Recent advances in MEMS technology have brought forward a new class of high-density stretchable/flexible electronics as well as large displacement MEMS devices. The in-situ electro-mechanical characterization of such devices is challenging since it requires: (i) highly delicate sample handling, (ii) controlled application of large (hundreds of µm) multi-axial displacements to mimic service conditions, (iii) integrated electrical testing and (iv) fast actuation for cyclic testing. Techniques already developed for small-scale testing in literature fall short to meet the combined set of requirements. To this end, a characterization methodology that fulfills all these requirements is developed and presented here. The technique is based on a piezo-driven micro-tensile stage, which provides large multi-axial displacements with high resolution and fast actuation (4000 µm/s). This is combined with a method for sample microfabrication on a test-chip to warrant delicate sample handling. Proof-of-principle experiments are shown for multi-axial mechanical characterization, electrical characterization and high cycle fatigue testing of micron-sized highly stretchable interconnects. Experiments are conducted under in-situ microscopic observation using optical microscopy, scanning electron microscopy, and high-resolution profilometry. The generic platform proposed here can be used for other problems where similar requirements are faced, e.g. other miniaturized, large displacement electro-mechanical applications that are currently being developed.@en

A device for studying the mechanical and electrical behavior of free-standing micro-fabricated metal structures, subjected to a very large deformation, is presented in this paper. The free-standing structures are intended to serve as interconnects in high-density, highly stretchable electronic circuits. For an easy, damage-free handling and mounting of these free-standing structures, the device is designed to be fabricated as a single chip/unit that is separated into two independently movable parts after it is fixed in the tensile test stage. Furthermore, the fabrication method allows for test structures of different geometries to be easily fabricated on the same substrate. The utility of the device has been demonstrated by stretching the free-standing interconnect structures in excess of 1000% while simultaneously measuring their electrical resistance. Important design considerations and encountered processing challenges and their solutions are discussed in this paper.@en

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.@en

The authors found that oxygen plasma etching of polyimide (PI) with aluminum (Al) as a hard-etch mask results in lightly textured arbitrary shaped “fur-like” residues. Upon investigation, the presence of Al was detected in these residues. Ruling out several causes of metal contamination that were already reported in literature, a new theory for the presence of the metal containing residues is described. Furthermore, different methods for the residue free etching of PI using an Al hard-etch by using different metal deposition and patterning methods are explored. A fur-free procedure for the etching of PI using a one step-reactive ion etch of the metal hard-etch mask is presented.@en

This paper reports the use of rubber—Polybutadiene as an intermediate adhesive layer for improving the adhesion between polyimide (PI) and silicone polydimethylsiloxane (PDMS) which is required for a reliable fabrication of flexible/stretchable body patches for various applications. The adhesive bond initiated by the butyl rubber (BR), apart from being extremely strong, is also chemically resistant and mechanically stable as compared to the state of the art processes of improving adhesion between PI and Silicone.@en

The exciting field of stretchable electronics (SE) promises numerous novel applications, particularly in-body and medical diagnostics devices. However, future advanced SE miniature devices will require high-density, extremely stretchable interconnects with micron-scale footprints, which calls for proven standardized (complementary metal-oxide semiconductor (CMOS)-type) process recipes using bulk integrated circuit (IC) microfabrication tools and fine-pitch photolithography patterning. Here, we address this combined challenge of microfabrication with extreme stretchability for high-density SE devices by introducing CMOS-enabled, free-standing, miniaturized interconnect structures that fully exploit their 3D kinematic freedom through an interplay of buckling, torsion, and bending to maximize stretchability. Integration with standard CMOS-type batch processing is assured by utilizing the Flex-to-Rigid (F2R) post-processing technology to make the back-end-of-line interconnect structures free-standing, thus enabling the routine microfabrication of highly-stretchable interconnects. The performance and reproducibility of these free-standing structures is promising: an elastic stretch beyond 2000% and ultimate (plastic) stretch beyond 3000%, with <0.3% resistance change, and >10 million cycles at 1000% stretch with <1% resistance change. This generic technology provides a new route to exciting highly-stretchable miniature devices.

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This paper focuses on the implementation of a new technique for the fabrication of stretchable electronic patches that can be used for medical applications. The technique is based on the Electronics on Plastics by Laser Release (EPlaR) technology which enables a one-step release of a stack of flexible/stretchable layers incorporating the active layers like interconnects and embedded devices. As a proof of concept meander shaped polyimide (PI) structures are fabricated on top of a glass substrate and then transferred to a PDMS substrate with the use of this technology. The stretchability in the device is enhanced by fabricating these meander shaped structures free from the PDMS substrate hence giving them the freedom to move out of plane.@en

Silicon wafers coated with a 5μm thick layer of polyimide were treated with different surface modification techniques such as chemical adhesion promoters, oxygen plasma and an Ar+ sputter etch. After surface modification, the wafers were molded with a 1mm thick layer of PDMS. The adhesion of the PDMS was tested by peel testing and by using a Nordson DAGE wedge shear tester. It was found that commercially available chemical adhesion promoters and oxygen plasma treatment resulted in a very poor PI/PDMS adhesion, whereas the Ar+ sputter etch resulted in an adhesion so strong that the PDMS could not be delaminated from the PI surface without the failure of the material.@en