Lars Büthe

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.

Fabrication and transfer of flexible few-layers MoS2 thin film transistors to any arbitrary substrate.

Recently, transition metal dichalcogenides (TMDCs) have attracted interest thanks to their large field effective mobility (>100 cm(2)/V · s), sizable band gap (around 1-2 eV), and mechanical properties, which make them suitable for high performance and flexible electronics. In this paper, we present a process scheme enabling the fabrication and transfer of few-layers MoS2 thin film transistors from a silicon template to any arbitrary organic or inorganic and flexible or rigid substrate or support. The two-dimensional semiconductor is mechanically exfoliated from a bulk crystal on a silicon/polyvinyl alcohol (PVA)/polymethyl methacrylane (PMMA) stack optimized to ensure high contrast for the identification of subnanometer thick flakes. Thin film transistors (TFTs) with structured source/drain and gate electrodes are fabricated following a designed procedure including steps of UV lithography, wet etching, and atomic layer deposited (ALD) dielectric. Successively, after the dissolution of the PVA sacrificial layer in water, the PMMA film, with the devices on top, can be transferred to another substrate of choice. Here, we transferred the devices on a polyimide plastic foil and studied the performance when tensile strain is applied parallel to the TFT channel. We measured an electron field effective mobility of 19 cm(2)/(V s), an I(on)/I(off)ratio greater than 10(6), a gate leakage current as low as 0.3 pA/μm, and a subthreshold swing of about 250 mV/dec. The devices continue to work when bent to a radius of 5 mm and after 10 consecutive bending cycles. The proposed fabrication strategy can be extended to any kind of 2D materials and enable the realization of electronic circuits and optical devices easily transferrable to any other support.

Metal oxide semiconductor thin-film transistors for flexible electronics

The field of flexible electronics has rapidly expanded over the last decades, pioneering novel applications, such as wearable and textile integrated devices, seamless and embedded patch-like systems, soft electronic skins, as well as imperceptible and transient implants. The possibility to revolutionize our daily life with such disruptive appliances has fueled the quest for electronic devices which yield good electrical and mechanical performance and are at the same time light-weight, transparent, conformable, stretchable, and even biodegradable. Flexible metal oxide semiconductor thin-film transistors (TFTs) can fulfill all these requirements and are therefore considered the most promising technology for tomorrows electronics. This review reflects the establishment of flexible metal oxide semiconductor TFTs, from the development of single devices, large-area circuits, up to entirely integrated systems. First, an introduction on metal oxide semiconductor TFTs is given, where the history of the field is revisited, the TFT configurations and operating principles are presented, and the main issues and technological challenges faced in the area are analyzed. Then, the recent advances achieved for flexible n-type metal oxide semiconductor TFTs manufactured by physical vapor deposition methods and solution-processing techniques are summarized. In particular, the ability of flexible metal oxide semiconductor TFTs to combine low temperature fabrication, high carrier mobility, large frequency operation, extreme mechanical bendability, together with transparency, conformability, stretchability, and water dissolubility is shown. Afterward, a detailed analysis of the most promising metal oxide semiconducting materials developed to realize the state-of-the-art flexible p-type TFTs is given. Next, the recent progresses obtained for flexible metal oxide semiconductor-based electronic circuits, realized with both unipolar and complementary technology, are reported. In particular, the realization of large-area digital circuitry like flexible near field communication tags and analog integrated circuits such as bendable operational amplifiers is presented. The last topic of this review is devoted for emerging flexible electronic systems, from foldable displays, power transmission elements to integrated systems for large-area sensing and data storage and transmission. Finally, the conclusions are drawn and an outlook over the field with a prediction for the future is provided.

IGZO TFT-Based All-Enhancement Operational Amplifier Bent to a Radius of 5 mm

An all-enhancement operational amplifier operating at 5 V and comprising 16 n-type amorphous indium-gallium-zinc-oxide thin-film transistors (TFTs) is fabricated on a 50 μm thick flexible polyimide substrate. The operational amplifier has an open loop voltage gain of 18.7 dB and a unity-gain frequency of 472 kHz while the common-mode rejection ratio (CMMR) is larger than 40 dB. The mechanical flexibility of the amplifier is demonstrated by bending the circuit to a radius of 5 mm, which corresponds to a tensile strain of 0.5% parallel to the TFT channels. The bent amplifier shows the same output behavior as when flat. The power consumption of the operational amplifier is 900 μW, regardless whether the circuit is flat or bent.

Flexible Self-Aligned Double-Gate IGZO TFT

In this letter, flexible double-gate (DG) thin-film transistors (TFTs) based on InGaZnO4 and fabricated on free standing plastic foil, using self-alignment (SA) are presented. The usage of transparent indium-tin-oxide instead of opaque metals enables SA of source-, drain-, and top-gate contacts. Hence, all layers, which can cause parasitic capacitances, are structured by SA. Compared with bottom-gate reference TFTs fabricated on the same substrate, DG TFTs exhibit a by 68% increased transconductance and a subthreshold swing as low as 109 mV/dec decade (-37%). The clockwise hysteresis of the DG TFTs is as small as 5 mV. Because of SA, the source/drain to gate overlaps are as small as ≈ 1 μm leading to parasitic overlap capacitances of 5.5 fF μm-1. Therefore a transit frequency of 5.6 MHz is measured on 7.5 μm long transistors. In addition, the flexible devices stay fully operational when bent to a tensile radius of 6 mm.

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).

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.

Demo: touch-less interaction with medical images using hand & foot gestures

Sterility restrictions in surgical settings make touch-less interaction an interesting solution for surgeons to interact directly with digital images. The HCI community has already explored several methods for touch-less interaction including those based on camera-based gesture tracking and voice control. In this paper, we present a system for gesture-based interaction with medical images based on a single wristband sensor and capacitive floor sensors, allowing for hand and foot gesture input. The first limited evaluation of the system showed an acceptable level of accuracy for 12 different hand & foot gestures; also users found that our combined hand and foot based gestures are intuitive for providing input.

Contact resistance and overlapping capacitance in flexible sub-micron long oxide thin-film transistors for above 100 MHz operation

In recent years new forms of electronic devices such as electronic papers, flexible displays, epidermal sensors, and smart textiles have become reality. Thin-film transistors (TFTs) are the basic blocks of the circuits used in such devices and need to operate above 100 MHz to efficiently treat signals in RF systems and address pixels in high resolution displays. Beyond the choice of the semiconductor, i.e., silicon, graphene, organics, or amorphous oxides, the junctionless nature of TFTs and its geometry imply some limitations which become evident and important in devices with scaled channel length. Furthermore, the mechanical instability of flexible substrates limits the feature size of flexible TFTs. Contact resistance and overlapping capacitance are two parasitic effects which limit the transit frequency of transistors. They are often considered independent, while a deeper analysis of TFTs geometry imposes to handle them together; in fact, they both depend on the overlapping length (LOV) between source/drain and the gate contacts. Here, we conduct a quantitative analysis based on a large number of flexible ultra-scaled IGZO TFTs. Devices with three different values of overlap length and channel length down to 0.5 μm are fabricated to experimentally investigate the scaling behavior of the transit frequency. Contact resistance and overlapping capacitance depend in opposite ways on LOV. These findings establish routes for the optimization of the dimension of source/drain contact pads and suggest design guidelines to achieve megahertz operation in flexible IGZO TFTs and circuits.

Flexible In–Ga–Zn–O Thin-Film Transistors on Elastomeric Substrate Bent to 2.3% Strain

In this letter, a photolithographic fabrication process is used to manufacture indium-gallium-zinc-oxide thin-film transistors (TFTs) with mobilities > 10 cm2/Vs directly on a 80 μm thick polydimethylsiloxane (PDMS) substrate. Once the fabrication is completed, the PDMS is detached from a silicon wafer used as carrier substrate. Due to the thermal mismatch between silicon and PDMS, the release results in a reduction of the PDMS area by 7.2%, which leads to the formation of out-of-plane wrinkles on the TFT surface. The reflattening of the wrinkles under tensile strain enables device functionality, while the TFTs are bent up to 2.3% strain. Mechanical stability of the TFTs with our wrinkled approach is shown by electrically characterizing them at bending radii down to 6 mm.