ETH - IfE-Wearable Computing - Flexible and Organic Electronics

Flexible and organic electronics are key components for a multitude of applications, such as smart tags, displays, functionalized surfaces, smart textiles etc. To enable these kinds of applications, we investigate thin-film transistors (TFTs), sensors made of organic and inorganic materials and the fabrication of interconnect lines made of metal to contact sensors, TFTs and circuits on flexible substrates. Many future applications of TFTs, sensors, circuits and interconnect lines require bending of the devices, we study the influence of bending on the devices and optimize processes and fabrication towards reliable device functionality under flexion.

Thin-Film Transistors

TFT Fabrication Thin-film transistors (TFTs) fabricated at low process temperatures (< 150 °C) on flexible plastic substrates are an integral part of future applications of flexible electronics. As semiconductor material we use amorphous Indium-Gallium-Zinc-Oxide (a-IGZO), because it can be deposited at room temperature and provides an electron mobility larger than 10 cm²/Vs. The whole TFT fabrication process is compatible with the tool available in the FIRST clean-room.	Cross-section of a TFT on a flexible substrate Single a-IGZO TFT
TFT Bending To test the influence of bending on our TFTs, we use a computer controlled, custom built bending tester. This device allows us to apply tensile and compressive strain to the TFTs. Our experiments show, that tensile strain parallel to TFT channel length increases the electron mobility and additionally, the ambient light boosts this effect.	Custom built bending tester
	Mobility change of strained TFT
TFT Circuits and Testing To enable flexible electronics, digital and analog circuits are required for signal processing, computational tasks etc. Therefore, we fabricated basic digital building blocks such as NANDs, NORs, Inverters and 1-bit SRAM cells as well as a transimpedance amplifier to convert an input current into an output voltage.	Unit cell of circuits based on a-IGZO TFTs
As for the TFTs, the influence of bending on the circuits is investigated. To apply bending and hence strain to the circuits, the flexible plastic substrate with circuits is wound around cylindrical rods and placed in a probe station for applying input signals and measuring output signals.	Circuits are tested under bending using probe tips
As an example for the influence of bending on a circuit, a bode plot of a bent transimpedance amplifier is shown. Decreasing the bending radius of the circuit decreases the gain and increases the -3dB frequency.	Bode plot of bent transimpedance amplifier
As a second example, NAND gates made of a-IGZO TFTs are bent to a radius of 3.5 mm. The output signal of the bent NAND gate behaves differently according to the direction of the applied strain. Strain applied parallel to the TFT channel of the load transistors has a smaller effect on the output signal compared to strain applied perpendicular to the channel of the load transistors. This effect is attributed to the development of micro-cracks in the channel region. For strain perpendicular to the channel of the load transistor, micro-cracks can disconnect a part of the gate contact leading to an increased resistance and therefore a drop of the high voltage of the output signal. However, reflattening the substrate restores the circuit functionality and the output signal is the same as before bending.	Bending NAND gates

Flexible Sensors

We fabricate flexible sensors on the same substrate as the TFTs for future combination of both technologies. The sensors are made of organic and inorganic materials.

Inorganic Flexible Sensor

As an example of an inorganic sensor, we fabricated a resistive temperature device (RTD). It consists of a meandering gold conductor with four contacts enabling a Kelvin measurement to decouple the interconnect lines to the sensor from the measured resistance values.

Microfabricated RTDs

Organic Flexible Sensor

Organic sensors are fabricated with either PEDOT or PANI. Both materials are conductive polymers that react on changes in humidity.

PEDOT Sensor

Gas sensing

To detect traces of gases in air, polymers are suitable materials. We fabricated finger electrodes on flexible substrates and covered the electrodes with a conductive polymer (PANI). PANI is sensitive to CO molecules and changes its resistances when exposed to CO.

PANI Sensor

PANI sensor response

Interconnect Lines

One targeted application are smart textiles. We investigated the mechanical influence of the threads on flexible substrates woven into textiles. Our measurements show that the flexible substrates are bent to radii smaller than 170 microns. This value corresponds to tensile or compressive strains in metal layers the flexible substrates of up to 12 %. Such strain values lead to cracks in metal layers. Because sensors, TFTs and circuits made of TFTs require interconnects, we adjusted our fabrication process by a substrate pre-treatment to enable metal layers on flexible substrate that can withstand strain values larger than 12 %.

Cross-section through textile with woven flexible stripe

Applications

Smart textiles, these are textiles that contain sensors, actuators and signal processing capabilities, are one important field of application for flexible electronics.

We integrate electronics on flexible substrates, cut in narrow stripes (2 mm wide or less) into textiles during the weaving process. In one of our first demonstrators, we integrated RTDs on flexible stripes into a woven textile.	Stripe with RTD woven into textile
A second application is a textile that contains digital temperature sensors. The sensors, silicon based ICs, are mounted on flexible plastic stripes. The plastic stripes provide contact pads and interconnect lines to these ICs and are woven into a textile during the weaving process. The individual stripes with the ICs are connected with conductive threads that are woven as well forming a bus structure. This allows a read-out of all sensors with only two external contacts on the textile.	Textile with integrated digital temperature sensors
The combination of thin-film a-IGZO TFTs and SMDs on flexible plastic stripes is demonstrated with a textile integrated 3 by 3 LED matrix. Beneath each LED, a-IGZO TFT is located to control the LED. Conductive threads are used to control the gates of the TFTs enabling to display different patterns.	LED_Matrix

Publications

1.	Development and evaluation of temperature sensors for textile integration Thomas Kinkeldei, Christoph Zysset, Kunigunde Cherenack and Gerhard Tröster Proceedings of the IEEE Sensors Conference 2009, Christchurch New Zealand, pages 1580-1583, IEEE, 2009
2.	Woven thin-film metal interconnects Kunigunde Cherenack, Thomas Kinkeldei, Christoph Zysset and Gerhard Tröster IEEE Electron Device Letters, 21:7(740-742), 2010
3.	Wearable Electronics: Woven Electronic Fibers with Sensing and Display Functions for Smart Textiles Kunigunde Cherenack, Christoph Zysset, Thomas Kinkeldei, Niko Münzenrieder and Gerhard Tröster Advanced Materials, 22:45(5071), 2010
4.	Woven Electronic Textiles: An Enabling Technology for Health-Care Monitoring in Clothing Christoph Zysset, Thomas Kinkeldei, Kunigunde Cherenack and Gerhard Tröster 5th International Workshop on Ubiquitous Health and Wellness (UbiHealth 2010), www.creat-net.org, 2010
5.	Weaving Integrated Circuits into Textiles Christoph Zysset, Kunigunde Cherenack, Thomas Kinkeldei and Gerhard Tröster Proceedings of the 14th IEEE International Symposium on Wearable Computers (ISWC 2010), Seoul, Korea, IEEE Computer Society, 2010
6.	Textile electronics for monitoring the room climate Thomas Kinkeldei, Christoph Zysset, Kunigunde Cherenack and Gerhard Tröster Plastic Electronics, Dresden, Germany, 2010
7.	Crack prevention of highly bent metal thin films in woven electronic textiles Thomas Kinkeldei, Christoph Zysset, Kunigunde Cherenack and Gerhard Tröster 3rd International Symposium on Flexible Organic Electronics, Greece, Halkidiki, 2010
8.	Indium-gallium-zinc-oxide based mechanically flexible transimpedance amplifier Christoph Zysset, Niko Münzenrieder, Thomas Kinkeldei, Kunigunde Cherenack and Gerhard Tröster Electronics Letters, vol. 47, no. 12 pp. 691 - 692, 2011
9.	A textile integrated sensor system for monitoring humidity and temperature Thomas Kinkeldei, Christoph Zysset, Kunigunde Cherenack and Gerhard Tröster International Conference on Solid-State Sensors, Actuators and Microsystems, Beijing, China, 2011
10.	A Flexible Gas Sensor for the Integration into Smart Textiles Thomas Kinkeldei, Christoph Zysset, Kunigunde Cherenack and Gerhard Tröster International Symposium on Olfaction and Electronic Nose, New York, USA, 2011
11.	A flexible InGaZnO based 1-bit SRAM under mechanical strain Niko Münzenrieder, Christoph Zysset, Thomas Kinkeldei, Kunigunde Cherenack and Gerhard Tröster Semiconductor Conference Dresden (SCD), 2011
12.	Testing of flexible InGaZnO-based thin-film transistors under mechanical strain Niko Münzenrieder, Kunigunde Cherenack and Gerhard Tröster The European Physical Journal Applied Physics, 55(23904), 2011
13.	The Effects of Mechanical Bending and Illumination on the Performance of Flexible IGZO TFTs Niko Münzenrieder, Kunigunde Cherenack and Gerhard Tröster Transactions on Electron Devices, IEEE, 58(2041-2048), 2011
14.	Woven Active-Matrix Display Christoph Zysset, Niko Münzenrieder, Thomas Kinkeldei, Kunigunde Cherenack and Gerhard Tröster IEEE Transactions on Electron Devices, 59:3(721-728), 2012

Oral and poster presentations at

International Symposium on Flexible Organic Electronics, ISFOE, 2010, Halkidiki, Greece
Plastic Electronics, Dresden, Germany, 2010
Materials Research Centre Symposium, Zürich, Switzerland, 2009 and 2010
International Thin-Film Transistor Conference, Cambridge, UK, 2011

Project Links

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