Kevin Dowling

Stretchable Inorganic‐Semiconductor Electronic Systems

Electronic and optoelectronic semiconductor components are the building blocks of modern instrumentation and equipment for sensing, computation, display, and communication. Systems incorporating these components are typically made on mechanically rigid printed circuit boards (PCBs). These systems can also be built on polymer-based fl exible PCBs, [ 1 ] which offer a bending radius of several centimeters about a single axis but are subject to fracturing from excessive bending or fatigue strain. Systems that are highly bendable (millimeter scale), stretchable, conformable to any surface topology, and mechanically insensitive to fatigue strain would greatly expand the application space of electronics. For example, in medicine there is a need for electronics to conform to and deform with the human body [ 2 ] to perform accurate diagnosis and deliver therapy. Other application spaces include renewable energy, [ 3,4 ] robotics, [ 5 ] military, [ 6 ] and lighting. [ 7 ] These applications have motivated research in fl exible and/or stretchable organic electronics [ 6 , 8–9 ] and inorganic electronics assembled on stretchable substrates. [ 2,4 , 7 , 10 ] One approach to building stretchable inorganic electronics is to connect thin electronic components together with stretchable spring-like metal interconnects and embed the entire interconnected structure into a stretchable (rubber) substrate. [ 2–3 , 7 ] Whereas prior work based on this approach used custom microfabrication of the electronic components and interconnects, here we present a process that uses commercially available electronic components and fl ip-chip bonding processes. Therefore, this fabrication process is a platform that can be used without modifi cation to create stretchable electronic systems incorporating any set of electronic components. As a demonstration, we fabricated stretchable light-emitting diode (LED) arrays containing up to 50 LEDs and show that the arrays can survive repeated stretching of 90 000 cycles and also tightly conform to a human thumb tip. The general concept behind the fabrication process is to separately manufacture the electronic components and the stretchable interconnects, then combine them using fl ip-chip bonding technology. We term this process “CINE” (combination of interconnects and electronics). Specifi cally, the process involves three steps: 1) the fabrication of metal contact pads and stretchable interconnects using standard microfabrication techniques; 2) transfer printing the contact pads and stretchable interconnects to a stretchable substrate using dissolvable adhesives as the intermediate transfer material; and 3) fl ip-chip bonding the electronic components onto the metal contact pads using anisotropic conductive fi lm (ACF). A detailed description of the fabrication process is presented in the Experimental Section. Figure 1 shows a stretchable LED array fabricated with this process. The stretchable LED array consists of ten pairs of gold contact pads, connected by serpentine-shaped metal interconnects. The interconnects are fully encapsulated in polyimide, whereas the contact pads have openings to allow electrical contact. Five blue and fi ve red LEDs were fl ip-chip bonded to the pairs of contact pads (in opposing polarities so the array can be powered with either a positive or negative voltage bias). The array was made on a silicone substrate with an elastic modulus of 10 kPa (we used the material EcoFlex made by Smooth-On Inc.). When the array is stretched, the serpentine-shaped interconnects deform to accommodate most of the strain, minimizing the strain seen by the LEDs and allowing the LEDs to maintain their optoelectronic properties (Figure 1 c,e). When the substrate is stretched, the interconnects accommodate strain via out-of-plane buckling as well as lateral deformation; this out-of-plane deformation is possible because the EcoFlex substrate is extremely compliant. We measured the resistance of individual interconnects while being stretched and found no signifi cant change in electrical resistance. To test the mechanical robustness of the arrays, we repeatedly stretched them in the length-wise direction using a mechanical actuator (additional details are provided in the Supporting Information). In the initial state, the array was without any strain, and we measured the distance between two adjacent LEDs as L 0 . When the array was fully stretched, we remeasured the distance between the two adjacent LEDs as L 1 . We defi ne the strain as ( L 1 – L 0 )/ L 0 . With a peak stretching strain of 67%, the arrays survived up to 90 000 stretching cycles (at an oscillating frequency of 1 Hz). With a peak stretching strain of 200%, the arrays survived up to 5000 cycles. We determined the failure mechanism to consistently be fracture–breakage of the serpentine interconnects near the contact pads used for powering the array. These contact pads were too large, at about 1 cm 2 in size, and created regions of high localized strain around their edges. The interconnects connecting adjacent LEDs never failed and neither did the fl ip-chip bonds made between the LEDs and the contact pads. Therefore, we expect the mechanical robustness of the arrays to dramatically increase simply by redesigning the end-most contact pads to be smaller by a factor of about four. To examine the electrical robustness, we measured the current-voltage ( I – V ) relation of an LED array prior to being stretched, after being stretched 1000 cycles, and after being stretched 10 000 cycles. We found no signifi cant variation in the I – V characteristics; the results are presented in Figure 2 a. We also measured the current fl owing through an array of 15 LEDs (arranged as three parallel sets of fi ve LEDs in-series), biased at 20 V. The current fl ow was a constant 93 mA as the

Novel Strain Relief Design for Multilayer Thin Film Stretchable Interconnects

Most electronic systems are rigid and inflexible. Many applications, however, require or benefit from conformable designs. To create efficient conformable systems, multilayer stretchable interconnects are necessary. A novel strain relief structure for multilayer stretchable interconnects is proposed. The numerical analysis shows that the proposed structure will function indefinitely when stretched as much as 20% of its initial length. Electromechanical measurements demonstrate that the onset of microcrack formation in the interconnects occurs, on average, after 89% elongation. These measurements also show that the structures are able to withstand elongations of up to 285%. Additionally, precise failure mechanisms, including interconnect straightening and microcrack formation are documented.

Epidermal electronics: Skin sweat patch

An ultrathin, stretchable, and conformal sensor system for skin-mounted sweat measurement is characterized and demonstrated in this paper. As an epidermal device, the sweat sensor is mechanically designed for comfortable wear on the skin by employing interdigitated electrodes connected via stretchable serpentine-shaped conductors. Experimental results show that the sensor is sensitive to measuring frequency, sweat level and stretching deformation. It was found that 20kHz signals provide the most sensitive performance: electrical impedance changes 50% while sweat level increases from 20 to 80. In addition, sensor elongation from 15 up to 50% affected the measurement sensitivity of both electrical impedance and capacitance.

A conformal sensor for wireless sweat level monitoring

A conformal, wearable and wireless system for continuously monitoring the local body sweat loss during exercise is demonstrated in this work. The sensor system includes a sweat absorber, an inter-digitated capacitance sensor, and a communication hub for data processing and transmission. Experimental results show that the sensor has excellent sensitivity and consistent response to sweat rate and level. A 150% variation in the sensor capacitance is observed with 50μL/cm2 of sweat collected in the absorber. During wear tests, the sensor system is placed on the subjects right anterior thigh for measuring the local sweat response during exercise (eg. running), and the measured sweat loss (147μL) was verified by the weight change within the absorbent material (144mg). With a conformal and wireless design, this system is ideal for applications in sport performance, dehydration monitoring, and health assessment.

Design for reliability of multi-layer thin film stretchable interconnects

To date, nearly all electronic systems have been rigid and inflexible. However, there are many areas such as in biomedical devices in which these rigid electronics are less than ideal and which require new conformable electronic systems. In order to create effective, compact, and complex systems, stretchable interconnects must be designed to overlap one another in multiple layers. The circular strain relief structure described in this paper effectively redistributes the strain to the crest of the horseshoes of the interconnects themselves. Numerical analysis and simulations of the strain relief structures described in this paper indicate that the structures will function indefinitely when stretched up to a 20% elongation. In-situ electromechanical measurements show that the structures are able to withstand elongations of 285% or more before failing. Precise failure mechanisms including straightening of the interconnects and micro-crack formation are documented with images taken during the electromechanical tests.

Epidermal electronics for seamless monitoring of biopotential signals

Medical deployment of electronics is often hampered by boxy and rigid packaging. Biological tissues are soft and curved, while electronic components are hard and angular. The mechanical mismatch can be improved by re-packaging electronics in radical new form factors. We present a technology platform using ultra-thin components linked with conformal interconnects and embedded in low modulus polymers to provide an excellent match to biological tissues. This technology platform builds on the pioneering work by Prof. John Rogers @ UIUC. [1, 2] Rather than developing novel semiconducting, conducting and insulating materials, the platform exploits the concept that only the top 5-15 μm of a silicon IC contributes to functional behavior. Similar considerations apply to other high performance components such as LEDs and photodiodes. The thin active layer can be removed and transferred to polymer by various processes, which we discuss below. The resulting thin and flexible silicon islands can be interconnected using metallization patterned to permit substantial macro-scale deformation while experiencing minimal micro-scale deformation, just as a coiled spring can stretch several times its own length while keeping the local metal strain within the elastic limit. On-body and in-body applications are both well suited to the technology platform. Epidermal electronics are skin-mounted systems that resemble electronic tattoos, and can be worn for extended periods without discomfort while providing continuous monitoring. In this paper, we discuss in detail, the following technologies and concepts that enable epidermal electronics: (1) Advanced die preparation methodologies that allow for thinning, placement, and attachment of sub-50μm commercial IC devices (COTS); (2) die embedding methods in flexible polymer substrates; (3) use of conformal metal interconnects to connect components; and (4) elastomer stacking optimized for application strain and compatibility with biological tissue. We present data of thinned COTS ICs embedded in flex test circuits to demonstrate the technology.