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.

Lath martensite is important in industry because it is the key strengthening component in many advanced high strength steels. The study of crystallography and chemistry of lath martensite is extensive in the literature, however, mostly based on fully martensitic steels. In this work, lath martensite in dual phase steels is investigated with a focus on the substructure identification of the martensite islands and microstructural bands using electron backscattered diffraction, and on the influence of the accompanied tempering process during industrial coating process on the distribution of alloying elements using atom probe tomography. Unlike findings for the fully martensitic steels, no martensite islands with all 24 Kurdjumov-Sachs variants have been observed. Almost all martensite islands contain only one main packet with all six variants and minor variants from the remaining three packets of the same prior austenite grain. Similarly, the martensite bands are typically composed of connected domains originating from prior austenite grains, each containing one main packets (mostly with all variants) and few separate variants. The effect of tempering at ~450 °C (due to the industrial zinc coating process) has also been investigated. The results show a strong carbon partitioning to lath boundaries and Cottrell atmospheres at dislocation core regions due to the thermal process of coating. In contrast, auto-tempering contributes to the carbon redistribution only in a limited manner. The substitutional elements are all homogenously distributed. The phase transformation process has two effects on the material: mechanically, the earlier-formed laths are larger and softer and therefore more ductile (as revealed by nanoindentation); chemically, due to the higher dislocation density inside the later-formed laths, which are generally smaller, carbon Cottrell atmospheres are predominantly observed.

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.