Flexible Bonding Method for Creating Flexible Electronics System with a Plasma Activation

On-skin devices are a critical technology for realizing next-generation advancements like the Internet of Things (IoT) and Digital Twins. To achieve this, researchers are focusing on developing high-performance, long-term stability, and flexible electronics that can conform to skin topography and movement. Common approaches to achieve this flexibility include utilizing organic materials and minimizing device thickness. Beyond performance enhancement, achieving robust and flexible bonding is crucial for the development of practical on-skin devices. Traditional bonding methods for flexible electronics include the use of adhesive layers and direct metal bonding.<br/>However, the bonding method of simultaneously achieving flexibility and robustness is very difficult because of material characteristics differences at interconnection. Adhesive bonding offers superior mechanical strength due to its full surface, bonding both the metal electrode and the polymer substrate. This full-coverage approach compromises flexibility at the connection owing to the increased thickness. Conversely, direct metal bonding methods preserve flexibility at the connection by eliminating additional thickness. This method suffers from inconsistent mechanical robustness due to variations in the bonding area caused by fluctuations in the ratio of electrode to substrate material at the interconnection. (Conventional direct bonding methods were limited to either metal or polymer bonding.)<br/>In this work, we developed a multipurpose direct bonding method for Au direct bonding and Parylene polymer direct bonding. This method exhibited metallic bonding or polymer fusion bonding under ambient air through plasma surface modification, water plasticizer effect and low-temperature heating. Therefore, by using this method as a bonding method for flexible electronics, we achieved ultra-flexible bonding of flexible electronics with the entire surface including the substrate area and the electrode area. A water vapor plasma treatment was applied to a sample in which 100 nm of gold was deposited on a 2-micron-thick parylene substrate. Next, the surface-modified surfaces were brought into contact with each other in the air. In this state, a water droplet of less than 5 µL was injected into the interface of the parylene substrate, which had been hydrophilized by plasma treatment. The sample was then heated in an oven at 85°C for 4 hours. When the cross-section of the bonded sample was observed, the interface of the gold electrode area disappeared, and a strong metallic bond was formed. The interface of the parylene substrate area showed the highest electron density, and crystallization or aggregation of polymer chains occurred. Thin film specimens with different wiring widths were created and bonded using the conventional gold direct bonding method, and our developed bonding and tensile tests were performed. As a result, the gold direct bonding method could not bond parylene substrates, so the bonding strength decreased to about 10% of the original when the width of the gold wiring was narrowed. On the other hand, our method could bond not only gold electrodes but also parylene polymer substrates, so the bonding strength remained constant even when the electrode width was changed. Moreover, Performance fluctuated by only 3% even after 10,000 bending cycles. The bonded area maintained a minimum bending radius below 0.5 mm due to the lack of an additional adhesive layer.<br/>By using a new versatile direct bonding method using water vapor plasma and the water plasticizer effect, we have succeeded in flexible and robust bonding whole interconnection surface without adhesives. It can be applied to fabricate the multifunctional integrated flexible electronics.