DOI: 10.1021/cg9012747
Growth and Branching Mechanisms of Electrochemically Self-Organized Mesoscale Metallic Wires
2010, Vol. 10 1455–1459
Sheng Zhong,*,†,§ Di Wang,§ Thomas Koch,†,§ Mu Wang,# Stefan Walheim,†,§ and Thomas Schimmel†,§ †
Institute of Applied Physics and Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology, Campus-S€ ud, D-76128 Karlsruhe, Germany, §Institute of Nanotechnology, Karlsruhe Institute of Technology, Campus-Nord, D-76021 Karlsruhe, Germany, and #National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China Received October 14, 2009; Revised Manuscript Received December 23, 2009
ABSTRACT: Electrochemical metallization with copper has been developed for on-chip interconnection in microelectronics. Therefore, the need for a detailed understanding of copper electrodeposition on the micrometer and nanometer scale is essential. In particular, approaches for the self-organized growth of mesoscale wires have become of great interest recently. Here, we investigate the details of the deposit morphology of such mesoscale self-organized copper wires, focusing especially on the origin of the branching mechanism. The details of the morphology inform how the wires develop and why the branching occurs. It also shows that branching is not only associated with system noises but also with the electrical and concentration field and their coupling with interface growth. We demonstrate that by carefully adjusting the growth parameters branching can be avoided. In this way, highly ordered, periodic parallel arrays of metallic mesoscale wires were fabricated using this novel approach for self-organized growth by electrodeposition in ultrathin electrolyte layers. The results provide a general explanation for morphology formation during nonequilibrium electrocrystallization in thin electrolyte layers. This knowledge helps to develop processes for the controlled fabrication of micro- and nanostructured metallic patterns by electrochemistry. Electrochemical metallization with copper is used in microelectronics for on-chip interconnection due to the superior conductivity and the high electromigration resistance of copper.1-3 However, the electrodeposition of metals usually produces ramified deposits.4 Though ramified deposits are useful in some cases,5-7 the formation of homogeneous and uniform deposition patterns is strongly desired for industrial applications, especially in micro- and nanoelectronics.3,4 Usually, fabrication of uniform metal patterns needs the aid of templates.8-14 With the help of ordered channels in anodized aluminum oxide,10,11 lithographic technologies,8,9 and patterned Langmuir-Blodgett (LB) films,13,14 a regular metallic deposition pattern can be fabricated. This strategy, however, involves complicated processes to prepare or remove, if needed, the templates, a process that is time-consuming, costly, and arduous. The other strategy is self-organization, by which one can directly and efficiently form a regular pattern on substrates.15-25 We have developed a new approach, known as ultrathin layer electrochemical deposition (ULECD), allowing for the self-organized growth of regular patterns of copper and other metals.13-21,25,26 However, there is a need for the fundamental understanding of the detailed mechanisms of copper electrodeposition and pattern formation, a prerequisite for the controlled growth of defined regular patterns. One of the key questions in self-organized growth of patterned mesoscale metallic arrays is the branching mechanism of metallic wires. The studies of crystal growth suggest that branching in electrochemistry is related to the noise in transport fields. In electrodeposition, this means the concentration field and the electrical field which are both Laplace fields.27-31 It is important to know if the ramified features remain when the external agitations are suppressed or if regular patterns can be electrodeposited directly for this case. In order to suppress this noise, agents such as an agarose gel were introduced into the electrolyte.32 *To whom correspondence should be addressed. Address: Institute of Nanotechnology, Karlsruhe Institute of Technology, Postfach 3640, D-76021 Karlsruhe, Germany. Tel/Fax: þ49 7247 82 2511/þ49 7247 82 6368. E-mail:
[email protected].
Though the experimental results showed that the deposits were tentatively more regular, side effects in using these additives in these cases made the whole experiment much more difficult to interpret. Our ULECD system can suppress the noise without any additives due to a very thin electrolyte layer (about 300 nm). In this way, more regular morphologies are obtained, allowing fabrication of a nonbranching parallel filament pattern. A conclusion one could draw is that the noise is one important factor that contributes to ramification.15,16,26 Yet, noise cannot explain the formation of the parallel filament pattern. The profile of the electrical field in front of the growth interface should be considered to understand the formation of regular patterns.17,18 But the mechanism of how branching initially occurs and consequently develops has not yet been shown. On the other hand, a detailed understanding of the branching mechanism is a prerequisite for effectively developing antibranching concepts. So far, the suggested explanations for pattern formation (see above) have not taken into account details of nucleation and interface kinetics, although both play critical roles in crystal growth.33 The central goal of this study is to understand the mechanism of branching in copper electrocrystallization. The details of the deposition give information about how nucleation occurs at the growing tip of the filaments and how branches develop. A mechanism of bifurcation is discussed, in which the transport fields in front of the growth interface control the nucleation and the splitting of the filaments and play a key role in pattern formation. The experimental procedure is summarized in Figure 1. The cell for electrodeposition consists of two parallel glass slides separated by spacers. As spacers usually the electrodes are used. The thin electrolyte layer is maintained between two glass slides. As shown, the electrodes are made by copper rods. Two kinds of cell geometries were used in our experiments: one is the “circular cell”, with a circular ring-shaped anode (diameter 20 mm) and a point cathode (diameter 0.5 mm) within the center of the cell; the other one is the “parallel cell” with two parallel wire-like electrodes facing each other (distance 8 mm and diameter 0.25 mm). The
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Figure 1. Schematic diagrams of two types of electrodeposition cells (A) and the process of generating copper deposits (B-E). (A) Circular cell and parallel cell seen from above. (B) Slow freezing of the CuSO4 electrolyte. Because of the segregation effect, a thin aqueous layer of electrolyte forms between the glass slide and the ice. (C) By applying a constant voltage between the two electrodes, the copper deposition starts at the cathode, the deposits subsequently growing from the cathode into the electrolyte. At the end of the experiment, the copper deposits propagate into the whole electrodeposition cell. (D) After deposition, cooling is stopped and as a result, the ice melts. (E) Finally, the copper deposits are taken out of the electrodeposition cell, rinsed with deionized water, and subsequently dried for further studies or application.
electrolyte was CuSO4 with a concentration of 0.05 M and pH of 4.5. Before the external potential was applied, the electrolyte between the glass slides was solidified by decreasing the temperature to a preset value (e.g., -5 °C). During solidification, CuSO4 is partially expelled from the ice into the electrolyte due to the segregation effect. When equilibrium is reached, an ultrathin aqueous layer of concentrated CuSO4 electrolyte is trapped between the ice interface and the upper glass slide (Figure 1A). The thickness of the electrolyte layer can be easily tuned by changing the temperature. It is variable from tens of nanometers to hundreds of nanometers.26 The electrodeposition will take place within this ultrathin layer of electrolyte. Subsequently, either a constant voltage or a constant current is applied between the two electrodes. As a consequence, the deposits emerge on the cathode and grow laterally along the surface of the glass substrate into the direction of the anode with a growth rate of several μm/s (Figure 1B). When the electrodeposition is finished, the temperature is increased to melt the ice (Figure 1C). The deposits are robustly adhesive on the glass substrate and can be easily taken out of the cell for further studies and experiments (Figure 1D). For further characterization of the samples, atomic force microscopy (AFM) was carried out in the intermittent contact mode (“tapping mode”). The optical microscopy (OM) images were taken by LEICA DMRE. The scanning electron microscopy (SEM) images were taken with a LEO Gemini2 System (ZEISS). The transmission electron microscopy (TEM) analysis was performed by FEI TITAN System. Figure 2 shows AFM images of branching metallic Cu structures deposited by the procedure described above. The images reveal that the fingerlike branches of the Cu structures consist of many straight filaments with periodical corrugated nanostructures on the surface. Numerous bifurcations are found in Figure 2A. Figure 2B shows the morphology of the copper
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Figure 2. Details of the morphology of the copper filaments grown in a circular cell (ring-shaped anode diameter is 20 mm) observed by AFM. (A) AFM-image of the finger pattern. A periodic structure can be seen on the surface of the filaments with the periodical length of approximately 70-80 nm. (B) Morphology of the copper filaments at higher magnification. The corrugated structures on the neighboring branches are correlated in position, which can be easily identified in the region of bifurcation, as indicated by the circles made of the white dashed line. (C) and (D) illustrate the morphology of the tip region of the filament. It can be seen in (C) that the filaments develop by heterogeneous nucleation on the tip. As indicated by the two arrows in (C), more than one nucleus can be initiated simultaneously on the tip. The morphology at the early stage of bifurcation can be seen in (D), where crystallites nucleate on two corner regions of the tip simultaneously. The size of the crystallites is only about several tens of nanometers. The experiment was carried out at a temperature of T = -5 °C, applying a constant voltage of V = 4.0 V at pH = 4.5.
filaments at higher magnification. A detailed analysis of the data of this and further images shows that the periodical corrugations of neighboring filaments correlate in their position, which can be easily identified at the branch-splitting sites. Since the periodical nanostructures reflect corresponding processes at the growth interface, the coherence of these structures indicates that they were generated simultaneously. The coherent periodical growth of the filaments is associated with an spontaneous oscillation of the electric current which, in turn, gives evidence for the coherent periodical growth and vice versa.15,16 Keeping this in mind, we can conclude that when a bifurcation happens, two crystallites nucleate on two corner regions of the same growth tip at the same time, which can be indeed observed experimentally, as shown in Figure 2C (two arrows). We find that no nucleation occurs behind the tips, a fact that can be explained because of the lower Cu2þ ion concentration behind the growth front and thus behind the tips of growing filaments, in agreement, for example, according to the theory of Chazalviel.34 As a direct consequence of the development of two nuclei at one and the same tip at the growth front a bifurcation occurs. An example is given in Figure 2D. To find out more about the origin of branching, we analyzed the changes of morphology along a filament. Figure 3 shows a typical wire and the analysis of the width of the stacked periodic structures (“slices”) as a function of the number N of the periodical structures, counted from an arbitrary fixed point. Figure 3A shows that bifurcation always occurs at the widest diameter of the growing filament. As a consequence, bifurcation is always connected with a discontinuous decrease in the width of the stem. Prior to bifurcation, the width of the filament increases continuously. A quantitative analysis of the width of the stem versus
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Figure 3. (A) Detailed morphology of the copper filaments imaged by AFM. Before bifurcation the width of the filaments is gradually increasing. After bifurcation, the width of each split branch drops discontinuously. Usually one of the split branches is terminated after a short length. The correlation of the corrugated structures on the neighboring branches can be seen clearly. (B) The width of the filaments measured along a main branch plotted against N, which is the number of corrugations counted from a fixed point, which scales with the distance of the measuring site from this fixed point. The measurements in (B) were carried out by selecting a “surviving” branch (i.e., along the main branch of the filament and not along one of the “dead end branches”). The experiment was carried out in a circular cell at a temperature of T = -5 °C, applying a constant voltage of V = 4.0 V at pH = 4.5.
position is shown in Figure 3B. Each time bifurcation occurs, the width of the main branch suddenly drops resulting in a maximum of the width at the point of bifurcation. Subsequently, a new cycle starts. After bifurcation, one side-branch finally gets thinner and thinner and then stops growing, most probably due to the screening effect.29 The other branch continues to grow, increasing continuously in diameter until a new bifurcation occurs. In the following, we will discuss the branching mechanism in more detail. It has been discussed previously that the branching in crystal growth is noise-controlled. Yet computer simulations show that even at zero-noise, tip splitting still appears.35 The experimental investigation of this problem is not trivial, as it is not easy to control the noise inside the commonly used electrodeposition cells. Our special experimental design provides an effective way to suppress noise during electrochemical growth. In this way, the role of the transport fields (here including the concentration field and the electrical field) on morphology formation becomes prominent. If the interfilament distance is much smaller than the depth of the diffusion layer, the tip of the filament experiences a one-dimensional (1D) concentration field. So in this special case, the width of the filaments could be maintained constant during growth. On the other hand, if the interfilament distance is much larger than the depth of the diffusion layer, the tip actually encounters a two-dimensional (2D) concentration field. “Nutrient” in the form of Cu2þ will be transported to the growing tip from the region around the tip from different directions corresponding to an opening angle of approximately 180°. The width of the filaments now can increase gradually. Thus, the number of potential nucleation sites and the maximum distance between them increase, allowing for the independent heterogeneous nucleation of two tips at the same time. It is well-known in the theory of crystal growth36,37 that the concentration gradient on the two corners of the filament tip is larger than that in its middle part. This difference increases with the widening of the growing interface. Figure 4 shows the simulated concentration gradient distribution for two different values of the tip width. The simulation was done using the Matlab PDE toolbox. The basic equation is the Laplace equation. Left and right boundary conditions both
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Figure 4. Computer simulation of the distribution of the concentration gradient for two different tip widths: (A) the narrow tip; (B) the wide tip. Arrows indicate the flux of ions. The concentration gradient gets stronger on the two corners, the difference to the center area increasing with the width of the growing interface, thus leading to a significant increase of the nucleation probability at these two corner points.
are Neumann conditions. For all other boundaries (top, bottom, and wire surface), Dirichlet conditions were used. With increasing tip width, the concentration gradient at the two corners increases, driving more ions into these two points. For a sufficiently wide filament tip the nucleation on the corner regions will have much higher probability than in the center part. So a bifurcation of the filaments occurs, as shown in Figure 2. This also explains why we only rarely observe events of a splitting of the filaments into three or more parts. We expect that this bifurcation mechanism plays the key role in noise-reduced ramified growth. In addition, the copper branches in Figure 3 are much more regular than those from previous reports,38-40 which can also be related to the suppression of noise in our experimental geometry. The macroscopic morphology of the copper deposits presented in this paper actually depends on both the bifurcation rate and the extinction rate of the filaments, the extinction rate being the rate of events when a filament is screened by the neighboring branches and stops growing. When the bifurcation rate is higher than the extinction rate, a gradual widening of the cellular pattern is observed. The local electrical field strongly influences the balance of these two rates. In the circular electrical field, which is generated by circle and dot electrodes (see above), nutrient is driven to the growing front from different directions, which promotes the widening of the tips and supports the bifurcation of the copper filaments. If, however, the electric field at the growth front is homogeneous in space, the bifurcation rate will not be higher since the tendency to broaden the filaments is limited. This situation can be realized with two parallel electrodes. Changing the electrical field (by changing the applied potential) also influences the morphology.17,18 The depth of the diffusion layer in front of the growth interface is determined by the competition between the ion transport into the diffusion layer and the ion consumption at the growth interface between metal and electrolyte. If the depth of the diffusion layer is thick, the individual diffusion layers of adjacent growing filaments will interpenetrate each other resulting in 1D transport fields. Otherwise, the growing filaments will face 2D transport fields, leading to bifurcations. Obviously, the applied potential can influence this critical depth of diffusion layer. Therefore, by applying a suitable voltage cross two parallel electrodes of the parallel cell, it is possible to fabricate on purpose deposits with parallel filament (wire) morphology. Figure 5 shows the different patterns that form under
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Figure 5. OM images (left) and SEM images (right) demonstrating copper film, parallel filaments, and branching filaments prepared in a parallel cell under different applied voltages: (A and A0 ) 1.5 V; (B and B0 ) 1.8 V; (C and C0 ) 2.4 V. From SEM image, the wavelike periodic structures can be observed on the surface of deposition, reflecting the growth interface shape. The deposition pattern strongly depends on the applied voltage. The white arrows in the SEM image indicate the growth direction. The experiment was carried out in the parallel cell at a temperature of T = -1 °C, and pH = 4.5. The distance of the two electrodes is 8 mm.
different applied voltages. The left column shows OM images and the right column shows SEM images. By increasing the applied voltage, the pattern changes from film, over parallel filaments (wires), to branching filaments. From the SEM images, the wavelike corrugated periodic structures are observed on the surface of deposition. These periodic structures reflect the growth interface and its development. The results show that the morphology of deposits strongly depends on the applied voltage. The high potential means high driving force for crystallization. With the increasing driving force, the growing front with flat interface comes to be destabilized and leads to formation of dendrites.41 Therefore, the growth pattern changes from film with a flat growth interface to the filaments when increasing the applied voltage. To avoid the branching, the key point is to control the depth of the diffusion layer. As mentioned above, if the diffusion layer depth is much larger than the interfilament distance, the tip of the filament experiences a 1D concentration field. Otherwise, the tip experiences a 2D concentration field. Under the 2D condition, the bifurcation mechanism takes effect. The tip of filaments widens and hence the branching emerges. By carefully tuning the experimental conditions (here is the applied voltage) to influence the ratio of the depth of diffusion layer and the interfilament distance, we can induce the growth of long, straight copper filaments. Figure 5B demonstrates parallel copper filaments almost completely without branching. The microstructure of crystals can influence their growth morphology.42 The microstructures for respective growth patterns have been investigated by TEM. Figure 6 shows the bright field images (left), corresponding diffraction patterns (inset), and dark field images (right) of the different growth patterns: film, parallel filament (wire), and branching filament, respectively (top to bottom). Dark particles on bright field images are Cu2O particles generated from transient deposition after switching off the applied voltage. The TEM results confirm that the deposits are polycrystalline, showing perfect rings on diffraction patterns (insets of Figure 6). There is no significant difference in grain size for respective growth patterns visible in the dark field images as well. Thus, the microstructure seems not to play a key role on
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Figure 6. TEM analysis of microstructures’ respective growth patterns. Bright field images (left) and corresponding diffraction patterns (insets). The corresponding zoom-in dark field images (right). (A and A0 ) copper film, V = 1.5 V. (B and B0 ) copper filament without branching, V = 1.8 V. (C and C0 ) copper branching filament, V = 2.4 V. The numbers labeled on the first diffraction pattern represent: Cu2O{111}(1); Cu{111}(2); Cu{200}(3); Cu2O{220}(4); Cu{220}(5); Cu{311}(6). Copper deposits are typical polycrystalline, indicated by perfect rings shown in diffraction patterns. The grain sizes for different patterns (A-C) are not significantly different. The growth pattern does not show a strong relationship with their microstructures. Other experimental parameters are T = -1 °C, pH = 4.5, the parallel cell with 8 mm electrode distance.
Figure 7. (A) An example of the profile plot of the film pattern (integrated intensities of first three rings around concentric circles as a function of the distance from a point from film diffraction pattern). The black, green, and red lines represent as follows: a profile plot, three fitting peak curves, and a summary fitting curve. (B) The ratio of the area of the Cu2O{111} peak versus that of the Cu{200} peak, which indicates the proportion of Cu2O and Cu concentration, decreases from 1.37 to 0.18.
pattern formation. It is worth noting that the intensity of the first and fourth ring in diffraction patterns change. From the diffraction patterns, there are the rings attributed by Cu2O (1 and 4) besides the rings by Cu (2, 3, 5, 6). The intensity of Cu2O rings reduce from a film pattern to branching pattern. We examine the concentration of Cu2O by analyzing the diffraction intensity of different patterns (Figure 7). First, we integrate intensities of the
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first three rings around concentric circles as a function of distance from a point to produce a profile plot. An example of a profile plot from the film diffraction pattern is given in Figure 7A. To separate the plot into three peaks, we use Origin multipeaks function. The ratio of the area of the Cu2O{111} peak versus that of the Cu{200} peak, which indicates the proportion of the concentration of Cu2O and Cu, decreases from 1.37 to 0.18 (Figure 7B). Although the data in Figure 7B are qualitative, they do indicate that the concentration of Cu2O in film is higher than that in the branching filament. This can be easily explained because the Cu deposition potential is more negative than that of Cu2O, and therefore the copper deposition under a high voltage condition is much higher than that under a low voltage condition. We have studied the growth and branching of copper filaments from an ultrathin layer of solution by electrodeposition. The effective suppression of noise was achieved by using ultrathin layer electrochemical deposition (ULECD). In this way, it was possible to study the role of the transport fields on the nucleationcontrolled deposition and to pinpoint the branching mechanism in ramified electrochemical growth. We have shown that for a sufficiently wide filament tip the nucleation on the corner regions will have much higher probability than in the center part, resulting in a bifurcation of the filament, accompanied by a simultaneous drop in filament width. In general, the transport fields govern the pattern formation in the electrodeposition by controlling both the local nutrient transport in front of the growing interface and the nucleation behavior on the growing tips. The unique and tunable growth and bifurcation behavior in restricted geometries demonstrated in our experiments opens intriguing perspectives for the controlled morphology-selective, self-organized growth of metallic patterns on the micrometer and nanometer scale.
(5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27)
Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft within the DFG-Center for Functional Nanostructures (CFN) and by the Landesstiftung Baden-Wuerttemberg within the Research Network on Functional Nanostructures.
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