Robust Full-Surface Bonding for Integrating Flexible Optical Electronics

Integrating multiple flexible electronics is a pivotal facilitator for practical applications in ultra-flexible wearable electronics. Robust and flexible bonding across the entire connection area is essential when amalgamating several electronics as a single system, regardless of electrode or substrate regions. However, due to distinct material properties, prevailing direct bonding methods are confined to the metal electrode or substrate direct bonding. This study introduces a direct bonding technique that can be used in two applications: gold metallic bonding and parylene polymer bonding without the use of adhesives despite the low process temperature of 85°C. By applying this bonding method to integrate flexible electronics fabricated on polymer substrates, the entire bonding surface, including the Au-electrode region and parylene substrate region, was bonded directly. Therefore, we have demonstrated, for the first time, the creation of three-dimensional stacked flexible electronics without the need for an adhesive layer, which exhibits robustness while retaining high flexibility. This hybrid bonding technology demonstrated bonding strength more than ten times that of conventional gold bonding integration using adhesive-free bonding. The performance only fluctuated by 3% even after 10000 bending cycles. Additionally, it showcased steadfast electrical performance even with a 5 µm pitch in high-resolution bonding. The bonded area maintained a minimum bending radius below 0.5 mm due to the lack of an additional adhesive layer. Furthermore, by depositing parylene polymer on other material partially, we have demonstrated the creation of stretchable organic photovoltaics. Therefore, our bonding method represents a promising integration technique for developing various structured flexible electronics applications.