A Flexible Chip-Film Patch and a Flexible Strain Gauge Sensor Suitable for a Hybrid System-in-Foil Integration

Hybrid Systems-in-Foil (HySiF) encompass large-area thin-film electronics and ultrathin, high performance CMOS chips, which are integrated into a flexible polymeric foil substrate. In this work, the individual components of a customized flexible sensor system are characterized in detail,
i.e.
, a strain gauge and a readout ASIC, and the system functionality is proved. We employ the Chip-Film Patch (CFP) technology, where an ultra-thin readout ASIC is embedded in spin-coated polyimide foil in order to monitor resistance variations of strain gauges. These two components are suitable to be integrated on the same foil, whereas the envisioned system is intended to monitor the respiratory system of newborns and premature infants. The latest developments in the CFP process using the aromatic polyimide PI-2611 are shown. In particular, the optimization of the spin-coating and evaluation of polyimide uniformity above the embedded chip are highlighted. Furthermore, we demonstrate a flexible strain gauge sensor using thin-film metals embedded in polyimide PI-2611, which is usable at up to 30% strain under electromechanical load. This also demonstrates the mechanical reliability of the CFP circuit interconnects. The electrical characterization of the readout ASIC, after back-thinning and embedding into the CFP, reveals no stress induced performance degradation.