Yuan-Nan Tsaiab,
Shih-Chieh Chinc,
Hsin-Yo Chenc,
Ta-I. Yangd,
Mei-Hui Tsai
*bc and
I.-Hsiang Tseng
*e
aDepartment of Electronic Engineering, Lunghwa University of Science and Technology, Guishan, Taoyuan 333326, Taiwan
bGraduate Institute of Precision Manufacturing, National Chin-Yi University of Technology, Taichung, 411030, Taiwan. E-mail: tsaimh@ncut.edu.tw
cDepartment of Chemical and Materials Engineering, National Chin-Yi University of Technology, Taichung, 411030, Taiwan. E-mail: tsaimh@ncut.edu.tw
dDepartment of Chemical Engineering, Chung-Yuan Christian University, Chungli, Taoyuan 320314, Taiwan
eDepartment of Chemical Engineering, Feng Chia University, Taichung, 407102, Taiwan. E-mail: ihtseng@fcu.edu.tw
First published on 5th May 2023
In order to shield the electronic circuits on a transparent polyimide (PI) substrate, an anti-reflection (AR) layer was deposited on a PI film via DC reactive magnetron sputtering. The effects of sputtering power and thickness of AR layer on the optical property and adhesion strength of the PI were investigated. The composition of the AR layer influences the bonding between layers. Sufficient thickness of the AR layer is essential to strengthen the adhesion between the PI and copper (Cu) layers. The sputtered AR layer on the PI also improves the barrier property for water vapor. The AR layer-sputtered PI substrates remain transparent and exhibit high peel strength to the Cu layer, suggesting their potential applications as reliable transparent substrates for modern electronic devices.
From our previous work, the deposition of AR layers on colorless PI substrates successfully enhances the adhesion strength between PI and the copper (Cu) layer, which mimics the electronic circuits.8 By adjusting the parameters, such as gas composition, of DC reactive magnetron sputtering, the AR layer-coated transparent PI substrates not only exhibit high adhesion strength to the Cu layer but low reflectance, indicating its potential as advanced flexible printed circuit material. In this work, we further investigate the effects of the AR-layer thickness and the sputtering power on the adhesive and optical properties of PI composite films. An optimum thickness of the AR layer, which contains copper and nickel oxides, is critical for the following deposition of conductive copper layer with sufficient adhesion to the PI substrate. The correlation of the bonding at the interface to the peeling strength of the PI substrate was also investigated. The AR-layer deposited transparent electronic substrates show the potential for future applications in soft panel displays and wearable devices.
A PI strip with a fixed dimension of 6 cm by 9 cm was taped to a glass substrate, and then purged with air to clean the surface. As illustrated in Scheme 1, a DC-reactive magnetron sputtering machine (KAO-DUEN, SPLTTER) equipped with a Cu–Ni alloy target (Cu/Ni = 75/25 wt% and purity = 99.95%) was used to deposit Cu–Ni oxides as the AR layer on transparent PI. The working pressure of the sputtering chamber was fixed at 6 × 10−3 torr, and the substrate temperature was 50 °C. The gas flowrate was 27 sccm for Ar and 3 sccm for O2. The thickness of the AR layer deposited on PI was 100 ± 25 Å, 300 ± 40 Å, or 500 ± 50 Å, respectively, under the sputtering power of 300 W (83.93 W/in2). For the AR layer with the thickness of 500 Å, the sputtering power was further changed, ranging from 225 W (62.95 W/in2) to 450 W (125.90 W/in2). On the top of each AR layer, the conductive Cu layer with a thickness of 1000 Å was then deposited. The sample name was denoted as S-PI-x-y, where x represents the thickness of AR layer (x = 100, 300, or 500 Å) and y represents the sputtering power (y = 225, 300, 450 W), respectively.
The elemental composition of the delaminated surface of the PI side was further determined by EDS. As shown in Fig. 2, the ratio of Cu and Ni significantly decreases with the increasing thickness of AR layer. Notably, copper and nickel oxides are the components of AR layer, and consequently the results further confirm the location of the peeling surface. When the thickness of AR layer increases to 300 Å, the delamination occurs at the interface of AR and PI that the residual Cu or Ni on PI dramatically decreases. At the same time, the content of F increases due to the exposure of fluorinated PI matrix.
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Fig. 2 The peel strength and atomic ratio of Ni, Cu, F, O, and C from the PI side of S-PI-x-300 W after peel tests. The thickness (x) of the AR layer is 100, 300, and 500 Å, respectively. |
As a reflectance of less than 15% is a requirement for fabricating the AR layer to effectively shield the conductive copper layers of electronic circuits on transparent PI, the reflectance from the PI surface was verified and measured as shown in the insets of Fig. 4. The reflectance of the PI composite films, which contain the PI substrate and coated AR and Cu layers, decreases with the increasing thickness of the AR layer under the same sputtering power of 300 W (Fig. 4(a)). Sputtering the AR layer with a thickness of 500 Å revealed the lowest reflectance, as well as the highest peel strength as shown in Fig. 2. When the thickness of AR layer is fixed to 500 Å, the films sputtering under the power of 450 W exhibits the lowest reflectance at 550 nm as shown in Fig. 4(b). Notably, when the sputtering power is 225 W, the reflectance is greater than 20% for S-PI-500 Å-225 W. In contrast, the reflectance in the visible light region (400 to 700 nm) is less than 10% for S-PI-500 Å-300 W.
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Fig. 4 Reflectance from the PI surface of S-PI-x-y films. (a) x = 100, 300, 500 Å; y = 300 W. (b) x = 500 Å; y = 225, 300, 450 W. |
The results of the peel strength and reflectance of S-PI-500 Å-y films are summarized in Table 1. The peel strength of films increases with the sputtering power. As the sputtering power increases, more target particles eject from the sputter target, resulting in a higher deposition rate on the substrate. In addition, the high sputtering power also leads to the ejected target particles with high energy to break more bonds on the substrate.11,12 The high energy deposition process will improve the interaction between the substrates and the deposited films. According to the EDS results listed in Table 1, the oxygen contents from delaminated surface of the AR side slightly increase with the sputtering power. The high energy deposition process results in more oxygen in the film that made the film darker. Therefore, the improved absorption of incident light makes the film reflect less lights.13,14
Sample | Peel strength (kgf cm−1) | Oxygen content of AR layer (%) | WVTR (g-mil/m2-day) | Transmittance @ 550 nm (%) | Haze (%) |
---|---|---|---|---|---|
PI | — | — | 242.9 ± 23.64 | 90.3 | 0.31 |
S-PI-500 Å-225 W | 0.422 ± 0.006 | 10.90 | 4.023 ± 0.626 | 90.2 | 0.68 ± 0.07 |
S-PI-500 Å-300 W | 0.428 ± 0.013 | 12.19 | 1.715 ± 0.826 | 90.1 | 0.70 ± 0.08 |
S-PI-500 Å-450 W | 0.441 ± 0.019 | 12.33 | 1.505 ± 0.167 | 90.1 | 1.07 ± 0.30 |
The XPS spectra of the delaminated surface of PI side from S-PI-500 Å-y are compared to elucidate the relationship between the adhesion strength and the bonding at the interface. The XPS full scan spectra of films shown in Fig. 5(a) suggest the existence of C, N, and O signals, which are the elements of PI substrate, as well as the signals of Cu and Ni, which indicate the remaining elements from AR layer after peeling test. The XPS spectra of C 1s region shown in Fig. 5(b–d) are deconvoluted into the following binding energies of 284.7 eV, 285.6 eV, 286.3 eV, and 288.6 eV, which represent the C–C, C–N, C–O, and CO bonding, respectively.15–18 The effects of the sputtering power on the bonding ratio on the peel strength was plotted in Fig. 6. The ratio of C–N bonding decreases with increasing sputtering power, while the content of C–O bonding increases. The high energy sputtering process leads to the breakage of C–N bonds and promotes the formation of C–O bonds.17–19 As Ni or Cu is easily bonded with C–O on PI, the interaction between AR layer and PI is strengthened to enhance the peel strength.3,7,19,20 Therefore, the increasing trend of peel strength is consistent with the increasing trend of C–O bonding ratio on PI.
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Fig. 5 (a) XPS full scan spectra of S-PI-500 Å-y. Deconvoluted XPS C1s spectra of (b) S-PI-500 Å-225 W, (c) S-PI-500 Å-300 W and (d) S-PI-500 Å-450 W. |
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Fig. 6 Effects of sputtering power (y) on the chemical bonding ratio and peel strength of S-PI-500 Å-y films. |
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Fig. 7 UV-vis spectra of original PI and remaining PI substrates after etching AR layer from S-PI-500 Å-y films. Etching the AR layer. |
The barrier property is another critical property for electronic materials. High water vapor transmission rate (WVTR) will reduce the adhesive strength or reliability of the film.5,21 The WVTR values of S-PI-500 Å-y films were also verified in this work, and the results were listed in Table 1. The WVTR of the original PI substrate is about 242.9 g-mil/m2-day. After sputtering with AR layer, a significant decrease in WVTR is revealed. For the same thickness of 500 Å, the deposition of AR layer at higher sputtering power exhibits better barrier property. The lowest WVTR value of 1.505 g-mil/m2-day is obtained from S-PI-500 Å-450 W. The high-energy deposition process provides more energy to the ejected target particles, making them easier to migrate on the PI substrate. Therefore, a denser AR layer structure is formed on PI, which effectively inhibits water penetration.12,22
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