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Fabrication of Foldable Metal Interconnections by Hybridizing with Amorphous Carbon Ultrathin Anisotropic Conductive Film Monalisa Pal,† Anupam Giri,† Dong Wook Kim, Sangbaie Shin, Minsik Kong, Kaliannan Thiyagarajan, Junghyeok Kwak, Odongo Francis Ngome Okello, Si-Young Choi, and Unyong Jeong* Department of Materials Science and Engineering, Pohang University of Science and Technology, Cheongam-Ro 77, Nam-Gu, Pohang, Gyeongbuk 790-784, Korea S Supporting Information *
ABSTRACT: With the advent of foldable electronics, it is necessary to develop a technology ensuring foldability when the circuit lines are placed on the topmost substrate rather than in the neutral plane used in the present industry. Considering the potential technological impacts, conversion of the conventional printed circuit boards to foldable ones is most desirable to achieve the topmost circuitry. This study realizes this unconventional conversion concept by coating an ultrathin anisotropic conductive film (UACF) on a printed metal circuit board. This study presents rapid large-area synthesis of hydrogenated amorphous carbon (a-C:H) thin films and their use as the UACF. Since the synthesized a-C:H thin film has electrical transparency, the metal/a-C:H hybrid board reflects the complexity of the underlying metal circuit board. The a-C:H thin film electrically connects the cracked area of the metal line; thus, the hybrid circuit board is foldable without resistance change during repeated folding cycles. The metal/UACF hybrid circuit board can be applied to the fabrication of various foldable electronic devices. KEYWORDS: foldable interconnection, ultrathin amorphous carbon film, anisotropic conductive film, electrical transparency, solution-based synthesis, hybrid electrode hin film foldable electronics are considered the next generation electronics.1−4 As foldable smart phones are on the verge of release, there is a fast-growing interest in foldable thin film electronics.5−7 Electrodes are usually placed between the encapsulation layers for stable operation of the device and insulation of the circuit. The current approach to manufacturing foldable devices is to use the neutral plane in which the bending stress is negligible because the inward contraction and the outward expansion cancel each other out.8,9 When using a neutral plane, there are several problems that arise in the process of making the thickness of the upper and lower encapsulation layers the same. Since the neutral plane requires a thick polymer layer on both sides, it not only makes the device manufacturing process difficult but also prevents heat dissipation of the device and limits the foldability.8 The ability to fabricate a foldable device without the need for a neutral plane, such as the use of a very thin top encapsulation layer or no use of a top layer, can be a
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© 2019 American Chemical Society
significant advantage in terms of heat dissipation and stable folding. In the display, it is preferable that the light-emitting layer is formed on the substrate surface in order to prevent the optical deflection and simplify the fabrication process. If a printed circuit board fabricated on the substrate surface maintains the electrical performance unchanged during extreme folding cycles, the device structure may be simplified and the manufacturing costs will be reduced. In addition, many foldable devices, such as skin-mounted healthcare systems, require the electrodes to be positioned on the top surface to receive the electrical signals from the skin.10,11Although graphene and conducting polymer-based hybrid electrodes have been proposed as a means of fabricating topmost foldable Received: April 6, 2019 Accepted: May 31, 2019 Published: May 31, 2019 7175
DOI: 10.1021/acsnano.9b02649 ACS Nano 2019, 13, 7175−7184
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Cite This: ACS Nano 2019, 13, 7175−7184
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ACS Nano
Figure 1. (a) Schematic illustration showing how an ultrathin anisotropic conductive film (UACF) coated on a flexible printed circuit board can electrically connect a cracked metal line at the folded state. The UACF is conductive in the thickness direction but insulating in the transverse direction. The UACF is highly conductive when the distance is less than 2 μm. Active units can be attached on the foldable interconnection. (b) Schematic representation of the microwave-assisted synthesis of the nitrogen-doped amorphous carbon (a-C:H) film used as the UACF. The process includes branched poly(ethylenimine) (b-PEI) and ammonia borane (AB). The as-synthesized a-C:H film was transferred onto the entire surface of a flexible substrate with metal circuit lines.
film mainly depends on the electron tunneling between the localized graphitic clusters.20−22 It is well-known that nitrogen doping reduces the optical band gap and the defect density of a-C:H, thus lowering the electrical tunneling barrier.21 If the aC:H film is ultrathin and electrically tunneled only in the thickness direction, the foldable metal/a-C:H hybrid circuit described in Figure 1a can be achieved. One of the technical challenges for the metal/a-C:H hybrid electrode is low-cost, large-area synthesis of uniform a-C:H thin films. So far, a-C:H films have been produced by the dry processes, including pulsed laser deposition,23 filtered cathodic carbon arc evaporation,21 sputtering (ion beam, radio frequency, magnetron, etc.),14 and physical or chemical vapor deposition (thermal, plasma, etc.).16,24 These processes typically request sophisticated instrumentation and controlled precursor flow at a high temperature and thus have limitations in robust scale-up and precise control of thickness in nanometers.25 There was a report about the solution-phase electrolysis synthesis of the aC:H film;26 however, the film was thick (800 nm) and its surface was rough. Despite the possibility of easy scale-up, solution-phase synthesis has rarely been investigated. This study reports a facile solution-based synthesis of uniform nitrogen-doped a-C:H ultrathin films and uses them as the UACF.
electrodes and interconnections, they are not suitable for highly integrated devices due to their low conductivity.12,13 An alternative approach is needed to achieve foldable circuit boards. Considering well-developed circuit technology and existing facilities, the major ripple challenge is to use the conventional metal deposition process to make the topmost foldable circuit boards. This study presents a simple way to convert conventional topmost metal circuit boards to foldable ones. The schematic drawing in Figure 1a describes the approach proposed in this study. An ultrathin anisotropic conductive film (UACF) having high conductivity in the vertical direction but limited conductance (
