This Fig. 2. STM image of 20 A image clearly shows that gold layer does not percolate.
of circular dots (Fig. 1(b)), located at approximately 1 cm from the cathode, for further electric or SEM characterizations. The electrolyte used is SnCl2 or SnSO4 in aqueous solution. The concentrations in this study are between 102 and 101 mol l1 . Electrodeposition of tin is more difficult than that of many other metals. Indeed, formation of whiskers and dendrites is easily observed [6]. The electrolyte, which is generally recommended in the literature, contains brighteners and levellers that are supposed to limit this phenomenon [6]. In this study, we purposely aim at forming the ramified growth, since we are interested in tailoring a re-
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
trievable polycrystalline tree structure, adsorbed on a glass slide. This is why we use the simple binary electrolyte, which would be a bad solution for plating. In our set-up, the grain size is a function of the electrochemical conditions of the electrolysis and of the cell geometry, because of concentration (point effect) of the electric field at the edge of the growing deposit. The current density is not uniform, and it is very much concentrated at the tip of the deposit, which favours nucleation in that region. The thickness of the cell and the concentration also influence the deposit thickness; since the total metal entering the deposit is (1 þ lc =la ) times the metal present in the solution [7], the thickness of the deposit is eventually equal to ð1 þ lc =la Þ hCsolution =Cbulkmetal , where la (lc ) is the anion (cation) mobility, h is the cell thickness, Csolution the concentration in the solution, and Cbulkmetal , the concentration of bulk metal (generally of the order of 100, for tin it is 48 mol l1 for white tin and 40 mol l1 for gray tin). The current density influences both the size of the individual crystals and the average growth rate of the front of growth [8]. In the experiments presented here, a constant current is used. Therefore, an average constant growth rate proportional to the current is observed, which, of course, corresponds to a constant flux of atoms. The actual values of the constant current used in this study are between 1.6 and 20 mA/cm2 . During the experiment, growth is monitored through an optical microscope used in transmission mode. After opening of the cell, the deposit is examined by SEM in secondary mode. When the grain size is less than 20 nm, an AFM is used, in this case the instrument uses a SiN4 sensor tip.
75
diameter of approximately 1 lm. This is observed with tin for the lowest currents explored here, which were in the same range. For somewhat higher values, with tin, and quite surprisingly, we observe the growth of a uniform carpet of very small grains (20 nm), with only one grain in thickness (Fig. 3). This was not reported with copper. In the high current range (>20 mA/cm2 ) the deposit shows a very rapid growth. This growth usually leads to whiskers and dendrites. In this case, volume (3D) growth is observed which leads to powdery growth. In this high current regime, the deposit is not in contact with the intermediary gold coating, nor attached to the substrate, so that it is lost when opening the cell. It looks very much like the free-floating dendrites obtained classically with other metals [11–13]. For currents between 1.5 and 20 mA/cm2 , which will now on be our working conditions, a growth is achieved which either is a uniform carpet of grains as described above, or it combines a fine grain structure and an acceptable growth rate for 2D, flat deposition along the substrate. This leads to the formation of dendritic tin trees (Fig. 4) which cover the substrate and can be recovered upon opening and dismantling of the cell. However, these dendritic grains are in fact preceded by the carpet of smaller tin grains. The coating texture is then bi-modal: it is a dendrite with grains of the
3. Experimental results 3.1. Dendritic deposits The aim of the experiment is to form dendritic deposits, which cover the glass slide. This regime is limited by two other regimes, for either low or high currents. For low currents, of the order of 1.5 mA/ cm2 , we expected a massive structure composed of big grains as reported by one of us for copper [10]. For copper, the grains have, in such conditions, a
Fig. 3. SEM view of tin deposit, in a situation where the bigger tin cristals deposited over smaller (5–20 nm) tin crystals (SnCl2 , 102 mol/l, 10 mA/cm2 ).
76
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
Fig. 4. SEM image of aspect of the tree like tin deposit. These deposits were imaged without additional coating (SnCl2 , 102 mol/l, 10 mA/cm2 ).
Fig. 5. SEM view within deposit, in a region of dense crystals packing (near anode, same experimental procedure).
order of 0.1 lm, which is actually covering a uniform deposit of grains 5–10 nm thick, which appears quite visible in Fig. 3. The instance in which only the carpet of grains is obtained just corresponds to an experiment which is stopped prior to the onset of growth of the dendritic crystals. 3.2. Microstructural characterization of deposits We have studied the samples with a SEM, at magnifications between 70 000 and 100 000. We focused on the reproducible deposits obtained in the optimal operating conditions. These all show the characteristics displayed on Figs. 3 and 5. We can barely notice the very uniform sub-layer composed of very small grains, whose sizing is impossible with SEM, above which some grains of a bigger size have germinated. Those grains which are interconnected (see Fig. 3), show a 3D structure of several tens of nanometers. AFM observation of the sub-layer (Fig. 6) shows that the deposit is uniform in height. The size of the small grains composing that polycrystalline layer is approximately 5 nm in height and 50 nm in width. The height resolution is much better than the horizontal one for AFM analysis (the horizontal resolution is limited by the tip radius), so the value of 50 nm for the width should be considered as an upper bound.
Fig. 6. AFM image of tin showing grains that the deposit is only grain thick.
3.3. Experimental potential curves A study carried out on a great number of samples has revealed that, notwithstanding opti-
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
mal current conditions, some deposits did not have the previous characteristics, with mixed or apparently aberrant morphologies difficult to interpret. This kind of irreproducibility is often encountered in dendritic growth, and generally not commented on. These observations have led us to study the chronopotentiographs of the electrolyses, in order to understand the origin of the bi-modal texture, and its window of existence. A statistical survey on 46 samples allowed to determine the characteristics of these chronopotentiographs and to compare the measurements collected from these curves to the morphologies of the deposits themselves. These potentials as a function of time, all show a variation similar to the one represented on Fig. 7(a). The chronopotentiograhs of experiments performed at constant current show two distinct parts (Fig. 7(b)): (i) A first roughly linear part which comes to an end at the so-called Sand time.
Potential ( V)
0. 6
Nucleation overpotential value (η)
0. 4 0. 2 0
Time Sand
- 0,45V
1000
1500
2000
2500
Time (s)
Potential (V)
(a)
E (2000)
Sand time
(b)
∆V
E (1000)
1000
2000
Time (s)
Fig. 7. (a) Plot of chronopotentiograph of the electrolyses. (b) Schematic plot of same chronopotentiograph (see text).
77
(ii) A second part showing a much weaker potential variation. In the statistical survey, we consider that the deposit has succeeded if the growth shows a dendritic pattern laying flat on the glass, such as shown in Fig. 4. 3.4. Interpretation of the curves in light of the morphologies In the first part, the straight line has a departure point, which has a negative potential of the order of )0.45 V. From there, the potential increases steadily. In agreement with Chazalviel model [14], as the concentration gradients settle in, the potential should increase. The potential rises because of progressive depletion along the electrodes. This rise will continue until Sand time, which corresponds to a change of slope (Fig. 7(b)). The expected Sand time for the conditions which we have used in Fig. 4 is 575 s. It is the observed Sand time, and it is quite typical of the Sand times in the window of parameters which we have explored. The depletion to zero of the ionic concentration close to the electrode generates, in principle, a divergence of the potential. It is generally believed that this divergence is the true cause of dendritic growth [15,16]. This has been convincingly demonstrated on 3D growth of copper [9] and zinc [11]. The origin of the dendritic growth is rooted in the high current demanded, which generates these even higher local electric fields (much higher than the average across the cell) in the vicinity of the deposit. After an induction time corresponding to the Sand time, the local diverging field induces rapid and unstable nucleation and growth. There are two ingredients in this instability: the first one is the possibility of actually nucleating grains, and the other is the tendency of the front of grains to be unstable. (The main difference between dendritic growth in solidification from a melt and a dendritic growth in electrochemistry is the polycrystalline nature of the electrodeposit.) The interplay of these two effects is not yet clearly understood. One of the reasons, is that, since the divergence of the potential is, in theory, and for an uniform electrode, exponential, it is very difficult
78
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
to distinguish, in this rapid rise, the triggering of different nucleation mechanisms (not even mentioning the fact that, in 3D, they would be impossible to visualize). That there should be different nucleation mechanisms is evident, since a constant current is demanded. At start, the current will flow through the most accessible and least demanding sites, in terms of nucleation overpotential. The growth begins when the nucleation overpotential (g) is reached (Fig. 7(a)), this value corresponds to the Sand time. As the overpotential varies and/or easy sites are consumed, the current will have to flow through other sites. The fact that the growth occurs at preferred locations has been mostly studied in the context of dendritic growth of lithium, for electric vehicle batteries, where it was shown that dendritic growth is not at all uniform, and that there are bursts of dendrites at some specific locations [16]. Sites of favourable nucleation are a very important (detrimental) technological issue in this context. How the preferred sites are selected is not known. More recently, Rosso et al. [17] were able to show, in the case of copper, that inhomogeneities of the electrode surface selected the most favoured sites of growth of dendrites. The interplay of nucleation kinetics, and out-of-equilibrium growth, is as yet not understood. One model was proposed by Fleury [18], but for a single family of nucleation sites. In the experiment with tin, we clearly see a bimodal texture, one of which is obviously surface deposition. The deposit shows small grains growing with a smooth envelope, and secondarily, bigger grains growing on top, and showing the more classical dendritic instability. An important observation is that the first thinner deposit is only one grain thick, i.e., it is composed of a layer of tin spanned by the diameter of only one-grain. These grains, therefore, all nucleate on the substrate, along the triple line tin/substrate/electrolyte, this is evidence of easy activation for the growth along this line. Then, nucleation on top of them is more difficult. This means that homogeneous nucleation on tin is less easy than heterogeneous nucleation, in this context. As long as only heterogeneous nucleation is possible, the deposit grows in the shape of the thin sub-layer. The homogeneous nucleation (tin on tin), leading to the dendritic
growth, is only possible after some time, which, in terms of the Sand time, correlates to a higher local field. This is the simplest explanation of both the time delay between the two kind of growths, and the distance separating the front edge of the small grains, and the front edge of the bigger grains which lag behind. To explain more clearly the logic, we see that, as the potential rises, it scans higher values, which switch on different possible nucleation mechanisms. Those correspond to the sites where current actually flows. The potential across the depleted zone is associated to a local high electric field, so in these out-of-equilibrium conditions, it is equivalent to consider that nucleation is a consequence of a threshold potential (as classically termed nucleation overpotential [14]), or of a threshold electric field [18]. As a constant current is demanded, the nucleation overpotential in fact rises, so several nucleation-growth phenomena around the cathode may be triggered. As the nucleation overpotential increases more, nucleation phenomena of different nature are rendered possible. The catalytic properties of the surface then read directly on the potential curve, as the location of the change in slope, where heterogeneous tin deposition by small grains nucleating on the triple line is possible, which arrests the divergence of the potential. The outof-equilibrium growth of metals in this set-up provides, in fact, a tool for measuring the nucleation potential of different nucleation mechanisms, in the context of rapid growth. This measurement should be done at the very beginning, where the onset of growth, corresponding to the most favourable mechanism of growth, will be associated to a limiting value of the rise in potential measured across the cell. When comparing the value of the slope (dV =dt) in the first regime to the morphology of our deposits the former does not seem to show a criterion to predict the success or failure of our deposits. Nevertheless, a high slope value (1.2 mV/s) will allow us to reach the second stage more rapidly. After the slope change, an almost stationary growth rate settles in corresponding to dendritic growth. The absolute magnitude of the potential measured between the anode and the cathode at a given moment during the stationary regime does
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
not correlate to the pattern evolution in any obvious way. Either there is a lack of descriptive tools for the minutiae of the pattern, or simply, as the dendritic regime forms, it has a well recognizable pattern in all the window of growth parameters. As explained above, the pattern is a function of both the nucleation mechanisms and the surface instabilities. We worked out the correlation between the potential variation, noted DV (Fig. 7(b)) between t ¼ 1000 and 2000 s, and the patterns which are observed. The statistical survey of the success probabilities for our deposits has shown that after 1000 s, an increment potential comprised between 0.4 and 0.6 V, was correlated with a success rate of our deposits in the order of 80%. When this potential difference is weaker, the growth is in the form of dense deposits of scattered big grains. The deposits obtained through higherlevel potential lead to a 3D structure, without any gold deposit, making the adherence to the 20 A deposit irretrievable after opening the cell. We correlated the onset of this 3D growth to values of DV higher than 0.6 V. Hence, a potential difference comprised between 0.4 and 0.6 V seems to be the first condition of success for dendritic growth adhering to the substrate. We ascribe this range of values to the generic value of the nucleation overpotential along the triple line tin/substrate/ electrolyte, where indeed most of the growth occurs. Below that value, the nucleation is not possible, and growth will occur at some random spots, where impurities allow a more favourable nucleation. Above the value of 0.6 V, we systematically see 3D growth: 3D nucleation is favoured at the expense of 2D growth. The second condition is linked to the variation of potential. The potential variation measurements between 1000 and 2000 s have shown that if that variation is too rapid, the deposits shows a 3D morphology. The analysis of the data leads one to consider that this rate of potential variation cannot exceed 0.33 mV/s. We ascribe this unexpected condition to the bi-modal texture of the deposit. In order for the dendritic deposit to remain stuck on the surface, it needs to nucleate first the thin layer (condition one), and afterwards only should direct homogeneous nucleation be possible. But homo-
79
geneous nucleation should be limited, to avoid 3D runaway growth. This is clearly obtained by conditions in which the dendritic regime lags behind the thin deposit, by a distance of the order of the cell thickness (because of screening). If the current is too high, then the the dendritic regime does not lag so far behind, and fluctuations will provoke a 3D growth starting on the tin dendritic deposit, and will eventually overcome (screen) the 2D growth. The third condition is linked to the very appearance of the quasi-stationary regime: a minimal time of 900 s proves necessary. The statistical survey of the 46 deposits studied leads to the definition of three criteria to obtain a fine grain dendritic structure: (i) the chronopotentiograph of the electrolytical deposit must have in quasi permanent regime a potential comprised between 0.4 and 0.6 V, (ii) a minimal duration of 900 s and (iii) a rate of potential variation inferior to 0.33 mV/s. This certainly explains the difficulty of the experiment, because the fluctuations in cell preparation (especially, the inhomogeneities of the surface state e.g. scratches) provoke spurious deposits which are a mixture of several modes, and then are quite difficult to classify.
4. Conclusion This study has allowed us to define electrodeposition operating conditions in order to obtain dendritic tin nanoaggregates composed of small grains in the 50–100 nm, themselves deposited on a sub-layer of grains in the range 5–50 nm. These conditions correspond to a rather narrow window of parameters. The cell design makes the deposits recoverable and enables a microstructural characterization. The potential curve and the theory of out-of-equilibrium growth without supporting electrolyte allow one to find the nucleation overpotentials of the different nucleation mechanisms, and hence to understand the origin of the windows of stability. This experiment paves the way to possible applications of tin thin films obtained directly on insulating substrates by electrodeposition (galvanic, instead of electroless) routes. It also gives a more general understanding of how
80
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
different nucleation mechanisms come into play as a function of time, in constant current conditions, to maintain constant the total current while the microscopic surface states evolve. More specifically, it helps to understand the activation properties of gold particles used as nucleation sites. Similar particles made of Pd are commonly used in electroless deposition for surface plating. The understanding provided by our experiment can certainly be transposed to other metals. This study is part of a program carrying Patent number FR 00/10147 (2000). Acknowledgements The authors wish to thank the Region Centre Territorial Collective Bodies for their financial help and M. Bonora, E. Fichou, Th. LeGrives, and N. Sagot for the technical help they provided. References [1] Y. Shimizu, M. Egashira, MRS Bull. 24 (1999) 8.
[2] G. Williams, G.S.V. Coles, MRS Bull. 24 (1999) 25. [3] G. Martinelli, C. Carotta, E. Traversa, G. Ghiotti, MRS Bull. (1999) 30. [4] V. Fleury, Patent PCT/FROO/02757, 2000. [5] V. Fleury, W.A. Watters, L. Allam, T. Devers, Nature 416 (2002). [6] V. Jordan, The Electrodeposition of Tin and its Alloys, Eugen G. Leuz, Ville, 1995. [7] V. Fleury, J.-N. Chazalviel, M. Rosso, B. Sapoval, J. Electroanal. Chem. 290 (1990) 249. [8] V. Fleury, J.-N. Chazalviel, M. Rosso, Phys. Rev. Lett. 68 (1992) 2492. [9] V. Fleury, J.-N. Chazalviel, M. Rosso, B. Sapoval, Phys. Rev. A 44 (10) (1991) 6693. [10] P. Pelce, Dynamics of Curved Fronts, Academic Press, London, 1991. [11] M. Matsushita, M. Sano, Y. Hayakawa, H. Honjo, Y. Sawada, Phys. Rev. Lett. 53 (1984) 286. [12] J.R. Melrose, D.B. Hibbert, R.C. Ball, Phys. Rev. Lett. 65 (24) (1990) 3009. [13] T. Viscek, Fractal Growth Phenomena, 2nd Ed., World Scientific, Singapore, 1992. [14] J.-N. Chazalviel, Phys. Rev. A 42 (1990) 7355. [15] E. Ben-Jacob, P. Garik, Nature 343 (1990) 523. [16] P. Meakin, in: C. Domb, J.L. Leibowitz (Eds.), Phase Transitions and Critical Phenomena, vol. 12, Academic Press, London, 1988. [17] Rosso et al., in preparation. [18] V. Fleury, Nature 390 (13) (1997) 145.
Preparation of dendritic tin nanoaggregates by electrodeposition T. Devers a
a,*
, I. Kante a, L. Allam a, V. Fleury
b
Laboratoire de Physique Electronique de Chartres, Universit e d’Orl eans, IUT de Chartres, 1 place Pierre Mendes France, 28000 Chartres, France b Laboratoire de Physique de la Mati ere Condens ee, CNRS-Ecole Polytechnique, 91128 Palaiseau, France Received 13 February 2002; received in revised form 6 January 2003
Abstract We have used a new galvanic technique to obtain tin nanoaggregates on an insulating surface. This study describes the morphology and the size of these nanoaggregates and the operating parameters. The nanoaggregates show a dendritic structure composed of polycrystalline grains with grain size down to 20 nm. The microstructural characterization has been achieved by scanning and atomic force microscopes. Potential curves allow one to describe the onset of nucleation. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 47.53+n; 68.70+w; 81.15+Lm
1. Introduction The tin coating on insulating substrates plays a very important role in electronics and often requires techniques such as chemical vapour phase deposition, laser ablation technique, or screen printing [1–3]. A new technique recently suggested by one of us [4,5] allows forming metallic coatings on insulators using electrodeposition. While electrodeposition is usually used to coat metallic substrates with very pure metals, this technique allows one to deposit uniform films as well as dendrites
*
Corresponding author. Tel.: +33-2 37 91 83 19/00; fax: +332 37 91 83 19/01. E-mail address: [email protected] (T. Devers).
laying flat on an insulating substrate. It makes possible the fabrication without subsequent treatment of very thin polycrystalline and conducting layers (thickness < 50 nm). Here, we present the influence of the operating parameters on the grain size and the growth speed in a branching regime. The nanoaggregates were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM).
2. Experimental method The electrodeposition cell has been described in part in Ref. [4] (Fig. 1(a)), in the context of copper deposition. Here, the cell is composed of two glass plates (microscope slides) separated by foils of
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(03)00091-7
74
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80 Tin deposit Glass plates Electrolyte
(a)
Copper foil (cathode)
20 Å
Tin foil (anode)
(b)
1000 Å
). Fig. 1. (a) Schematic view of the cell. (b) Glass plates with a golg layer (20 and 1000 A
copper and tin acting respectively as an anode and a cathode. This paper presents the conditions in which such a set-up makes possible a growth that starts from the cathode, and proceeds horizontally towards the anode, in the form of a very thin deposit. The 12.5 lm thickness of the metal foils determines also the electrolyte volume. The thickness of the cell is much bigger than the final thickness of the deposit. The lower glass plate is gold coating at both its ends covered with a 1000 A ) in and covered with a very thin gold coating (20 A gold coating is in the shape the middle. The 20 A of little islands of gold, which do not percolate (Fig. 2). As the experiment described here consists in electrodepositing along a surface, the 1000 A coating is actually necessary to give a better chance to the deposit to start right away on the surface, and remain directly in contact with it. It may happen, however, that growth proceeds on the surface even without this coating. For example, growth may, although rarely, start and continue on the other glass plate, which is the non-coated one. gold coating which is The non-conducting 20 A added all across the glass slide to be coated is used as activation layer to enable the electrolytic deposit to nucleate on the surface, as the deposit progressively invades the glass slide. It also improves the adherence, which allows one to recover the deposit after opening the cell, and rinsing the electrolyte. The gold coatings are obtained by evaporation in a custom evaporator jar. In different operating configurations, the lower plate can gold offer two types of configuration: the 20 A deposit is either uniformly deposited (Fig. 1(a)) or thick contacts in the shape it contains four 1000 A
gold thick layer on glass slide. This Fig. 2. STM image of 20 A image clearly shows that gold layer does not percolate.
of circular dots (Fig. 1(b)), located at approximately 1 cm from the cathode, for further electric or SEM characterizations. The electrolyte used is SnCl2 or SnSO4 in aqueous solution. The concentrations in this study are between 102 and 101 mol l1 . Electrodeposition of tin is more difficult than that of many other metals. Indeed, formation of whiskers and dendrites is easily observed [6]. The electrolyte, which is generally recommended in the literature, contains brighteners and levellers that are supposed to limit this phenomenon [6]. In this study, we purposely aim at forming the ramified growth, since we are interested in tailoring a re-
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
trievable polycrystalline tree structure, adsorbed on a glass slide. This is why we use the simple binary electrolyte, which would be a bad solution for plating. In our set-up, the grain size is a function of the electrochemical conditions of the electrolysis and of the cell geometry, because of concentration (point effect) of the electric field at the edge of the growing deposit. The current density is not uniform, and it is very much concentrated at the tip of the deposit, which favours nucleation in that region. The thickness of the cell and the concentration also influence the deposit thickness; since the total metal entering the deposit is (1 þ lc =la ) times the metal present in the solution [7], the thickness of the deposit is eventually equal to ð1 þ lc =la Þ hCsolution =Cbulkmetal , where la (lc ) is the anion (cation) mobility, h is the cell thickness, Csolution the concentration in the solution, and Cbulkmetal , the concentration of bulk metal (generally of the order of 100, for tin it is 48 mol l1 for white tin and 40 mol l1 for gray tin). The current density influences both the size of the individual crystals and the average growth rate of the front of growth [8]. In the experiments presented here, a constant current is used. Therefore, an average constant growth rate proportional to the current is observed, which, of course, corresponds to a constant flux of atoms. The actual values of the constant current used in this study are between 1.6 and 20 mA/cm2 . During the experiment, growth is monitored through an optical microscope used in transmission mode. After opening of the cell, the deposit is examined by SEM in secondary mode. When the grain size is less than 20 nm, an AFM is used, in this case the instrument uses a SiN4 sensor tip.
75
diameter of approximately 1 lm. This is observed with tin for the lowest currents explored here, which were in the same range. For somewhat higher values, with tin, and quite surprisingly, we observe the growth of a uniform carpet of very small grains (20 nm), with only one grain in thickness (Fig. 3). This was not reported with copper. In the high current range (>20 mA/cm2 ) the deposit shows a very rapid growth. This growth usually leads to whiskers and dendrites. In this case, volume (3D) growth is observed which leads to powdery growth. In this high current regime, the deposit is not in contact with the intermediary gold coating, nor attached to the substrate, so that it is lost when opening the cell. It looks very much like the free-floating dendrites obtained classically with other metals [11–13]. For currents between 1.5 and 20 mA/cm2 , which will now on be our working conditions, a growth is achieved which either is a uniform carpet of grains as described above, or it combines a fine grain structure and an acceptable growth rate for 2D, flat deposition along the substrate. This leads to the formation of dendritic tin trees (Fig. 4) which cover the substrate and can be recovered upon opening and dismantling of the cell. However, these dendritic grains are in fact preceded by the carpet of smaller tin grains. The coating texture is then bi-modal: it is a dendrite with grains of the
3. Experimental results 3.1. Dendritic deposits The aim of the experiment is to form dendritic deposits, which cover the glass slide. This regime is limited by two other regimes, for either low or high currents. For low currents, of the order of 1.5 mA/ cm2 , we expected a massive structure composed of big grains as reported by one of us for copper [10]. For copper, the grains have, in such conditions, a
Fig. 3. SEM view of tin deposit, in a situation where the bigger tin cristals deposited over smaller (5–20 nm) tin crystals (SnCl2 , 102 mol/l, 10 mA/cm2 ).
76
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
Fig. 4. SEM image of aspect of the tree like tin deposit. These deposits were imaged without additional coating (SnCl2 , 102 mol/l, 10 mA/cm2 ).
Fig. 5. SEM view within deposit, in a region of dense crystals packing (near anode, same experimental procedure).
order of 0.1 lm, which is actually covering a uniform deposit of grains 5–10 nm thick, which appears quite visible in Fig. 3. The instance in which only the carpet of grains is obtained just corresponds to an experiment which is stopped prior to the onset of growth of the dendritic crystals. 3.2. Microstructural characterization of deposits We have studied the samples with a SEM, at magnifications between 70 000 and 100 000. We focused on the reproducible deposits obtained in the optimal operating conditions. These all show the characteristics displayed on Figs. 3 and 5. We can barely notice the very uniform sub-layer composed of very small grains, whose sizing is impossible with SEM, above which some grains of a bigger size have germinated. Those grains which are interconnected (see Fig. 3), show a 3D structure of several tens of nanometers. AFM observation of the sub-layer (Fig. 6) shows that the deposit is uniform in height. The size of the small grains composing that polycrystalline layer is approximately 5 nm in height and 50 nm in width. The height resolution is much better than the horizontal one for AFM analysis (the horizontal resolution is limited by the tip radius), so the value of 50 nm for the width should be considered as an upper bound.
Fig. 6. AFM image of tin showing grains that the deposit is only grain thick.
3.3. Experimental potential curves A study carried out on a great number of samples has revealed that, notwithstanding opti-
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
mal current conditions, some deposits did not have the previous characteristics, with mixed or apparently aberrant morphologies difficult to interpret. This kind of irreproducibility is often encountered in dendritic growth, and generally not commented on. These observations have led us to study the chronopotentiographs of the electrolyses, in order to understand the origin of the bi-modal texture, and its window of existence. A statistical survey on 46 samples allowed to determine the characteristics of these chronopotentiographs and to compare the measurements collected from these curves to the morphologies of the deposits themselves. These potentials as a function of time, all show a variation similar to the one represented on Fig. 7(a). The chronopotentiograhs of experiments performed at constant current show two distinct parts (Fig. 7(b)): (i) A first roughly linear part which comes to an end at the so-called Sand time.
Potential ( V)
0. 6
Nucleation overpotential value (η)
0. 4 0. 2 0
Time Sand
- 0,45V
1000
1500
2000
2500
Time (s)
Potential (V)
(a)
E (2000)
Sand time
(b)
∆V
E (1000)
1000
2000
Time (s)
Fig. 7. (a) Plot of chronopotentiograph of the electrolyses. (b) Schematic plot of same chronopotentiograph (see text).
77
(ii) A second part showing a much weaker potential variation. In the statistical survey, we consider that the deposit has succeeded if the growth shows a dendritic pattern laying flat on the glass, such as shown in Fig. 4. 3.4. Interpretation of the curves in light of the morphologies In the first part, the straight line has a departure point, which has a negative potential of the order of )0.45 V. From there, the potential increases steadily. In agreement with Chazalviel model [14], as the concentration gradients settle in, the potential should increase. The potential rises because of progressive depletion along the electrodes. This rise will continue until Sand time, which corresponds to a change of slope (Fig. 7(b)). The expected Sand time for the conditions which we have used in Fig. 4 is 575 s. It is the observed Sand time, and it is quite typical of the Sand times in the window of parameters which we have explored. The depletion to zero of the ionic concentration close to the electrode generates, in principle, a divergence of the potential. It is generally believed that this divergence is the true cause of dendritic growth [15,16]. This has been convincingly demonstrated on 3D growth of copper [9] and zinc [11]. The origin of the dendritic growth is rooted in the high current demanded, which generates these even higher local electric fields (much higher than the average across the cell) in the vicinity of the deposit. After an induction time corresponding to the Sand time, the local diverging field induces rapid and unstable nucleation and growth. There are two ingredients in this instability: the first one is the possibility of actually nucleating grains, and the other is the tendency of the front of grains to be unstable. (The main difference between dendritic growth in solidification from a melt and a dendritic growth in electrochemistry is the polycrystalline nature of the electrodeposit.) The interplay of these two effects is not yet clearly understood. One of the reasons, is that, since the divergence of the potential is, in theory, and for an uniform electrode, exponential, it is very difficult
78
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
to distinguish, in this rapid rise, the triggering of different nucleation mechanisms (not even mentioning the fact that, in 3D, they would be impossible to visualize). That there should be different nucleation mechanisms is evident, since a constant current is demanded. At start, the current will flow through the most accessible and least demanding sites, in terms of nucleation overpotential. The growth begins when the nucleation overpotential (g) is reached (Fig. 7(a)), this value corresponds to the Sand time. As the overpotential varies and/or easy sites are consumed, the current will have to flow through other sites. The fact that the growth occurs at preferred locations has been mostly studied in the context of dendritic growth of lithium, for electric vehicle batteries, where it was shown that dendritic growth is not at all uniform, and that there are bursts of dendrites at some specific locations [16]. Sites of favourable nucleation are a very important (detrimental) technological issue in this context. How the preferred sites are selected is not known. More recently, Rosso et al. [17] were able to show, in the case of copper, that inhomogeneities of the electrode surface selected the most favoured sites of growth of dendrites. The interplay of nucleation kinetics, and out-of-equilibrium growth, is as yet not understood. One model was proposed by Fleury [18], but for a single family of nucleation sites. In the experiment with tin, we clearly see a bimodal texture, one of which is obviously surface deposition. The deposit shows small grains growing with a smooth envelope, and secondarily, bigger grains growing on top, and showing the more classical dendritic instability. An important observation is that the first thinner deposit is only one grain thick, i.e., it is composed of a layer of tin spanned by the diameter of only one-grain. These grains, therefore, all nucleate on the substrate, along the triple line tin/substrate/electrolyte, this is evidence of easy activation for the growth along this line. Then, nucleation on top of them is more difficult. This means that homogeneous nucleation on tin is less easy than heterogeneous nucleation, in this context. As long as only heterogeneous nucleation is possible, the deposit grows in the shape of the thin sub-layer. The homogeneous nucleation (tin on tin), leading to the dendritic
growth, is only possible after some time, which, in terms of the Sand time, correlates to a higher local field. This is the simplest explanation of both the time delay between the two kind of growths, and the distance separating the front edge of the small grains, and the front edge of the bigger grains which lag behind. To explain more clearly the logic, we see that, as the potential rises, it scans higher values, which switch on different possible nucleation mechanisms. Those correspond to the sites where current actually flows. The potential across the depleted zone is associated to a local high electric field, so in these out-of-equilibrium conditions, it is equivalent to consider that nucleation is a consequence of a threshold potential (as classically termed nucleation overpotential [14]), or of a threshold electric field [18]. As a constant current is demanded, the nucleation overpotential in fact rises, so several nucleation-growth phenomena around the cathode may be triggered. As the nucleation overpotential increases more, nucleation phenomena of different nature are rendered possible. The catalytic properties of the surface then read directly on the potential curve, as the location of the change in slope, where heterogeneous tin deposition by small grains nucleating on the triple line is possible, which arrests the divergence of the potential. The outof-equilibrium growth of metals in this set-up provides, in fact, a tool for measuring the nucleation potential of different nucleation mechanisms, in the context of rapid growth. This measurement should be done at the very beginning, where the onset of growth, corresponding to the most favourable mechanism of growth, will be associated to a limiting value of the rise in potential measured across the cell. When comparing the value of the slope (dV =dt) in the first regime to the morphology of our deposits the former does not seem to show a criterion to predict the success or failure of our deposits. Nevertheless, a high slope value (1.2 mV/s) will allow us to reach the second stage more rapidly. After the slope change, an almost stationary growth rate settles in corresponding to dendritic growth. The absolute magnitude of the potential measured between the anode and the cathode at a given moment during the stationary regime does
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
not correlate to the pattern evolution in any obvious way. Either there is a lack of descriptive tools for the minutiae of the pattern, or simply, as the dendritic regime forms, it has a well recognizable pattern in all the window of growth parameters. As explained above, the pattern is a function of both the nucleation mechanisms and the surface instabilities. We worked out the correlation between the potential variation, noted DV (Fig. 7(b)) between t ¼ 1000 and 2000 s, and the patterns which are observed. The statistical survey of the success probabilities for our deposits has shown that after 1000 s, an increment potential comprised between 0.4 and 0.6 V, was correlated with a success rate of our deposits in the order of 80%. When this potential difference is weaker, the growth is in the form of dense deposits of scattered big grains. The deposits obtained through higherlevel potential lead to a 3D structure, without any gold deposit, making the adherence to the 20 A deposit irretrievable after opening the cell. We correlated the onset of this 3D growth to values of DV higher than 0.6 V. Hence, a potential difference comprised between 0.4 and 0.6 V seems to be the first condition of success for dendritic growth adhering to the substrate. We ascribe this range of values to the generic value of the nucleation overpotential along the triple line tin/substrate/ electrolyte, where indeed most of the growth occurs. Below that value, the nucleation is not possible, and growth will occur at some random spots, where impurities allow a more favourable nucleation. Above the value of 0.6 V, we systematically see 3D growth: 3D nucleation is favoured at the expense of 2D growth. The second condition is linked to the variation of potential. The potential variation measurements between 1000 and 2000 s have shown that if that variation is too rapid, the deposits shows a 3D morphology. The analysis of the data leads one to consider that this rate of potential variation cannot exceed 0.33 mV/s. We ascribe this unexpected condition to the bi-modal texture of the deposit. In order for the dendritic deposit to remain stuck on the surface, it needs to nucleate first the thin layer (condition one), and afterwards only should direct homogeneous nucleation be possible. But homo-
79
geneous nucleation should be limited, to avoid 3D runaway growth. This is clearly obtained by conditions in which the dendritic regime lags behind the thin deposit, by a distance of the order of the cell thickness (because of screening). If the current is too high, then the the dendritic regime does not lag so far behind, and fluctuations will provoke a 3D growth starting on the tin dendritic deposit, and will eventually overcome (screen) the 2D growth. The third condition is linked to the very appearance of the quasi-stationary regime: a minimal time of 900 s proves necessary. The statistical survey of the 46 deposits studied leads to the definition of three criteria to obtain a fine grain dendritic structure: (i) the chronopotentiograph of the electrolytical deposit must have in quasi permanent regime a potential comprised between 0.4 and 0.6 V, (ii) a minimal duration of 900 s and (iii) a rate of potential variation inferior to 0.33 mV/s. This certainly explains the difficulty of the experiment, because the fluctuations in cell preparation (especially, the inhomogeneities of the surface state e.g. scratches) provoke spurious deposits which are a mixture of several modes, and then are quite difficult to classify.
4. Conclusion This study has allowed us to define electrodeposition operating conditions in order to obtain dendritic tin nanoaggregates composed of small grains in the 50–100 nm, themselves deposited on a sub-layer of grains in the range 5–50 nm. These conditions correspond to a rather narrow window of parameters. The cell design makes the deposits recoverable and enables a microstructural characterization. The potential curve and the theory of out-of-equilibrium growth without supporting electrolyte allow one to find the nucleation overpotentials of the different nucleation mechanisms, and hence to understand the origin of the windows of stability. This experiment paves the way to possible applications of tin thin films obtained directly on insulating substrates by electrodeposition (galvanic, instead of electroless) routes. It also gives a more general understanding of how
80
T. Devers et al. / Journal of Non-Crystalline Solids 321 (2003) 73–80
different nucleation mechanisms come into play as a function of time, in constant current conditions, to maintain constant the total current while the microscopic surface states evolve. More specifically, it helps to understand the activation properties of gold particles used as nucleation sites. Similar particles made of Pd are commonly used in electroless deposition for surface plating. The understanding provided by our experiment can certainly be transposed to other metals. This study is part of a program carrying Patent number FR 00/10147 (2000). Acknowledgements The authors wish to thank the Region Centre Territorial Collective Bodies for their financial help and M. Bonora, E. Fichou, Th. LeGrives, and N. Sagot for the technical help they provided. References [1] Y. Shimizu, M. Egashira, MRS Bull. 24 (1999) 8.
[2] G. Williams, G.S.V. Coles, MRS Bull. 24 (1999) 25. [3] G. Martinelli, C. Carotta, E. Traversa, G. Ghiotti, MRS Bull. (1999) 30. [4] V. Fleury, Patent PCT/FROO/02757, 2000. [5] V. Fleury, W.A. Watters, L. Allam, T. Devers, Nature 416 (2002). [6] V. Jordan, The Electrodeposition of Tin and its Alloys, Eugen G. Leuz, Ville, 1995. [7] V. Fleury, J.-N. Chazalviel, M. Rosso, B. Sapoval, J. Electroanal. Chem. 290 (1990) 249. [8] V. Fleury, J.-N. Chazalviel, M. Rosso, Phys. Rev. Lett. 68 (1992) 2492. [9] V. Fleury, J.-N. Chazalviel, M. Rosso, B. Sapoval, Phys. Rev. A 44 (10) (1991) 6693. [10] P. Pelce, Dynamics of Curved Fronts, Academic Press, London, 1991. [11] M. Matsushita, M. Sano, Y. Hayakawa, H. Honjo, Y. Sawada, Phys. Rev. Lett. 53 (1984) 286. [12] J.R. Melrose, D.B. Hibbert, R.C. Ball, Phys. Rev. Lett. 65 (24) (1990) 3009. [13] T. Viscek, Fractal Growth Phenomena, 2nd Ed., World Scientific, Singapore, 1992. [14] J.-N. Chazalviel, Phys. Rev. A 42 (1990) 7355. [15] E. Ben-Jacob, P. Garik, Nature 343 (1990) 523. [16] P. Meakin, in: C. Domb, J.L. Leibowitz (Eds.), Phase Transitions and Critical Phenomena, vol. 12, Academic Press, London, 1988. [17] Rosso et al., in preparation. [18] V. Fleury, Nature 390 (13) (1997) 145.