Fabrication of enhanced flexible electronics using gold and water vapor plasma

December 23, 2021

(News from Nanowerk) Researchers from the RIKEN Center for Emergent Matter Science (CEMS) and the RIKEN Cluster for Pioneering Research (CPR) in Japan have developed a technique to improve the flexibility of ultra-thin electronics, such as those used in foldable devices or the clothes.

Posted in Scientists progress (“Direct gold bonding for flexible integrated electronics”), the study details the use of water vapor plasma to directly bond attached gold electrodes to separate ultra-thin polymer films, without the need for adhesives or high temperatures.

(A) Evaporated gold surfaces on 2 μm thick parylene substrates were exposed to a water vapor plasma. (B) Bonding of water vapor plasma treated gold was achieved by layering the two substrates and storing them in ambient air for a few seconds to several hours without any applied pressure or heat. (Image: RIKEN)

As electronic devices get smaller and the desire for electronics that are foldable, wearable, and on the skin increases, conventional methods of building these devices have become impractical. One of the biggest issues is how to connect and integrate multiple devices or parts of a device that each reside on separate ultra-thin polymer films.

Conventional methods that use layers of adhesive to bond electrodes together reduce flexibility and require temperature and pressure that damage ultra-thin electronics. Conventional methods of direct metal-to-metal bonding are available, but require perfectly smooth and clean surfaces which are not typical of these types of electronics.

A team of researchers led by Takao Someya at RIKEN CEMS/CPR has developed a new method for securing these connections that does not use adhesives, high temperatures or high pressures, and does not require totally smooth or clean surfaces. In fact, the process takes less than a minute at room temperature, followed by around 12 hours of waiting.

The new technique, called water vapor plasma assisted bonding, creates stable bonds between gold electrodes that are printed onto ultra-thin (2 thousandths of a millimeter) polymer sheets using a thermal evaporator.

“This is the first demonstration of ultra-thin and flexible gold electronics made without any adhesives,” said lead researcher Kenjiro Fukuda of RIKEN CEMS/CPR. “Using this new direct bonding technology, we were able to fabricate an integrated system of flexible organic solar cells and organic LEDs.”

Experiments have shown that water vapor plasma assisted bonding performs better than conventional bonding or direct bonding techniques. In particular, bond strength and consistency were superior to those achieved by standard surface-assisted direct bonding. At the same time, the material conformed better to curved surfaces and was more durable than could be achieved using a standard adhesive technique.

According to Fukuda, the method itself is surprisingly simple, which might explain why they discovered it by accident. After attaching the gold electrodes to polymer sheets, a machine is used to expose the sides of the sheet electrodes to a water vapor plasma for 40 seconds. Then the polymer sheets are pressed together so that the electrodes overlap in the right place. After 12 hours of waiting at room temperature, they are ready to use.

Another advantage of this system is that after activation with a water vapor plasma, but before being glued together, the films can be stored in vacuum packs for days. This is an important practicality when considering the potential for ordering and dispensing pre-activated components.

Conformability test of a flexible gold electrode Conformability test. Images of the conformability test on a curved surface (radius: 0.5 mm). Ultra-thin films bonded using water vapor plasma assisted bonding (top) or standard adhesive (bottom). The conformability was much better for the water vapor plasma assisted bonding. (Image: RIKEN)

As a proof of concept, the team integrated ultra-thin organic photovoltaic modules and LED lighting modules that were printed on separate films and joined by five additional polymer films. The devices withstood extensive testing, including being wrapped around a stick and being wrinkled and twisted to extremes. In addition, the energy efficiency of the LEDs did not suffer from the treatment. The technique also made it possible to join pre-packaged LED chips to a flexible surface.

“We expect this new method to become a flexible wiring and mounting technology for next-generation wearable electronics that can be attached to clothing and skin,” Fukuda said. “The next step is to develop this technology for use with cheaper metals, such as copper or aluminum.”

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