Thomas Kinkeldei

Wafer-scale design of lightweight and transparent electronics that wraps around hairs

Electronics on very thin substrates have shown remarkable bendability, conformability and lightness, which are important attributes for biological tissues sensing, wearable or implantable devices. Here we propose a wafer-scale process scheme to realize ultra flexible, lightweight and transparent electronics on top of a 1-μm thick parylene film that is released from the carrier substrate after the dissolution in water of a polyvinyl- alcohol layer. The thin substrate ensures extreme flexibility, which is demonstrated by transistors that continue to work when wrapped around human hairs. In parallel, the use of amorphous oxide semiconductor and high-K dielectric enables the realization of analogue amplifiers operating at 12 V and above 1 MHz. Electronics can be transferred on any object, surface and on biological tissues like human skin and plant leaves. We foresee a potential application as smart contact lenses, covered with light, transparent and flexible devices, which could serve to monitor intraocular pressure for glaucoma disease.

Flexible Self-Aligned Amorphous InGaZnO Thin-Film Transistors With Submicrometer Channel Length and a Transit Frequency of 135 MHz

Flexible large area electronics promise to enable new devices such as rollable displays and electronic skins. Radio frequency (RF) applications demand circuits operating in the megahertz regime, which is hard to achieve for electronics fabricated on amorphous and temperature sensitive plastic substrates. Here, we present self-aligned amorphous indium-gallium-zinc oxide-based thin-film transistors (TFTs) fabricated on free-standing plastic foil using fabrication temperatures . Self-alignment by backside illumination between gate and source/drain electrodes was used to realize flexible transistors with a channel length of 0.5 μm and reduced parasitic capacities. The flexible TFTs exhibit a transit frequency of 135 MHz when operated at 2 V. The device performance is maintained when the TFTs are bent to a tensile radius of 3.5 mm, which makes this technology suitable for flexible RFID tags and AM radios.

Woven Temperature and Humidity Sensors on Flexible Plastic Substrates for E-Textile Applications

In this paper, a woven textile containing temperature and humidity sensors realized on flexible, plastic stripes is presented. The authors introduce two different sensors fabrication techniques: the first one consists of a conventional photolithography patterning technique; the second one, namely inkjet-printing, is here presented as an effective, low-cost alternative. In both cases, we obtain temperature and humidity sensors that can be easily integrated within a fabric by using a conventional weaving machine. All the sensors are fully characterized and the performances obtained with the two different fabrication techniques are compared and discussed, pointing out advantages and drawbacks resulting from each fabrication technique. The bending tests performed on these sensors show that they can be successfully woven without being damaged. A demonstrator, consisting of a mechanical support for the e-textile, a read-out electronic circuit, and a graphical PC interface to monitor the acquisition of humidity and temperature values, is also presented and described. This paper opens an avenue for real integration between printed electronics and traditional textile technology and materials. Printing techniques may be successfully used for the fabrication of e-textile devices, paving the way for the production of large area polymeric stripes and thus enabling new applications that, at the moment, cannot be developed with the standard lithography methods.

Integration Method for Electronics in Woven Textiles

This paper presents a technology to integrate electronics at the thread level in woven textiles. Flexible plastic substrates are cut into stripes and serve as carriers for electronics, including ICs, thin-film devices, interconnect lines, and contact pads. These functionalized plastic stripes, called e-stripes, are woven into textiles. Conductive threads perpendicular to the e-stripes electrically interconnect the devices on the individual e-stripes. The integration of e-stripes and conductive threads into the woven textiles is compatible with commercial weaving processes and suitable for large-scale manufacturing. We demonstrate the technology with a woven textile containing five e-stripes with digital temperature sensors. Conductive threads interconnect the e-stripes among each other to form a bus topology. We show that the contacts between the conductive threads and the pads on e-stripes as well as the contacts between the temperature sensors and e-stripes withstand shear forces of at least 20 N. The integration of the temperature sensors into the textile increases the bending rigidity of the textile by 30%; however, it is still possible to obtain a textile-bending radii of <;1 mm. This technology seamlessly integrates electronics into textiles, thus advancing the field of smart textiles and wearable computing.

Design Rules for IGZO Logic Gates on Plastic Foil Enabling Operation at Bending Radii of 3.5 mm

Findings obtained from bending experiments with mechanically flexible InGaZnO-based thin-film transistors are used to derive design rules for flexible InGaZnO-based n-channel metal-oxide-semiconductor logic circuits. Based on the developed design rules, flexible NAND gates, inverters, and five-stage ring oscillators are fabricated directly on free-standing plastic foils at temperatures ≤ 150 °C. Geometrically well-designed circuits operated at a supply voltage of 5 V are exposed to tensile mechanical strains, induced by bending, up to 0.72% without performance degradation. This corresponds to a bending radius of 3.5 mm. At the same time, increases in the rise time by a factor of ca 2 and reductions in the high and low output voltage levels by ca 10% and 50% have been observed for circuits with disadvantageous geometrical design. Ring oscillators designed to be operated under strain show an increase in oscillation frequency from 22.9 kHz (flat substrate) to 23.32 kHz (bending radius: 3.5 mm). This demonstrates the held-effect mobility increase in a-IGZO-based circuits under tensile mechanical strain. Long-term reliability is evaluated with 20000 cycles of repeated bending and reflattering without circuit failure.

A textile integrated sensor system for monitoring humidity and temperature

We report on the fabrication of a temperature and a humidity sensor for the integration into a smart textile as sensing device that monitors the room climate. For this purpose, we developed a sensor system on flexible polyimide substrates containing a gold resistance temperature sensor (RTDs) and conductive polymer (PEDOT-PSS) humidity sensor. The sensor is woven into a textile using a commercial band weaving machine. The measured resistance versus temperature shows a linear response with a temperature coefficient of 0.0028 °C−1. The measured resistance versus humidity for the PEDOT sensors shows a non-linear distribution over the range from 30 to 80 % RH. We successfully tested the devices in a controlled environment at 20 °C and 30–50 %RH. Both sensors show their potential for environmental monitoring of the room climate as integrated sensors in textiles.

Influence of Mechanical Bending on Flexible InGaZnO-Based Ferroelectric Memory TFTs

Future flexible electronic systems require memory devices combining low-power operation and mechanical bendability. Here, we present mechanically flexible amorphous InGaZnO (a-IGZO) memory thin-film transistors (TFTs) with a ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] gate insulator. Memory operation is demonstrated with a memory window of 3.2 V and a memory ON/OFF ratio of 1.5×106 (gate-source voltage sweep of ±6 V). The measured mobility of 8 cm2 V-1s-1 and the ON/OFF current ratio of 107 are comparable with the values for reference TFTs fabricated on the same substrate. To use memory TFTs in flexible applications, it is crucial to understand their behavior under mechanical strain. Flexible memory and reference TFTs are characterized under bending radii down to 5.5 mm, corresponding to tensile and compressive strain of ≈ ±0.6%. For both memory and reference TFTs, tensile strain causes negative threshold voltage shifts and increased drain currents, whereas compressive strain results in the opposite effects. However, memory TFTs, compared with reference TFTs, exhibit up to 8× larger threshold voltage shifts and 17× larger drain current variations. It is shown that the strain-dependent properties of a-IGZO can only explain the shifts observed in reference TFTs, whereas the variations in memory TFTs are mainly caused by the piezoelectric properties of P(VDF-TrFE).

Encapsulation for Flexible Electronic Devices

Flexible electronic devices are influenced by mechanical impacts or bending. This mechanical stress can damage brittle inorganic thin-film layers. One way to improve the stability of such devices is to encapsulate them. For this reason, we encapsulate brittle inorganic materials in our thin-film transistors (TFTs) on flexible polyimide (Kapton) substrates by gluing a second patterned flexible polyimide substrate on top. The developed encapsulation method has the advantages of low process temperatures and micrometer alignment between features on the two substrates and does not damage any thin-film device layer. We are able to bend encapsulated TFTs down to a radius of 125 μm without loss of functionality. At this bending radius, the saturation mobility decreases by 3%, and the threshold voltage shifts by 0.2 V. Compared with nonencapsulated TFTs, we decrease the bending radius before the devices fail by an order of magnitude.

Textile integrated sensors and actuators for near-infrared spectroscopy

Being the closest layer to our body, textiles provide an ideal platform for integrating sensors and actuators to monitor physiological signals. We used a woven textile to integrate photodiodes and light emitting diodes. LEDs and photodiodes enable near-infrared spectroscopy (NIRS) systems to monitor arterial oxygen saturation and oxygenated and deoxygenated hemoglobin in human tissue. Photodiodes and LEDs are mounted on flexible plastic strips with widths of 4 mm and 2 mm, respectively. The strips are woven during the textile fabrication process in weft direction and interconnected with copper wires with a diameter of 71 μm in warp direction. The sensor textile is applied to measure the pulse waves in the fingertip and the changes in oxygenated and deoxygenated hemoglobin during a venous occlusion at the calf. The system has a signal-to-noise ratio of more than 70 dB and a system drift of 0.37% ± 0.48%. The presented work demonstrates the feasibility of integrating photodiodes and LEDs into woven textiles, a step towards wearable health monitoring devices.

Locally reinforced polymer-based composites for elastic electronics.

A promising approach to fabricating elastic electronic systems involves processing thin film circuits directly on the elastic substrate by standard photolithography. Thin film devices are generally placed onto stiffer islands on the substrate surface to protect devices from excessive strain while still achieving a globally highly deformable system. Here we report a new method to achieve island architectures by locally reinforcing polymeric substrates at the macro- and microscale using magnetically responsive anisotropic microparticles. We demonstrate that the resulting particle-reinforced elastic substrates can be made smooth enough for the patterning and successful operation of thin film transistors with transfer characteristics comparable to state-of-the-art devices.