Free standing interconnects for stretchable electronics

Abstract

Advancements in stretchable electronic systems have changed the way modern electronics interact with their target systems, by their conformability to more complex shapes as compared to conventional rigid or flexible electronics. By utilizing this, limitless applications in the field of healthcare can be realized, such as wearable and implantable electronics. Medical devices that can be stretched/conformed to a certain limit, will reduce the effort by physicians and improve the user experience by providing enhanced dynamic shaping and matching mechanical properties to that of the human body. In literature, many methods for the realization of stretchable electronic systems are presented. In this Thesis, the design and micro fabrication of freestanding stretchable interconnect technologies for both large and small area devices is presented. Free-standing interconnects have the freedom to bend out of plane during stretching, thus enabling an increase in stretchability. This Thesis presents a reliable microfabrication technology for stretchable electronic circuits with high density interconnects, that can be considerably stretched even in densely packed/high fill-factor circuits. To fabricate and study the free standing interconnects, a demonstrator patch with a sparse horse-shoe shaped interconnect design is presented in the first part of the Thesis. To render such large structures free standing, several technology modules needed to be developed. After the first proof of principle showing free standing polyimidemeander structures, the poor adhesion of polyimide (PI) and polydimethylsiloxane (PDMS) led to failure of the devices. Therefore, two methods involving surface modification of polyimide, and using an intermediate adhesion layer for improving the adhesion between PI and PDMS, were tested and assessed. Finally, butyl rubber as an intermediate layer was selected and implemented in the final fabrication process. The adhesive bond initiated by the butyl rubber (BR), apart from being extremely strong, is also chemically resistant and mechanically stable. For the final fabrication flow of these structures with metal interconnects, technological modules like PDMS pillars to prevent drooping of the large horse-shoe shaped interconnects and PI-PDMS “stitches” to ensure a reliable adhesion of the pillars to the interconnects were developed and implemented. A demonstrator patch with reversible stretchability of 80% is presented. However, it was observed that the testing of such large free-standing structures on a patch is not straightforward. In the second part of the thesis, testing was made an integral part in the design of the device. A device with a high fill factor i.e. densely packed rigid islands, allows only for a very small footprint of the interconnects. Therefore, a sub-micron interconnect design that can be realized with standard fine-pitch photolithography based IC techniques was developed, and an interconnect pattern based on a design presented by S. Shafqat et. al was implemented. In the second part of this Thesis, a test device for the micro tensile testing of these micron sized free standing structures is designed and fabricated for their easy, damage- free handling and mounting in a test setup. The device is fabricated as a single chip that can be separated into two movable parts after fixing it on the micro tensile test stage. The test device successfully demonstrated the tensile testing of the micron sized free standing structures that show reversible stretchability up to 2000%, while simultaneously measuring the resistance. Moreover, the generic design of the device allows the implementation and testing of different size and shape free-standing structures. After the fabrication of the micron sized free standing structures, several “fur like” residues were observed after the oxygen plasma etching of polyimide using aluminumas a hard etch mask. Therefore, different methods for the residue free etching of the polyimide were explored and a “fur-free” procedure for the etching of PI using a one-step reactive ion etch of the metal hard-etch mask is presented. In conclusion, the results and technological advances presented in this PhD Thesis have led to an increased understanding of the technologies for the reliable fabrication of free standing interconnect structures and have resulted in an improved stretchability in conformal electronic devices.