Advances in soft and stretchable substrates and elastomeric materials is giving rise to an entirely new generation of flexible, printed and organic electronics. And while you might have been paying attention to other matters there has been a rapid development of applications utilizing flexible electronics, including bio/medical equipment such as wearable health monitoring devices, flexible active matrix OLED (AMOLED) displays for cell phones, bendable solar cells, and printed batteries. What’s more, flexible TFTs and passive components monolithically integrated on bendable plastic substrates promise to enable such applications as large area RF systems that can be directly mounted to the curved surfaces of military vehicles.
Market statistics seem to be supporting the belief that in the future many electronic assemblies on rigid printed circuit boards (PCBs) will be replaced by mechanically flexible or even stretchable alternatives. According to the Cambridge UK-based market research firm ID Tech EX, the global market for printed and potentially printed electronics will rise from $9.4 billion in 2012 (predominately organic electronics such as OLED display modules followed by thin film transistor circuits, sensors and batteries) to $63.28 billion in 2022.
Flexible electronics device manufacturing of basic electronic circuit components has the potential to reduce fabrication cost and eliminate time-consuming production steps. A traditional photolithographic patterning technique (e.g., plasma enhanced chemical vapor deposition, sputtering, laser annealing, etc.) can require more than six steps to deposit a single layer. Much of the material is then etched away. Flexible electronics combine graphics arts printing and microelectronics to enable machines to literally print circuits (such as printed thin film transistors) on top of plastic materials in a similar manner to the way that an inkjet printer sprays ink on paper. It requires as few as three steps which, in turn, could lead to dramatically reduced manufacturing costs.
Researchers around the globe are developing a wide range of flexible system components, sensors and power sources. Here’s a few for instance:
Memory, amps and ring oscillators
At the International Electron Devices Meeting (IEDM) last fall IBM researchers demonstrated CMOS circuits —including SRAM memory and ring oscillators—on a flexible plastic substrate. The extremely thin silicon on insulator devices had a body thickness of just 60 angstroms. IBM built them on silicon and then used a room-temperature process called controlled spalling, which essentially flakes off the Si substrate. Then they transferred them to flexible plastic tape.
The devices had gate lengths of <30 nm and gate pitch of 100 nm. The ring oscillators had a stage delay of just 16 ps at 0.9 V, believed to be the best reported performance for a flexible circuit.
In a recent edition of the journal Nature Communications a team of researchers from the University of Pennsylvania showed that nanoscale particles, or nanocrystals, of the semiconductor cadmium selenide can be "printed" or "coated" on flexible plastics to form high-performance electronics. Because the nanocrystals are dispersed in an ink-like liquid, multiple types of deposition techniques can be used to make circuits, wrote the researchers. In their study, the researchers used spin coating, where centrifugal force pulls a thin layer of the solution over a surface, but the nanocrystals could be applied through dipping, spraying or ink-jet printing as well, they report.
Using this process, the researchers built three kinds of circuits to test the nanocrystal’s performance for circuit applications: an inverter, an amplifier and a ring oscillator. All of these circuits were reported to operate with a couple of volts, according to the researchers an important point since If you want electronics for portable devices that are going to work with batteries, they have to operate at low voltage or they won’t be useful.
Health monitors and displays
Imec, based in Leuven, Belgium, recently announced that it has integrated an ultra-thin, flexible chip with bendable and stretchable interconnects into a package that adapts dynamically to curving and bending surfaces for wearable health monitors (e.g. electrocardiogram or temperature sensors), advanced surgical tools, or consumer electronics such as mobile phones. For the demonstration the researchers thinned a commercially available microcontroller down to 30µm, preserving its electrical performance and functionality. This die was then embedded in a slim polyimide package (40-50µm thick). Next, this ultrathin chip was integrated with stretchable electrical wiring.
Imec is also making thin film transistors on flexible plastic, combining the n-type transistors of the metal oxide AM backplane with organic p-type semiconductors to make RFID circuits and display line drivers. Imec further has teamed up with the HOLST Centre system-in-foil research program for next-generation flexible OLED displays. Apart from the application of backplanes for displays, thin-film electronics on flexible foils is being envisaged as technology for circuits. The research outfit is investigating thin-film circuits for small circuits ranging from transponders for high frequency radio-frequency identification (HF RFID), to reconfigurable computers and microprocessors.
PARC, a Xerox company, along with Thin Film Electronics ASA have been awarded a 12-month contract from the FlexTech Alliance to develop flexible, lightweight, printed integrated sensor systems comprising logic and memory. Applications of these sensors will include radiation tags, disposable medical sensors, pharmaceuticals tags to read and track temperature during shipping, and food tags, which help determine spoilage and contamination, among other uses.
The initial project will concentrate on measuring temperature and blood oxygen content. In the longer term, it will pave the way toward printed sensing platforms to monitor humidity, light, pressure, sound, and gas.
PARC has developed jet-printing processes for organic semiconductors (including all-printed TFT arrays), and conductors – resulting in increased functionality and reduced manufacturing costs. The printed transistors are said to have exceptional performance for polymers and meet all requirements to address displays. The company’s a-Si, low-temperature polysilicon (LTPS), and organic semiconductor TFTs – which have the advantage of low-temperature deposition and low-elastic modulus – have also been applied to various radiation detectors, including x-ray, ultrasound, and neutron imaging.
One of the things seemingly hampering advances in bendable electronics research is uncertainty surrounding a product’s power source. At the University of Delaware, Bingqung Wei and his colleagues are researching energy sources that are scalable and stretchable. In a report published in Nano Letters, a journal of the American Chemical Society, Wei’s research team reported significant progress in developing scalable, stretchable power sources using carbon nanotube macrofilms, polyurethane membranes and organic electrolytes. According to Wei, the supercapacitor developed in his lab achieved excellent stability in testing and the results will provide important guidelines for future design and testing of this leading-edge energy storage device.
Also in Nano Letters researchers from the Korea Advanced Institute of Science and Technology (KAIST) in Daejeon, South Korea published a study on a new bendable Li-ion battery for fully flexible electronic systems.
Although the rechargeable lithium-ion battery has been regarded as a strong candidate for a high-performance flexible energy source, compliant electrodes for bendable batteries are restricted to only a few materials, and their performance has not been sufficient for them to be applied to flexible consumer electronics including rollable displays.
The researchers presented a flexible thin-film lithium ion battery that enables the realization of diverse flexible batteries regardless of electrode chemistry. The result is a flexible Li-ion battery that can be made with almost any electrode material. Here, the researchers used lithium cobalt oxide as the cathode material, which is currently the most widely used cathode in non-flexible Li-ion batteries due to its high performance. For the anode, they used traditional lithium.
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Murray Slovick is Editorial Director of Intelligent TechContent, an editorial services company that produces technical articles, white papers and social media posts for clients in the semiconductor/electronic design industry. Trained as an engineer, he has more than 20 years of experience as chief editor of award-winning publications covering various aspects of consumer electronics and semiconductor technology. He previously was Editorial Director at Hearst Business Media where he was responsible for the online and print content of Electronic Products, among other properties in the U.S. and China. He has also served as Executive Editor at CMP’s eeProductCenter and spent a decade as editor-in-chief of the IEEE flagship publication Spectrum.