Effects of a micro pattern on Cu alloy electrodeposition and its application as an oil detector
© The Author(s) 2016
Received: 5 October 2016
Accepted: 17 October 2016
Published: 24 October 2016
In this study, the effects of open area ratio (OAR) variations by micro-patterns on Cu alloy electrodeposition were analyzed experimentally. To change the OAR of the samples, a strip-type micro-pattern was formed on a substrate through a photolithography process. Moreover, the OAR was controlled by adjusting the distance of the stripe pattern to a width of 20 μm. When electrodeposition was applied on a non-patterned substrate with an OAR of 100%, a pillar-type Cu alloy structure was produced. In addition, when the OAR was decreased to 40%, the height of the Cu alloy structures was increased. However, when the OAR was decreased to 20%, no electrodeposited structures were formed. To confirm the industrial effectiveness of the electrodeposited structures on a micro-pattern, the Cu alloy electrodeposited structures were applied to the formation of an oil detector.
Micro- and nanostructured metal substrates are widely used in various industrial fields including surface modification, anti-corrosion, solar cells, and microelectronic interconnection [1–5]. Representative methods for producing metallic micro-and nanostructured substrates include an electroplating method using a micro- or nano-patterned mold, a dry or wet etching method using an etching barrier, and laser machining [6–8]. However, these methods require considerable time and cost for producing metallic microstructures.
To overcome this problem, an electrodeposition technique without a mold was proposed [9, 10]. Using this electrodeposition method, the time and cost required for the fabrication of metallic microstructures can be significantly reduced. Moreover, electrodeposition methods allow the formation of various shapes of the metal alloy structure by controlling simple variables such as the stirring rate, temperature, and applied current density [11, 12].
Representative materials for producing a metallic microstructure through an electrodeposition technique include Cu, Au, Ni, Ag, and Sn [11–16]. Among them, Cu and Cu alloys are excellent engineering materials, and have significant advantages including a low chemical reactivity, low cost, high electrical conductivity, and good thermal conductivity [12, 17]. Therefore, many researchers have been studying methods for producing Cu or Cu alloy microstructures through an electrodeposition technique. However, most researches on Cu or Cu alloy microstructure formation have utilized non-patterned substrates.
In this paper, we produced a Cu alloy microstructure on a stripe-type micro-patterned substrate. Moreover, the effects of the open area ratio (OAR) variations from a micro-pattern on the formation and growth of Cu alloy structures was analyzed. Furthermore, to evaluate the effectiveness, our research group applied a Cu alloy electrodeposited structure on a micro-pattern for the fabrication of an oil detector.
The copper electrodeposition solution used was composed of 0.6 M CuSO4·5H2O (Dae Jung, Korea) and 1.0 M boric acid (H3BO3, Dischem, USA). To apply the electrodeposition, an electroplating machine (Sung Won Forming, Korea) was used. In addition, a copper plate (Daeguang metal, Korea) with dimensions of 3 × 3 cm was used as an anode. For the fabrication of a cathode, Cr (50 nm) and Cu (500 nm) were deposited sequentially on a silicon wafer (Win Win Tech, Korea). The silicon wafer was then diced to a sample size of 1 × 1 cm. In addition, the solution temperature, stirring rate, and applied current density were maintained at 60 °C, 200 rpm, and 50 mA/cm2, respectively. For the stirring of the solution, a magnetic stir bar (Cowie technology, UK) with a diameter of 0.8 cm and length of 5.0 cm was used. The morphology of the electrodeposited structure was observed using a scanning electron microscope (SEM) (S-4800, HITACHI, Japan). Furthermore, an energy dispersive spectroscope (EDS) (7593-H, HORIBA, Japan) was used to analyze the composition of the fabricated samples.
Results and discussion
Results of Cu alloy electrodeposition on non-patterned substrate
O-Cu composition ratio at each position shown in Fig. 1
O:Cu [atomic %]
O:Cu [atomic %]
When electrodeposition was conducted using a CuSO4·5H2O solution, Cu and Cu2O were deposited at the same time. The electrodeposition mechanism of Cu and Cu2O can be described through Eq. 1 (reduction of Cu2+ ions) and Eq. 2 (reduction of Cu+ ions) [11, 19].
As shown in Eqs. 1 and 2, Cu2+ ions in a CuSO4·5H2O solution can be precipitated into Cu2O or Cu. Moreover, the deposited Cu2O can be reduced to Cu metal through Eq. 2a. However, when Cu2O structures grow larger before a reduction, a larger Cu2O structure has difficulty converting into Cu metal . Because the resistance of the Cu2O is higher than that of Cu, a charge is difficult to transfer to large Cu2O structures. On the other hand, a small Cu2O structure is easily converted into Cu through Eq. 2a. Therefore, when electrodeposition is applied using a CuSO4·5H2O solution, a Cu and Cu2O structures are formed separately.
Figure 1b shows the shape of the electrodeposited structure formed when the solution was stirred at 200 rpm during the electrodeposition. When the solution was stirred, a pillar-type electrodeposited structure was generated. The promotion of Cu2O deposition through stirring is regarded as the reason for the pillar structure formation.
Because the equilibrium electrode potential of Cu2O (0.347 V) versus a standard hydrogen electrode is higher than that of Cu (0.297 V), Cu2+ ions in an electrodeposition solution are usually precipitated into Cu2O rather than Cu . However, when the diffusion rate of Cu2+ ions is lower than the charge supply rate, the deposited Cu2O is actively converted into Cu through Eq. 2 instead of insufficient Cu2+ ions. Under this condition, the deposition of the Cu2O is controlled through the diffusion of the Cu2+ ions. Moreover, Cu2O deposition is concentrated at the top of the electrodeposited structure with a short diffusion distance.
Therefore, when Cu2O deposition is controlled by the diffusion of the Cu2+ ions, Cu2O deposition promotes a vertically oriented growth of the electrodeposited structures. On the other hand, the reduction of the Cu2O is not affected by the diffusion of the Cu2+ ions. Cu precipitation through a reduction in Cu2O can be achieved throughout the entire area of the Cu2O structure. Therefore, a reduction in Cu2O promotes isotropic growth of the electrodeposited structures.
Results of Cu alloy electrodeposition on micro-patterned substrate
Design and electrodeposition conditions of the samples as a function of the OAR
Open area ratio (%)
Pattern width (μm)
Applied current (mA)
Applied current density (mA/cm2)
In addition to the variations in size, when the OAR was decreased to 40%, tree-type electrodeposited structures were actively formed (Fig. 4c). This phenomenon is thought to have originated from the growth rate increase of the electrodeposited structure. Figure 5 shows the formation mechanism of the tree-type structures. When the height of the electrodeposited structures was increased, the deposition of the Cu2+ ions was concentrated at the top-edge of the structure with a short diffusion distance (indicated by the red circle in Fig. 5b). This phenomenon can trigger the formation of new branch structures. The repetitive formation of branches produces the tree-shaped Cu alloy micro-structure (Fig. 5c).
When the OAR was decreased to 20%, no electrodeposited structure was formed. This is thought to have originated from the applied decrease in potential. Because the applied potential is proportional to the applied current, the applied potential is decreased with a decrease in the OAR (Table 2). For a reduction of the Cu2+ ions, a negative potential below the reduction potential needs to be applied. When the OAR is decreased to 20% (i.e., the applied current is 10 mA), an insufficient potential is applied to the sample for Cu or Cu2O deposition. Figure 4e proves this phenomenon. Figure 4e shows the electrodeposition results of the non-patterned substrate when the applied current is 10 mA. In this case, no electrodeposited structure was formed.
Application as an oil detector
On the other hand, when the oil detector was inserted into oil, the oil was easily diffused into the space between the two stripe patterns (Fig. 9c). Furthermore, when the detector was pulled out, residual oil remained in the space between the stripe patterns (Fig. 9d), which resulted in a variation of the detector’s capacitance. Owing to this phenomenon, Cu-alloy electrodeposited stipe patterns can be used as an oil detector.
Capacitance measurement results of the fabricated oil detector
No. 1 (nF)
No. 2 (nF)
No. 3 (nF)
After removing from the water
After removing from the water with a 3 mm thick light oil film
In this paper, the effects of the OAR variations using micro patterns on Cu alloy electrodeposition were analyzed. To discover the influence of the OAR variation, a stripe-shaped micro-pattern was formed through the lithography process. By adjusting the distance of the stripe patterns with a width of 20 μm, samples with an OAR of 100, 80, 60, 40, and 20% were produced. The applied current density of the electrodeposited sample was fixed to 50 mA/cm2. Under this condition, when the OAR was decreased from 100 to 40%, the height of the electrodeposited structures showed a tendency to increase. On the other hand, no electrodeposited structure was formed when the OAR was 20%. To confirm the industrial effectiveness of a Cu alloy electrodeposited structure on a micro-pattern, Cu alloy structures on a stripe-type micro-pattern were applied to the fabrication of an oil detector. The fabricated oil detector is able to detect an oil film of 3 mm in thickness by measuring the variations in capacitance.
JML participated in design, fabrication, and test the device and drafted the manuscript. JSK conceived of the study, reviewed all test methods and results, and finalized the drafted manuscript. Both authors read and approved the final manuscript.
This work was supported by a 2 year Research Grant of Pusan National University.
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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