Transfer, Assembly, and Embedding of Small CMOS-Die Arrays for the Build-Up of Flexible Smart Implants

Abstract

The integration of small wafer-based devices into flexible polymeric systems allows the fabrication of smart systems. Such systems are of high interest and are being developed in different fields. Active implants for biomedical applications promise new insights and therapy possibilities including the novel field of electroceuticals, which focuses on the treatment of systemic diseases by neuromodulation. For the fabrication of a new active flexible microelectrode array, we developed a process that allows the embedding of an array of small CMOS dies into a polymer foil. The process includes a transfer printing process based on adhesion, transferring the desired of the separated dies from UV-curable dicing tape to a substrate coated with sugar solution. The transfer printing process has shown to be highly reliable and selective, yielding a transfer success rate of 100% in most cases. The relative position of the transferred dies is conserved to less than $15~\mu \text{m}$ lateral displacement. To evaluate the process, the pull-off strength of the employed silicone stamps was measured. For stamps made of soft silicone, a pulling strength up to 83 kPa was obtained. In addition to this, a process for the embedding of the dies into a flexible foil system is presented. Using polydimethylsiloxane, the topography due to the dies’ height is reduced from more than $200~\mu \text{m}$ to approximately $10~\mu \text{m}$ , enabling the use of standard microelectromechanical fabrication processes on top.