Manufacturing of Flexible Electronics

Fabrication of Flexible Electronics Improved Using Gold and Water-Vapor Plasma

Researchers at RIKEN Center for Emergent Matter Science (CEMS) and RIKEN Cluster for Pioneering Research (CPR) in Japan have developed a technique to increase the flexibility of ultra-thin electronics, such as those used in bendable devices or clothing. A study published in Science Advances details the use of water-vapor plasma to directly bond gold electrodes fixed to separate ultra-thin polymer films without requiring adhesives or high temperatures. As electronic devices shrink and the desire for flexible, wearable and on-skin electronics grows, traditional methods of fabricating these devices have become less practical. However, one of the biggest challenges is how to bond and integrate multiple devices or parts of a device, each on separate ultra-thin polymer films. Traditional methods using adhesive layers to bond electrodes together reduce flexibility and require temperatures and pressures that can damage super-thin electronics. Traditional methods of direct metal-to-metal bonding exist, but they require perfectly smooth and clean surfaces, which are not typical in such electronics. A research team led by Takao Someya at RIKEN CEMS/CPR developed a new method to secure these bonds that uses no adhesives, high temperatures or high pressures and does not require completely smooth or clean surfaces. In fact, the process takes less than a minute at room temperature and is then left for approximately 12 hours. The new technique, called water-vapor plasma-assisted bonding, creates stable bonds between gold electrodes pressed onto ultra-thin polymer sheets 0.002 millimeters thick using a thermal evaporator. Kenjiro Fukuda, Senior Research Scientist at RIKEN CEMS/CPR, says, "This is the first demonstration of ultra-thin, flexible gold electronics produced without using any adhesives. Using this new direct bonding technology, we were able to produce an integrated system consisting of flexible organic solar cells and organic LEDs." Experiments showed that water-vapor plasma-assisted bonding performed better than conventional adhesive or direct bonding techniques. In particular, the strength and consistency of the bonds were greater than those achieved by standard surface-assisted direct bonding. At the same time, the material conformed better to curved surfaces and was more durable than what could be achieved using a standard bonding technique. According to Fukuda, the method itself is remarkably simple, which may explain why they discovered it by chance. After securing gold electrodes to polymer sheets, a machine is used to expose the electrode sides of the sheets to water-vapor plasma for 40 seconds. The polymer sheets are then pressed together so the electrodes overlap in the correct position. After waiting 12 hours at room temperature, it is ready for use. Another advantage of this system is that after activation with water-vapor plasma, but before being bonded together, the films can be stored in vacuum-sealed packages for days. This is an important practical aspect considering the potential to order and distribute pre-activated components. As proof of concept, the team integrated ultra-thin organic photovoltaic and LED light modules printed on separate films and bonded together with five additional polymer films. The devices underwent extensive testing including being wrapped around a rod, crumpled and bent severely. Additionally, the power efficiency of the LEDs was not affected by the process. The technique also succeeded in attaching pre-packaged LED chips to a flexible surface. Fukuda said, "We expect this new method to be a flexible wiring and assembly technology for a new generation of wearable electronics that can be attached to clothing and skin. The next step is to develop this technique for use with cheaper metals such as copper or aluminum."  

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Materials provided by RIKEN. Journal Source: Masahito Takakuwa, Kenjiro Fukuda, Tomoyuki Yokota, Daishi Inoue, Daisuke Hashizume, Shinjiro Umezu, Takao Someya. Direct gold bonding for flexible integrated electronics. Science Advances, 2021; 7 (52) DOI: 10.1126/sciadv.abl6228 RIKEN. "Fabrication of flexible electronics improved using gold and water-vapor plasma." ScienceDaily. www.sciencedaily.com/releases/2021/12/211222153005.htm (accessed 4 January 2022).