As part of the Japan Science and Technology Agency’s (JST) Exploratory Research for Advanced technology project, an international research team has manufactured the world’s thinnest and lightest soft organic transistor integrated circuits (ICs). These are on ultrathin polymeric films with a thickness of only 1.2µm. Possible applications range from healthcare and biomedicine to mobility equipment.
Professor Takao Someya, Associate Professor Tsuyoshi Sekitani, Dr Martin Kaltenbrunner, University of Tokyo, and their coworkers have succeeded in developing the world’s lightest (3g/m2) and, simultaneously, the world’s thinnest so far (2µm; one micrometre (µm) is 1/1,000,000 of a metre) mechanically flexible integrated circuits and touch sensor system. This was possible because the team developed a novel technique to form a high-quality 19nm-thick (one nanometre (nm) is 1/1,000,000,000 of a metre) insulating layer on the rough surface of the 1.2-µm-thick polymeric polyethylenenaphthalate (PEN) film. To put these values into perspective, the thickness is about one fifth of that of plastic kitchen wrap, and the weight is about 1/30 of that of standard weight office paper. The organic transistor IC can be used to manufacture a prototype touch sensor system consisting of 144 (12 × 12) sensor cells arranged in an effective area of 4.8 × 4.8cm2 with an intercell spacing of 4mm.
The 19nm-thick aluminium oxide layer shows good adhesion to the substrate based on anodic oxidation that is carried out at room temperature. High-energy oxygen plasma treatment, which has been commonly employed to form aluminium oxide layers on plastic films, damages ultrathin polymer foils and creates pinholes. The methods developed here avoid such high-energy processes and are fully compatible with the specific processing requirements of ultrathin and conformable foil substrates.
Sensors and electronic circuits for healthcare and medical applications are generally fabricated using silicone and other rigid electronic materials. To minimise the discomfort of wearing rigid sensors, soft electronic materials were the focus of the researchers, particularly for devices that come directly into contact with the skin. In this regard, electronics manufactured on thin polymeric films are said to provide a higher level of comfort as a thinner substrate provides better mechanical flexibility. However, directly manufacturing sensors or electronic circuits on ultrathin polymeric films with thicknesses of several micrometres or less is a difficult task if conventional semiconductor processes are used.
The organic transistor ICs are said to exhibit robustness in spite of being super-thin. Indeed, the electrical properties and mechanical performance of the transistor ICs were practically unchanged (no degradation was seen) even when squeezed to a bending radius of 5µm, dipped in physiological saline, or stretched to up to double their original size. Finally, these organic transistor ICs have been utilised to develop a flexible touch sensor system prototype. The ICs are claimed to be mechanically and electrically unbreakable, even when squeezed to reduce the bending radius to 5µm, crumpled like paper, or dropped from a height of a metre (or more).
The durability was also demonstrated in the study: after immersing the ICs in physiological saline (with components that are the same as bodily fluids or sweat) for more than two weeks, no obvious deterioration in the electrical properties was observed. Furthermore, the electric and mechanical performances of the organic transistor ICs were practically unchanged even when stretched by up to 233%.
Imperceptible electronics, namely, extremely thin, lightweight electronics whose presence cannot be perceived when worn, will open up a wide range of new applications in fields ranging from healthcare and biomedicine to mobility equipment. Many new applications could emerge including wearable healthcare sensor systems, stress-free (free of discomfort) input units for mobility equipment, such as wheelchairs, sensors for medical electronic equipment and tough sensors for sports usage. The soft sensor system can be applied to freely curved surfaces like human skin to continuously measure body temperature, blood pressure, and many other vital signs.
This project was carried out in collaboration with Jonathan Reeder, Dr Tomoyuki Yokota, Kazunori Kuribara and Takeyoshi Tokuhara at the University of Tokyo, Tokyo, Japan; and Professor Siegfried Bauer, Michael Drack, Dr Reinhard Schwödiauer, Assistant Professor Ingrid Graz and Dr Simona Bauer-Gogonea at Johannes Kepler University, Linz, Austria.
In 2011, the same team succeeded in fabricating high-quality organic solar cells on 1-µm-thick polymeric films, but in contrast, this development is the world’s first report of the successful fabrication of organic transistor ICs on 1-µm-thick polymeric films.