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A nanotechnologist's take on 'two sides of the same coin

(Nanowerk Spotlight) Implementation of sensor systems, in particular the envisaged monitoring devices for body functions and advanced healthcare applications, entail development of a low-cost sensor electrode array which is physically flexible; chemically, mechanically and thermally stable; and, in case of implantable systems, the electrode array has to be non-corrosive and bio-compatible. Finally, they have to be integrated in an efficient manner with the interface circuitry.
"Sensor arrays and control elements for flexible electronics devices are usually placed on the same plane, unnecessary requiring additional area, and causing problems of heat dissipation," Muhammad Mustafa Hussain, an Associate Professor of Electrical Engineering at King Abdullah University of Science and Technology (KAUST), tells Nanowerk. "These challenges motivated me to come up with an area-efficient solution for the problem of connecting sensors and electronics together in such a way that electronics can be kept away from the sensed surface. This is the first time ever the concept of double sided flexible 3D electronics has been introduced in the flexible and wearable electronics industry."
Hussain and his team developed a highly manufacturable integration strategy for making 3D flexible sensor arrays and connecting them to control electronics based on the widely popular phrase, 'Two sides of the same coin'.
They reported their findings in the July 25, 2016 online edition of Small ("Deterministic Integration of Out-of-Plane Sensor Arrays for Flexible Electronic Applications").
Out-of-Plane Sensor Arrays for Flexible Electronic Applications
(a) Illustration of an implanted electrode array for monitoring electrical activity in the brain (ECoG). (b) Cross-section of the fabricated TPVs connecting the sensor electrode. (c) Schematic process flow for fabrication of flexible electrode array with vertical TPVs. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
"The key innovation in our work is the fabrication of through polymer vias (TPVs) for interconnecting sensors made on one side of a polymer thin film to the control electronics fabricated on the other side," notes Aftab M Hussain, a PhD student in Hussain's lab and the paper's first author. "All the fabrication, including the sensor array, TPVs and control electronics can be done on the same substrate using CMOS compatible processes."
"Hence, the technology is applicable to any sensor array and any thin film transistor (TFT) including silicon, silicon germanium, gallium nitride, etc. based electronics," he adds.
The choice of materials used for this process development involved several challenges requiring innovative engineering solutions.
The polymer used in the process is polyimide, which is a mechanically stable, chemically inert and environmentally friendly polymer. The sensors are fabricated on one side of the polymer thin film, while the electronics are on the other side.
As the scientists explain, a distinct advantage of polyimide is that it can withstand temperatures of up to 350°C for long durations, which is sufficient for the fabrication of thin film based electronics circuits on the polymer base.
Also, another key advantage of polyimide is that it does not react with most of the common chemical reagents used in the CMOS industry, hence the base layer is not affected by the subsequent fabrication processes.
A further key advantage with the use of polymer after the fabrication of sensor layer is that the polymer covers the sensor layer conformally. This means that the top surface is smooth enough for fabrication of the electronics. Also, the polymer flows and settles around the sensor pads on the bottom surface, making the bottom surface smooth and conformal as well.
TPVs are basically interconnections made through this polymer layer – hence the name through polymer vias. The interconnections are made using copper which is a low-cost metal commonly used in the electronics industry.
This method reduces the overall area of the electronic device, increases the contact between the sensor layer and the sensed surface, and increases the distance between the control electronics and the sensed surface.
"This is an important advantage in case of biomedical applications because the sensed surface, which can be the skin or the brain surface, remains free of the heat dissipated by the control electronics," emphasizes Prof. Hussain.
One of the major applications of this technology is in the advanced healthcare industry. In particular, diagnostic devices made using this technology can have the sensor array and sensor pads on one side of a polymer thin-film and the control electronics on the other. This provides the opportunity to conformally contact the sensors with the sensed surface, while keeping the electronic circuits away from it.
Other industries such as automobile and aviation can benefit from the reduction in area of the electronics by fabricated electronics on both sides of a polymer substrate.
The next stage in the team's work will be to interconnect multiple layers of these two-sided electronic circuits to obtain multilayer flexible 3D electronic systems. This will result in further area reduction and improvement in interconnect delays and power dissipation.
"This work has the potential to revolutionize the electronics industry and opens doors to commercializing affordable high performance sensing devices," concludes Prof. Hussain. "The CMOS compatible fabrication process provides a way for deterministic assembly and fabrication of millions of device. This manufacturability and scalability can reduce the cost of advanced healthcare devices, thus making it affordable for the masses."
"I believe the field of flexible electronics, in particular for advanced healthcare applications, can greatly improve the lives of millions of people across the world by providing continuous and early detection of symptoms," he adds.
"However" he cautions, "there are several challenges that need to be addressed for the devices to have large scale viability and affordability. One of the key challenges is the development of a battery or power source for powering these devices continuously over several months, if not years."
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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