Transparent and flexible film for shielding electromagnetic interference

    Transparent and flexible film for shielding electromagnetic interference Dong-Hwan Kim, Youngmin Kim, Jong-Woong Kim PII: DOI: Reference:

S0264-1275(15)30551-7 doi: 10.1016/j.matdes.2015.09.142 JMADE 712

To appear in: Received date: Revised date: Accepted date:

6 April 2015 22 September 2015 26 September 2015

Please cite this article as: Dong-Hwan Kim, Youngmin Kim, Jong-Woong Kim, Transparent and flexible film for shielding electromagnetic interference, (2015), doi: 10.1016/j.matdes.2015.09.142

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ACCEPTED MANUSCRIPT Transparent and flexible film for shielding electromagnetic interference

Dong-Hwan Kim, Youngmin Kim and Jong-Woong Kim*

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Display Components & Materials Research Center, Korea Electronics Technology Institute 68 Yatap-dong, Bundang-gu, Seongnam 463-816, Korea

+82 31 789 7438, Fax: +82 31 789 7439, E-mail: [email protected]

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*Phone:

Abstract

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The miniaturization of components in modern electronics has made them more susceptible to electromagnetic interference (EMI); therefore, this study proposes a new low-cost method for

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fabricating highly efficient shielding materials. This approach uses a low cost combination of inverted layer processing of silver nanowires (AgNWs) to produce a conductive network in a

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polyimide surface, followed by electroless plating of Cu to further enhance conductivity. This results in a highly efficient (>55 dB) EMI shielding film that is less than 10 μm thick, transparent

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(>58 % transmittance), and sufficiently flexible to maintain its conductivity after bending to a

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radius of 3 mm for 10,000 times.

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Keyword Electromagnetic interference; flexible transparent electrode; shielding film; silver nanowire; electroless plating

ACCEPTED MANUSCRIPT 1. Introduction The increasing miniaturization and complexity of mobile electronic devices require a greater

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packing density that is more susceptible to electromagnetic interference (EMI) [1-4], which can lead to adverse effects such as malfunction or the leakage of information in wireless

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telecommunications [5-9]. More importantly, it can present a possible radiation hazard to the human body and the surrounding environment, which has led to increased interest in the development of EMI shielding materials/structures suitable for microwave and millimeter-scale

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wavelengths [10-13].

There are a number of factors that need to be considered in the design and development of any

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EMI shielding material for high-density electronic devices. First, the material needs to be sufficiently thin to be applied to even the smallest mobile device form factors, such as ultrathin

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laptop computers, smartphones, portable medical devices, and wearable electronics [14,15]. Second, it needs to be flexible and mechanically stable when bent to a curvature of less than 5

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mm, which is most easily achieved by reducing the film’s thickness. Finally, a thin, lightweight, and flexible film still needs to provide adequate shielding effectiveness (SE). One of the more

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popular approaches to fabricating shielding films is to use metal sheets adhered to a polymer

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film; however, this produces an excessive thickness (typically more than 50 μm) that makes it difficult to attach such a film to a complex structure of stacked microscale multilayers without producing defects [16-18]. An alternative approach is to evaporate or sputter much thinner metal layers onto a polymer film; however, even with this approach, the final thickness is still on the order of 50-120 μm, and incurs a much higher fabrication cost [19,20]. Composite structures consisting of conductive filler and a binding polymer have also been extensively investigated as a possible EMI shield, owing to their reasonable fabrication cost and improved flexibility [21-23]. Creating such materials relies on embedding conductive metal particles, metal flakes, or a carbon-based filler material in a polymer matrix, which means that their conductivity is dependent on the percolation of the filler. This creates a situation in which using a large amount of filler is advantageous in terms of achieving a high shielding efficiency;

ACCEPTED MANUSCRIPT however, it is detrimental in terms of fabrication cost and film thickness. Furthermore, the mixing of the conductive filler into a liquid polymer typically results in a heterogeneous structure

matrix increasing the weight and reducing the flexibility of the film.

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across the thickness of the film, with any increase in the interfacial area between the filler and

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Network structures based on silver nanowires (AgNWs) are well known to offer an intrinsically high conductivity, transparency, and flexibility. This is mainly the result of their anisotropic shape, which means that only a small fraction of percolated AgNW network is needed to form a highly

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conductive electrode [24-28]. This makes it possible to reduce the excessive use of expensive nanowires and their associated fabrication cost, and it has seen AgNW-based transparent

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electrodes used in a variety of optoelectronic devices such as touch sensors, organic light emitting diodes, organic solar cells, and thin film transistors. Interestingly, M. Hu et al. also used AgNWs

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as a flexible and transparent EMI shielding material by protecting them with an overcoat of poly(ethersulfone) [29]. This study therefore seeks to improve on this work by employing an

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inverted-processing method and additional electroless plating to create a more effective

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transparent EMI shielding film, the properties of which are discussed in this paper.

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2. Experimental procedures An inverted-processing method (Fig. 1) was used to embed percolated AgNWs into a polyimide (PI) surface to create a AgNW/PI composite. For this purpose, a glass substrate was first cleaned with detergent, de-ionized water, acetone, and isopropanol, and then, it was spin coated with a poly(methyl methacrylate) (PMMA) suspension in chlorobenzene. After baking at 120 °C to produce a 200 nm-thick sacrificial PMMA layer, the glass substrate was placed on a Mayer-rod apparatus and several drops (0.5 ml) of nanowire solution (Nanopyxis Ltd., Korea) were deposited onto it. These nanowires had an average diameter and length of approximately 35 nm and 30 μm, respectively, obtaining an aspect ratio of 950–1000. Next, a #8 Mayer rod (R.D. Specialties, Inc., USA) was rolled over the drops to evenly spread the nanowire solution over the PMMA surface, which was then carefully dried under infrared illumination for 10 min. A

ACCEPTED MANUSCRIPT poly(amic acid) precursor (Picomax, Korea) was then spin-coated onto the electrode and subsequently cured at 200 °C for 1 h. A thickness of 7-8 μm was selected to provide the best balance of flexibility and reliability, and once formed, the sample was soaked in acetone for 2 min

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to dissolve the PMMA layer and allow the film to be safely peeled from the supporting glass.

The sheet resistance of the composite electrode after it was first peeled from the glass substrate was found to be approximately 500 ohm/sq, a value that is too high for highly efficient EMI shielding. Electroless plating for 12 min was used to increase the conductivity of the AgNWs/PI

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composite; Cu being selected as the target because of its high conductivity. The composition of the plating bath was as following: 7 g/L of CuSO4‚ 5H2O, 25 g/L of potassium sodium tartrate,

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4.5 g/L of sodium hydroxide, and 9.5 g/L of formaldehyde. The plating solution was heated to 85 °C and the samples were dipped into it for varying time (from 6 to 12 min). Unfortunately,

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the low coverage of the electrode and thin layers of PI on the AgNWs meant that Cu did not form properly on the nanowire network. In order to overcome this, selective etching by plasma

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treatment for 60, 120, 240 or 360 s at a power of 300 W was performed to remove the PI layers

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and expose the AgNWs. Considering that the AgNWs could be easily oxidized, Ar was selected for this treatment without employing O2. For this, the gas flow rate and gas pressure were

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controlled to be 50 ml/min and 30 Pa, respectively.

The microstructure of the AgNW and Cu networks was analyzed by scanning electron microscopy (SEM, JEOL Ltd., JSM6700F, Japan), and their optical transmittance was measured using a UV–visible spectrophotometer (Jasco, V-560, Japan). The sheet resistance was measured by a noncontact system (Napson Corporation, EC-80P, Japan). The surface roughness was measured by atomic force microscopy (AFM, Park Systems, XE-100TM, USA), and a crosssection of each sample was prepared by a focused ion beam (FIB, JEOL Ltd., JIB-4601F, Japan) system. The mechanical stability of the film was evaluated by an automatic bend-testing machine (Bending tester, Jaeil Optical System, Korea); a bending radius of 3 mm being used to induce a ~0.15% tensile strain. The films were bent at a cycle rate of 0.3 Hz, and their resistance was

ACCEPTED MANUSCRIPT measured during outward bending cycles. The EMI shielding effectiveness was measured by a network/impedance analyzer (Agilent, 4396B, USA).

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3. Results and discussion

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The low adhesion of AgNWs to PI, which is one of the most commonly used materials in flexible electronics, not only limits the flexibility of the AgNW coating, but also hinders any additional processing aimed at enhancing the conductivity or creating patterns [30]. This could be

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alleviated by using an adhesive or binding material; however, this would only tend to increase the fabrication cost and could potentially induce a reaction with neighboring materials or chemicals

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used for additive processing. An inverted layer processing method was therefore adopted in this study (Fig. 1), as this dramatically increases the adhesion of AgNWs to PI and enhances both the

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mechanical and chemical stability of the electrode formed [31]. Figure 2 shows SEM and AFM micrographs of the surface produced by Sequence 3 in Figure 1,

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from which we see that the high aspect ratio of the AgNWs generally leads to a highly porous surface morphology with a large peak-to-valley (Rpv) and root-mean-square roughness (RRMS). For

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example, the Rpv value of 224.8 nm in Figure 2 (b) is more than five-times the nanowire diameter

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of 35 nm, which clearly indicates that they are not densely stacked or stuck to the substrate. However, by dissolving the sacrificial PMMA layer after fully imidizing the poly(amic acid), an extremely smooth surface is exposed (Figure 2 (c) and (d)). This can be directly attributed to the smoothness of the initial PMMA surface that was first in contact with the AgNWs and PI. Furthermore, this sacrificial layer helps to facilitate the smooth release of the composite electrode, thereby eliminating any unnecessary force that could pluck nanowires from the polymer. More importantly, because the PI precursor diffuses and completely infiltrates the nanopores formed between the AgNWs and PMMA, there is scarcely any evidence of nanoholes or steps being present. The Rpv (4.7 nm) and RRMS (0.5 nm) in Figure 2 (d) are an order of magnitude lower than that of a normal ITO film (30–50 nm), and comparable to ITO glass, suggesting that the surface roughness is dramatically improved by this simple approach.

ACCEPTED MANUSCRIPT The use of Ar gas for plasma etching was predicated on the fact that it allowed for precise control over the etching rate by varying the input power, gas flow rate, and etching time, and also provided selective etching. The plasma treatment was therefore varied to find the optimal

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conditions for removing the thin PI layer without oxidizing the AgNWs [31], through which it

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was found that the exposure of AgNWs that would otherwise have been buried produced a significant increase in the surface roughness of the composite, as is shown in Figure 2 (e-h). After 360 s of plasma etching, the Rpv value reached a maximum of approximately 108 nm; and although some nanowires were partially excavated, they proved practically impossible to detach

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by any physical means (e.g., a repeated tape test). Interestingly, the plasma treatment at the selected treating conditions did not deteriorate the conductivity of the electrode, which implies

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that the AgNWs were not largely affected by this approach. For the best balance of sufficient exposure of AgNWs with retaining their conductivity and an enough adhesion of nanowires to

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PI, we selected 360 s of treating time.

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Figure 3 (a)-(d) shows the effect of immersing the plasma treated samples (360 s at a power of 300 W) in an electroless Cu plating solution for different periods of time. It is evident from this

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that a plating time of less than 5 min produces no obvious evidence of Cu metallization, but a partially percolated Cu network is formed over the AgNWs at 6 min. Increasing the immersion

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time to 8 min increases the diameter and regularity of the Cu wires to the point at which the Cu network becomes virtually the same as the AgNW network. Figure 3 (e) shows a cross-sectional view of this Cu network, in which it appears that two intersecting wires were simultaneously etched by ion-milling and merged into a set of spherical wires. The thickness of the wires (500– 550 nm) is about 13 times greater than the diameter of the original AgNWs, which is significant from an engineering perspective in that it means there is less need to use expensive AgNWs to increase the conductivity and surface coverage. Further immersion, however, causes the Cu wires to fully connect with each other and form a dense network, thereby increasing the Cu coverage. The energy dispersive spectrometer (EDS) analysis results in Figure 3 (f) confirm that the plating produced was Cu. Given that one of the primary goals of this study was to make a transparent

ACCEPTED MANUSCRIPT conductive film, a plating time of 8 min was selected to ensure the Cu network is fully percolated, while still leaving about half of the PI surface area exposed.

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Figure 4 (a) shows how the sheet resistance of the Cu/AgNWs/PI film initially decreases quite gradually with increasing plating time, as metallization at this stage is largely random and lacking

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in long-range connectivity (Figure 3(a)). There is, however, a sharp drop after approximately 6 min, which is related to the percolation of Cu wires. At 8 min, the networked Cu wires formed in large areas continue to increase the conductivity of the film, giving a sheet resistance of 27.8

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ohm/sq. A similar trend was observed in the case of transmittance (Figure 4 (b)), which dropped to 58 % at 550 nm after 8 min.

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The EMI SE of the AgNW/PI and Cu/AgNW/PI samples was measured up to 1.5 GHz to evaluate the effects of Cu metallization. As seen in Figure 5 (a), EMI SE was generally only

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weakly dependent on the frequency across the measured range, which meant that average EMI SE values could be used to evaluate the amount of incident signal that is blocked. Thus, 8 min of

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Cu metallization produces a considerably significant increase in EMI SE from 22 to 55 dB, which can be mainly attributed to the increased conductivity of the Cu/AgNWs/PI composite. It

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should also be noted that the thickness and continuity of the metallic conductive network also

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influences the EMI SE, and therefore, the increased thickness and long-range connectivity produced by longer immersion times will likely have a stronger interaction with incoming electromagnetic waves, thereby improving the shielding effectiveness. However, the increased thickness also has a detrimental effect on the transparency of the film and the range of applications in which it can be used. The unique structure of the Cu/AgNWs/PI electrode was found through cyclic (0.3 Hz) bend testing to an outward radius of 3 mm (inset of Figure 5 (b)) to provide good bending fatigue strength, with Figure 5 (b) showing that despite a few initial fluctuations, the measured resistance was stable for up to at least 10,000 cycles. These fluctuations were observed in the testing results for most of the samples, suggesting that is caused by a phenomenon typical of the specific physical structure formed. The destruction and elimination of Cu oxides formed on the surface

ACCEPTED MANUSCRIPT of the Cu branches and/or the rearrangement of Cu and AgNW networks during the first stage of bending are considered possible causes; however, further work is needed to determine the true

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mechanism.

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4. Conclusion

A thin and flexible transparent film for shielding EMI has been successfully developed through

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inverted layer processing and electroless plating. By embedding AgNWs into the surface of PI to a thickness of less than 10 μm by inverted processing, a high adhesion and good mechanical

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stability was achieved. Subsequent Ar plasma treatment allowed the insulating PI material to be removed, thereby exposing any buried AgNWs. Finally, electroless plating with Cu was used to reduce the sheet resistance of the AgNWs/PI composite from 500 to 28 ohm/sq, while still

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maintaining a transmittance of more than 58 % at 550 nm. The EMI SE of the resulting

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Cu/AgNWs/PI film was measured to be 55 dB, which represents a 150 % increase over the AgNWs/PI composite electrode. This unique transparent film structure also maintained its

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conductivity over repeated bending cycles, and therefore, this method is considered an important step toward the low-cost fabrication of high-performance, flexible, and transparent films for

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EMI shielding. We believe that this structure could be used in the development of various future electronic devices such as rollable and semi-transparent television or wearable transparent sensing devices.

Acknowledgements This research was financially supported by the Ministry of Trade, Industry and Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT) through the Promoting Regional specialized Industry.

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Figure 1 Schematic depicting the fabrication of a transparent film for EMI-shielding.

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Figure 2 Surface topography of a conductive AgNW-PI electrode surface: (a) SEM and (b) AFM

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micrographs of AgNWs coated on PMMA/glass, and (c,d) of an as-fabricated AgNW/PI composite electrode. AFM morphology of a AgNW/PI composite after plasma treatment for (e)

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60, (f) 120, (g) 240, and (h) 360 sec at 300 W.

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Figure 3 SEM micrographs of Cu networks plated onto a AgNW/PI composite after (a) 6, (b) 8, (c) 10, and (d) 12 min of plating time. (e) Cross-sectional view of the Cu/AgNW/PI composite, and (f) EDS analysis of its composition.

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Figure 4 (a) Sheet resistance and (b) transmittance of a Cu/AgNWs/PI composite with

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increasing plating time.

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Figure 5 Physical properties of a Cu/AgNWs/PI film: (a) EMI shielding effectiveness up to 1.5

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GHz and (b) change in resistance with repeated bending to a radius of 3 mm.

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights 1. By employing plasma treatment and electroless plating, copper network could be

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formed on silver nanowires/polyimide composite.

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2. Semi-transparent and flexible film for shielding electromagnetic interference could be fabricated.

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was measured to be 55 dB up to 1.5 GHz.

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3. Shielding effectiveness of the fabricated copper/silver nanowires/polyimide film