Interface Adhesion in Implantable Chip-in-Foil Systems.

Abstract

Bioelectronic medicine requires miniaturized implants to selectively interface small target structures in the autonomous nervous system. Long-term stable non-hermetic packaging techniques have to be developed for smallest implantable electronics and interfaces. A process for the fabrication of chip-in-foil implants is proposed that combines a flip-chip approach for bare die embedding with a silicone rubber backbone. The conducting tracks are structured on polyimide (PI), enabling the use of microsystems fabrication technologies. The long-term stability of the interface between PI and silicone rubber is investigated by peel tests in phosphate buffered saline after prolonged soaking at $37 ^{circ}\mathrm {C}$. With a peel force of 721 mN after 14 days of soaking, the combination of 10-nm-thick titanium oxide and the adhesion promoter Dow Corning 1200 OS leads to the highest interface stability of the tested methods. This conforms to the results of atomic force microscopy measurements, where this treatment increased the surface roughness from 0.44 nm to 46.45 nm. The devised interface enables the construction of a chip-in-foil system with silicone rubber for height levelling in combination with polyimide-based micro structuring.