Direct Electrodeposition of Copper Ladder Structures on a Silicon Substrate Ke Wang, Lianping Niu, Zhaocun Zong, Mingzhe Zhang,* Chen Wang, Xinjing Shi, Yongfan Men, and Guangtian Zou
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 442–445
National Laboratory of Surperhard Materials and Institute of Atomic and Molecular Physics, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed February 28, 2007; ReVised Manuscript ReceiVed NoVember 20, 2007
ABSTRACT: We report a copper electrodeposition with a microscopic structure of a regular ladder pattern on a silicon substrate where the ladder formation and evolution can be controlled by varying the electric field intensity during the electrodeposition. The deposit morphology varies from a periodic square-island structure to a ladder structure when the applied current is changed. The formation of a ladder structure with nanoparticles is related to the periodic potential measured across the electrodes. This method provides a deep insight into pattern formation in electrodeposition. Introduction Regular-pattern microscopic structures are attractive for their potential applications as interconnects in future generation nanometer-scale electronics, such as microelectromechanical systems, microelectronic devices, photonic materials, microchip reactors, miniaturized sensors, and separation techniques.1–6 Electrodeposition is one of the methods used to produce such regular microstructures. It has been used to produce materials with nano/microstructures that cannot be built with other traditional techniques, from well-ordered structures to many unexpected forms.5–8 These nano/microstructure patterns can be deposited on a substrate, either with or without template template assistance or with induced additives.6–9 In the thinlayer electrochemical deposition, a variety of growth morphologies have been observed, depending on the quasi-twodimensional experimental conditions,10–12 which are mainly governed by the Laplacian fields.13,14 It was found that the deposited patterns can be generated on the substrate and withdrawn from the solution.15 In an ultrathin electrolyte layer, where the electric migration overcomes the diffusion, the electrodeposits may have a very low branching rate16,17 or form regular arrays of metallic wires.6,7 Electrochemical deposition occurring at the interface is determined by diffusion, migration, convection, and chemical processes.18–25 The complex interplay of the spatial variation of charge density and ion concentration leads to diverse patterns of electrodeposition. In this paper, we report a very unique pattern of ladder structure obtained by electrodeposition of copper on a silicon substrate. The deposit morphology varies from a periodic squareisland structure to a ladder structure when the current applied across the electrodes during the electrodeposition process is changed. We suggest a scenario for the microscopic processes to explain the formation of such ladder patterns.
Figure 1. Schematic diagrams showing the procedure used to prepare the substrate for electrodeposition. (a) Electrochemical cell. (b) The surface of a silicon wafer is first oxidized. (c) OTS molecules are selfassembled into the silicon dioxide. for 3 min at 300 W (Templa System 100-E plasma system) and rinsed again with deionized water. These treatments caused the silicon surface to become fully hydrophilic. A surface structure with a -CH3 termination was obtained by immersing a silicon substrate into a 1 mM octadecyltrichlorosilane (OTS) (APTMS, Aldrich) solution of 1-phenyloctane (Aldrich) or hexadecane (Aldrich) for 15 min. This procedure was followed by ultrasonic cleaning and rinsing in chloroform to remove the polymeric residuals.
Silicon substrates (WaterNet Co., type N, 0.5 mm thick, orientation [100], resistivity 2–5 Ω · cm) were cut into ∼20 × 20 mm2 pieces before they were successively cleaned ultrasonically in acetone, chloroform, ethanol, and water for 10 min, rinsed with Millipore water, and blown dry with high-purity nitrogen. Finally, they were treated with oxygen
The experimental setup for the electrodeposition is shown schematically in Figure 1. The electrochemical cell consisted of a piece of silicon substrate with a monolayer-thick OTS surface, on top of which two electrodes were placed and covered with a clean glass plate. The experiment was conducted in a cell with two parallel electrodes made of a 40-µm-thick copper foil (99.9%) and separated by a distance of 8 mm. The electrolyte solution of 0.07 M CuSO4 with a pH of 1.9 was prepared with the analytical reagent CuSO4 (Fluka) and Millipore water. A peltier element and a temperature selector were employed to reduce the temperature and to solidify the electrolyte. An optical microscope (Leitz) was employed in situ to observe the electrodeposition of the thin layer. Temperature was controlled by circulators with a programmable or digital controller (Polystat). The experiments were carried out at -2.0 °C. The solute, CuSO4, was partially expelled from the solid in the solidification process because of the partitioning effect. Eventually, an ultrathin layer of concentrated CuSO4 electrolyte was formed between the ice of the electrolyte and the lower silicon substrate. Copper filaments developed on the cathode, and applying a constant current across this layer caused the filaments to move toward the anode on the substrate.
* To whom correspondence should be addressed. E-mail: zhangmz@ jlu.edu.cn.
The structure and the phase composition of the electrochemical deposit formation were determined by X-ray diffraction (XRD), electron diffraction analysis, and high-resolution electron microscopy (FEG
Experimental Section
10.1021/cg070200n CCC: $40.75 2008 American Chemical Society Published on Web 12/29/2007
Cu Ladder Structures Electrodeposition on Si
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Figure 2. Scanning electron micrograph showing the copper deposit with a ladder structure growing from a pH 1.9, 0.07 M CuSO4 electrolyte on the modified silicon oxide surface with a constant current applied across the electrodes. The SEM image on the lower left shows the tip of the ladder structure when the power supply for electrodeposition was switched off. The inset on the lower right corner is an enlarged view of the ladder structure.
Figure 4. Relationship between the periodic ladder structure and the measured oscillating voltage across the electrodes. (a) Ladder structure; (a′) voltage measured under the following experimental conditions: I ) 5.5 µA, T ) -2.0 °C, pH ) 1.9, and C ) 0.07 M. (b) Modified ladder structure; (b′) voltage measured under the following experimental conditions: I ) 5.0 µA, T ) -2.0 °C, pH ) 1.9, and C ) 0.07 M. (c) Periodic square-island structure; (c′) Voltage measured under the following experimental conditions: I ) 4.6 µA, T ) -2.0 °C, pH ) 1.9, and C ) 0.07 M. Figure 3. (a) XRD pattern of the electrodeposits. (b) TEM images and electron diffraction of the copper wire, which confirm that the wire is polycrystalline. The diffraction rings can be indexed as copper crystallites. (c) High-resolution electron microscope image of a portion of the copper wire shown in (b). Locations I and II are two individual copper grains, and the insets are the enlarged images of the corresponding areas, indicating that the grains are oriented with their [011] and [111] directions perpendicular to the picture (i.e., pointing into the paper). 2100). Scanning electron microsope (SEM) imaging was obtained with a field-emission SEM (JSM-6700f).
Results and Discussion Figure 2 is a scanning electron micrograph of a typical copper ladder structure resulting from the fabrication process described above. The ladder structure grows on a substrate with a relatively regular shape within a two-dimensional plane. The wires of the ladder are polycrystalline and have a diameter of about 200 nm, as shown in the inset. Figure 3a shows the XRD pattern of the electrodeposits. According to the standard diffraction database, the peak locations and the relative intensities of (111), (200), (220), and (311) can be identified as those of copper. The transmission electron microscopy (TEM) diffraction pattern, shown in Figure 3b, contains the polycrystalline diffraction rings of copper. The fine polycrystalline features shown in the micrograph are made of grains arranged side by side. Figure 3c shows the high-resolution transmission electron micrographs of a copper wire, where two individual copper grains (I and II), their orientations, and the boundary between them can be identified.
When a constant current was applied across the two electrodes of the cell, the cell voltage changed to an oscillatory periodic behavior. Figure 4 shows the relationship between the deposited pattern and the measured cell voltage (V) as a function of time (t). Figure 4a shows the ladder structure which was obtained when the applied current (I ) 5.5 µA) was above a certain threshold value, and the oscillation amplitude of the measured voltage across the electrodes was within a certain finite width (as shown in Figure 4a′). A larger amplitude width can lead to the destruction of the ladder structure. The measured voltage was periodic, with a dynamic range of 1.2 V, varying from 3.5 to 4.7 V, and a period of 1.43 s. The cycle of the voltage variation determined the periodicity of the ladder structure. Figure 4b,b′ shows that, as the applied current was decreased to I ) 5.0 µA, the ladder structure became distorted, and the measured-voltage dynamic range was reduced to 0.9 V, varying from 3.1 to 4.0 V, with a period of 1.02 s. Figure 4c,c′ shows that when the applied current was decreased to I ) 4.6 µA, the ladder structure was replaced by a periodic square-island structure in which the island chain was connected at the base by a thin deposit, and the measured-voltage dynamic range was 0.7 V, varying from 2.6 to 3.3 V, with a period of 0.55 s. The fact that the periodic structure of the deposit is related to the periodic variation of the measured voltage could be verified by in situ observation of the growth process. It took 1.43 s in case a, 1.02 s in case b, and 0.55 s in case c for the aggregates to develop one unit in the periodic ladder structure. When a constant current was applied to the electrodes, the cathode as well as its overpotential became more negative with respect to the anode. The Cu2+ ions were driven to the cathode
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Figure 5. (a) and (b) Schematic diagrams showing the sites of nucleation on the growth front and the profile of ionic transport in front of the electrodeposit. (c) Suggested local electric field distribution near the copper tip which causes the deposit to develop a ladder unit in the ladder structure.
by the electric field. According to the Nernst equation, the equilibrium electrode potential becomes less negative as the Cu2+ concentration increases.6 When both the cathode overpotential and the equilibrium potential reached the threshold for reaction, the nucleation and growth started, and the electrolyte began to be depleted near the growing deposit. During the electrodeposition, the cation transfer is controlled by diffusion, electromigration, and convection. The convection is reduced drastically in an ultrathin film electrodeposition. Hence, electromigration and diffusion become the major factors contributing to the cation transfer. When the electric field is high, the electromigration overcomes the diffusion in the thin layer of the electrolyte, which results in a parabolic-shaped profile of migrating ions, as shown in Figure 5a. This leads to the nucleation of crystallites only in the middle of the copper deposit, which form aggregates floating in the electrolyte. When the electric field is low, the component of the drift velocity of the ions related to the electric field is also low, and the diffusion becomes as important as the electromigration for the cation transfer. This leads to a somewhat flat profile, as shown in Figure 5b. Hence, the nucleation occurs over the entire growth front, and the crystallites grow uniformly at the interface. They fill up the space of the thin electrolyte layer and are firmly anchored to the substrate. The electric field distribution is essential for the formation of the ladder pattern on the substrate. We believe that the distribution of the local field near the growth front looks like the one shown in Figure 5c. During the growth process, the electric field distribution along the growth direction has a sinusoidal-like variation, as reflected by the periodic variation of the measured voltage, whereas the electric field distribution perpendicular to the growth direction is determined by the charge distribution on the center front tip of the copper deposit as well as that on the neighboring tips.6,7 As a result, the local electric field near the tip of a copper wire (or ladder) has a sinusoidal-like distribution, both along and perpendicular to the growth direction, as illustrated in Figure 5c. This particular form of electric field distribution, where the field is higher in the middle than at the four borders, affects the growth process and leads to the formation of the ladder structure. When the growth front is exposed to the minimum potential (less than 3.8 V, a value determined through experimental observation) of the sinusoidal-like variation, that is, at the four borders as shown in Figure 5c, the electromigration is low, the ion diffusion is high, and the ion transport has the profile shown in Figure 5b. With a sufficient supply of ions, the crystallites nucleate first at the corner sites, where the nuclei formation energy is the lowest, and then propagate over the entire interface. They grow steadily along the four borders, forming aggregates that are strongly bonded to the growth front and firmly anchored to the substrate. When the
Wang et al.
measured potential is larger than 3.8 V, the electromigration9 caused by the high electric field is strong, forming an ion transport profile of parabolic shape as shown in Figure 5a. This results in a large probability for the nuclei at the center site of the copper deposit to catch the ions from the fluid,27 leading to a steady growth of these nuclei at the midlevel. On the other hand, because of an insufficient supply of ions, nucleation at the corner sites is practically prohibited. These center crystallites form aggregates floating in the electrolyte and weakly bonded to the growth front.11 When the sample is taken out of the system, the part that is firmly anchored to the substrate remains and forms the side rails of the ladder structure, whereas the part that is weakly bonded to the growth front falls off and forms the void of the ladder structure, as shown in Figure 4a. When the measured voltage is below a certain threshold value, most of the sinusoidal-like variation is less than 3.8 V, and the conditions become favorable for crystal growth. The nuclei growing at midlevel and at the corners merge into one piece that is firmly anchored to the substrate, forming a square-island chain structure. These islands are connected at the base when the potential is at its minimum and the growth is at its slowest rate, resulting in the pattern shown in Figure 4c. Conclusions In conclusion, we report observations that copper ladder patterns can be deposited on a silicon substrate under electric field intensity control. The ladder structure with nanoparticles can be tuned by varying the applied current. This method offers an elegant means to growing and wiring metal patterns on a substrate. By demonstrating the effect of dimensionality of the growth system on electrocrystallization, the phenomena shown here provide deeper insights on pattern formation in electrodeposition. Acknowledgment. This work was funded by the Ministry of Science and Technology of China, No. 2005CB724404. It was also supported by the National Science Foundation of China, No. 50672029, and the International Cooperation Project of the Ministry of Science and Technology of China, No. 2001CB711201. The authors would like to thank Professor KehJim Dunn for helpful discussions and careful review of the article.
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