Abstract
Flexible and stretchable electronics has emerged as a promising field in the past decade, especially towards healthcare and environmental monitoring applications. We have witnessed significant progress in developing flexible sensors, logic/memory devices (transistors), communication modules, energy sources, and alternate materials to enhance performance with added physical conformity. However, there are unaddressed challenges in matching status-quo CMOS-based electronics, packaging, reliable flexible interconnects, and lack of pragmatic integration schemes to readily complement the existing state-of-the-art technology. Moreover, often rigid ICs are used on flexible polymeric substrates placed sparsely, making the system bendable with a caveat of reduced area efficiency. Hence, we present a CMOS compatible heterogeneous integration scheme with a unique 3D coin architecture to obtain truly flexible electronic systems. This integration sequence has multiple polymeric layers that resemble a coin with two faces and a sandwich layer. The bottom face of the coin hosts flexible sensors while the top face is reserved for communication (antenna) and energy harvester (flexible solar cell), and the associated electronic ICs (microcontroller, RF IC, solid-state micro-batteries) are embedded in the sandwich layer (all components in the flexible form). We use the modular-LEGO approach for the assembly of bare die ICs like a lock-and-key mechanics in plug and play manner to obtain reliable, flexible interconnects and accurate assembly. These multiple layers are connected electrically using vertical through-polymer-via (TPV) that shows less than 4% overall change in the resistance across the longest metallic interconnect layer trace. This vertical TPV interconnection helps in maximizing area usage, thereby increasing area efficiency. We transformed the commercial CMOS based 16 MHz microprocessor bare die and an array of micro-batteries with a rating of \mathrm{5}\ \mu\text{Ah} @ 3.9V into ultrathin ( < \mathrm{20}\ \mu\mathrm{m} ) flexible form after assembly of thicker dies ( \mathrm{200}-\mathrm{300}\ \mu\mathrm{m} ) into the LEGO sites. It can integrate a solar cell on top face after following the corrugation approach of converting rigid solar cells into a flexible one. At the same time, the antenna for communication and the TPV are fabricated using electrochemical deposition technique. The overall thickness of the 3D-coin system is less than 1 mm and free from any rigid component making it flexible up to a bending radius of ∼1 mm. Soft-polymeric encapsulation provides mechanical strength, reduces stresses, and improves ruggedness for the stand-alone electronic system. Thus, we believe our 3D heterogeneous integration strategy offers a pragmatic approach to obtaining truly flexible electronic systems without compromising next-generation electronic systems' performance aspects.