Optimization of copper electroplating process applied for microfabrication on flexible polyethylene terephthalate substrate

Copper sulfate pentahydrate (CuSO₄.5H₂O), sulfuric acid (H₂SO₄), and hydrochloric acid (HCl) were bought from Merck (Germany). Cu-7979, a surface additive, was bought from the MinhChat Company. PET substrates are commercial products; their thicknesses are about 80 μm.

2.2.1. Experimental preparation

Firstly, PET substrates were cleaned and used for depositing a thin film of copper using a dc magnetron sputtering Leybold Univex 350 system with the appearance of inert argon (Ar) gas. These parameters were shown in table 1. Almost all samples have the same dimension of 5 × 5 cm², except samples for Hull cell testing, which have a dimension of 10 × 10 cm².

Table 1.  Parameters of sputtering system.

Power (W)	Time (min)	Argon (sccm)	Pressure (mbar)
100	15	5	2.1 × 10−3

Chemical compositions of four electroplating solutions are shown in table 2. The concentration of sulfate anion, hydrochloric acid, and Cu-7979 were kept invariable in all four solutions. Four electroplating solutions had the ratio of copper (II) sulfate (CuSO₄) concentration to sulfuric acid (H₂SO₄) concentration of 1.33, 0.75, 0.40, and 0.17 and were called 01–1.33, 02 − 0.75, 03 − 0.40, and 04 − 0.17, respectively.

Table 2.  Chemical composition of electroplating solutions.

	01–1.33	02–0.75	03–0.40	04–0.17
Sulfate anion (mol l−1)	1.4
CuSO4 (mol l−1)	0.8	0.6	0.4	0.2
H2SO4 (mol l−1)	0.6	0.8	1.0	1.2
CuSO4/H2SO4	1.33	0.75	0.40	0.17
HCl (μl l−1)	137
Cu-7979 (ml l−1)	10

2.2.2. Hull cell testing

Before electroplating, samples of copper thin film on a PET substrate were dipped in these solutions and were tested by a 267 ml Hull cell bath in order to find the range of suitable current density for each solution. Hull cell testing is considered to be a simple method to control and evaluate various parameters of electroplating processes [8]. The dimensions and top section of the Hull cell bath are shown in figure 1. The current densities along the cathode are shown in following equation [8]

$$\begin{eqnarray}&&{{j}_{c}}=I(51-52.4{\rm log} {{x}_{c}}),\end{eqnarray} \tag{ 1 }$$

where j_c is the cathode current density (mA/cm²), I is the total current electroplating (A), and x_c is a coordinate along the cathode (cm).

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The result of Hull cell testing with I = 1 A in a solution of 03 − 0.40 is shown in figure 2(a). This result has shown that the copper surface was good when x_c was between 1.5–6.5 cm. According to equation (1), the range of suitable electroplating current density was between 8.4–41.8 mA cm⁻². However, the experimental result has shown that the electroplating current density of 40 mA cm⁻² was not suitable for electronic devices with small scale, due to bad adhesion (figure 2(b)). Therefore, the electroplating current densities that were chosen to investigate for each of the four solutions were 10, 15, 20, and 30 mA cm⁻². These current densities were also suitable for three solutions 01–1.33, 02 − 0.75, and 04 − 0.17, due to the results of Hull cell testing of these solutions.

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2.2.3. Electroplating experiments

After finding a range of suitable electroplating current densities for four solutions by Hull cell testing, 16 samples of copper thin film on a PET substrate were electroplated in four solutions 01–1.33, 02 − 0.75, 03 − 0.40, and 04 − 0.17, corresponding with four current densities 10, 15, 20, and 30 mA cm⁻². All the samples were electroplated with the same electrical quantity of 0.248 A.h. The dimension of the samples is 5 × 5 cm².

Thicknesses and sheet resistances of samples were measured by a Dektak 6 M surface profiler and a four-point probe from Lucas Labs Division/QuadPro S302-8, respectively. Resistivities of samples were calculated by the following equation [9]

$$\begin{eqnarray}&&\rho ={{R}_{s}}t=\frac{\pi }{{\rm ln} 2}\frac{V}{I}t,\end{eqnarray} \tag{ 2 }$$

where ρ is the resistivity of sample (Ω m), R_s is the sheet resistance of sample (Ω/▯), and t is the thickness of sample (m). In addition, surfaces of samples were characterized by scanning electron microscopy (SEM) Jeol JSM-6480LV.

The thicknesses of samples (μm) are shown in table 3.

Table 3.  Thicknesses of electroplated samples (μm) in four solutions 01–1.33, 02 − 0.75, 03 − 0.40, and 04 − 0.17, corresponding to four electroplating current densities 10, 15, 20, and 30 mA cm⁻².

	Thicknesses of samples (μm)
	01–1.33	02 − 0.75	03 − 0.40	04 − 0.17
10	12.36 ± 0.97	8.92 ± 1.24	10.10 ± 0.50	8.45 ± 0.57
15	12.56 ± 1.22	8.80 ± 1.06	9.40 ± 0.78	8.01 ± 0.62
20	13.32 ± 1.55	10.46 ± 1.57	9.89 ± 0.97	7.67 ± 0.45
30	14.08 ± 1.78	12.32 ± 0.78	9.05 ± 0.79	9.91 ± 1.34

All the samples have the same electrical quantity of the electroplating step. According to Faraday's law, which is shown in the following equation, the weight of copper liberated at the anode is proportional to the electrical quantity

$$\begin{eqnarray}&&m=\frac{MQ}{Fz},\end{eqnarray} \tag{ 3 }$$

where m is weight of material liberated at the anode (g), M is the molar mass of material at the anode, Q is the electrical quantity passed through anode (C), F = 96 485 C mol⁻¹ is the Faraday constant, and z is the valence number of ions of the material at the anode.

All the samples were electroplated with the same electrical quantity so that the mass of copper liberated at the anode in all experiments has the same theoretical value. Nevertheless, the results of the measurement have shown that the thicknesses of samples were not the same (table 3). This can be explained by the difference in electroplating solutions, chemical compositions, and electroplating current densities. Indeed, when the electrons went to cathode, three reactions shown in the following equations could occur in the electroplating process:

$$\begin{eqnarray}&&C{{u}^{2+}}+2{{e}^{-}}\to Cu,\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&2{{H}^{+}}+2{{e}^{-}}\to {{H}_{2}},\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&2{{H}_{2}}O+2{{e}^{-}}\to {{H}_{2}}+2O{{H}^{-}}.\end{eqnarray} \tag{ 6 }$$

Therefore, the appearance of a H⁺ ion led to some side reactions so that the electroplating efficiency was reduced. The higher the sulfuric acid concentration was, the less efficient it was. Indeed, it is easy to notice the reduction of thickness when the ratio of copper (II) sulfate (CuSO₄) concentration to sulfuric acid (H₂SO₄) was decreased from 1.33 to 0.17 (figure 3(a)).

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However, the solution with a high concentration of sulfuric acid has the advantage of high surface uniformity. The lowest thickness error was 0.45 μm at solution 04 − 0.17 and a current density of 20 mA cm⁻². In addition, thicknesses of samples deposited in solution 03 − 0.40, which have the ratio CuSO₄/H₂SO₄ of 0.40 with various current densities, did not have the large difference (smallest value was 9.05 and largest value was 10.10) (figure 3(b)). This advantage gave this solution an opportunity to be applied to fabricate complicated copper details, which include many edges and polygons. In fact, when complicated copper details were electroplated, the electroplating current density was distributed unevenly: larger at the edge of polygon and smaller at the inside of the polygon. Solution 03 − 0.40, which had a small difference of thickness corresponding with various current densities, could be applied to solve this problem and fabricate complicated electronic details with high surface uniformity.

Thicknesses of the samples were also influenced by electroplating current densities (figure 3(b)). In general, thickness of the sample was increased when current density was increased. Indeed, when the current density increased, the current also increased, and the duration for the electroplating process decreased. It means that the number of electrons which went to cathode per second was higher, and the copper layer was electroplated faster. The faster the electroplating process was, the more pores and defects were formed. This conclusion is verified by SEM images of samples surfaces in figure 4.

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The electroplated surfaces with low current densities got higher surface uniformity and fewer defects than the electroplated surfaces with high current densities. When the sample had many defects, the inside structure became very porous. That is the reason why the sample thickness was increased while increasing the current density.

Electroplating solution 04 − 0.17 with current density of 20 mA cm⁻² was the best to obtain a copper layer with high surface uniformity. However, this solution was not suitable to fabricate complicated copper details in reality, because a small difference of current densities could lead to a large difference of thicknesses (figure 3(b)). Therefore, electroplating solution 03 − 0.4 with low current density between 10–20 mA cm⁻² was the most suitable electroplating solution in order to form a high surface uniformity copper layer.

The resistivities of samples (μΩ cm) are shown in table 4.

Table 4.  Calculated sample resistivities (μΩ cm) in four solutions 01–1.33, 02 − 0.75, 03 − 0.40, and 04 − 0.17, corresponding to four electroplating current densities 10, 15, 20, and 30 mA cm⁻².

	Resistivity (μΩ cm)
	01–1.33	02 − 0.75	03 − 0.40	04 − 0.17
10	2.683 ± 0.507	2.027 ± 0.698	2.260 ± 0.451	1.899 ± 0.420
15	3.061 ± 0.637	2.078 ± 0.602	2.291 ± 0.579	2.386 ± 0.566
20	3.309 ± 0.820	2.603 ± 0.871	2.586 ± 0.662	3.028 ± 0.821
30	3.675 ± 0.885	3.291 ± 0.868	2.642 ± 0.681	3.537 ± 1.021

Chemical concentration of electroplating solutions and current density influenced the resistivities of the electroplated samples. These results have shown that the high concentration of CuSO₄ or H₂SO₄ was not suitable in order to obtain a low-resistivity copper layer. Indeed, when the CuSO₄ concentration was so high, the efficiency of the electroplating step would be high, and the copper layer would be quickly electroplated. Therefore, the uniformity of surface was not good, because many pores and defects were formed inside the copper layer, as we concluded in the above paragraph. The more pores and defects that were formed, the higher the resistivity of the copper layer. While the H₂SO₄ concentration was so high, the formation of H₂ gas in the electroplating process would also form many pores and defects inside the copper layer. Optimized electroplating solutions to form a copper layer with high conductivity were solutions 02 − 0.75 and 03 − 0.40. The lowest obtained resistivity was 1.899 μΩ cm, nearly theoretical copper resistivity (1.678 μΩ cm), and could be used for communication devices. This value could be obtained when copper was electroplated in solution 04 − 0.17 with current density of 10 mA cm⁻². However, this solution was not suitable to be applied in reality, because a small difference of electroplating current densities could lead to a large difference in resistivities (figure 5(b)).

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When electroplating current densities had been increased from 10 and 15 to 20 and 30 mA cm⁻², the resistivities of the samples also increased generally (figure 5). These results have again proved that with a high velocity of copper layer deposition, many pores and defects were formed, and thus conductivities decreased. The most suitable solution to form the copper layer with acceptable low resistivity was 03 − 0.40, because of small difference of resistivities corresponded to various current densities (figure 5(a)).