Integration Techniques for Roll-to-Roll Compatible Fabrication of Flexible Hybrid Electronics
The emerging Internet-of-Everything (IoE) framework targets to enable communication between almost every physical object via billions of sensors and actuators. A substantial fraction of these devices will be placed on objects that would undergo repeated bending deformation (such as sensors for prosthetics, human body, and robots) or on curved surfaces (like interior as well as exterior of automobiles, buildings, and industrial equipment). Therefore, flexible sensors and actuators providing high performance at low power requirements and manufactured at low cost will be the key to a successful implementation of IoE. Though printed and organic electronics fulfill the flexibility and low-cost requirements of IoE applications, they often fall short of the high performance and low power requirements demonstrated by silicon ICs. Therefore, silicon ICs would be the automatic choice to satisfy the more demanding segments of IoE. However, silicon ICs commercially available as standard surface mount device (SMD) components lack the bendability and conformability required for these IoE applications. Thin and ultra-thin bare silicon ICs fabricated by thinning down standard ICs can be integrated into flexible polymer foils to create flexible hybrid electronic (FHE) components that can be used to replace rigid SMD components. We have developed a variety of roll-to-roll compatible techniques to integrate thin and ultra-thin silicon chips as well as MEMS sensors in polymer foils. An overview of the diverse processes that we have successfully implemented to fabricate devices such as flexible MEMS pressure sensor stripes (thickness t < 500 µm), flexible microcontroller (t < 30 µm) and QFN compatible chip-foil packages (t<150 µm) will be presented in the first part of the talk. The second part of the talk will be dedicated to various bending reliability tests. Static bending tests performed on ultra-thin chips (t-30 µm) indicated that their robustness could be enhanced by up to 350% by embedding them in polymer foils. Besides, dynamic or repeated bending tests conducted on ultra-thin chips integrated on/in polymer foils at a radius of 5 mm revealed that the failure of samples occurred after a few thousand bending cycles due to cracking of the redistribution layer (RDL). Nevertheless, we have also devised methods to enhance the reliability of RDL patterns that do not crack even beyond 100000 bending cycles.