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Printed Electronics World
Posted on June 4, 2008 by  & 

Scientists create stretchable silicon circuits

Scientists at the US University of Illinois and McCormick School of Northwestern University, have developed a new form of stretchable silicon integrated circuit that can wrap around complex shapes such as spheres, body parts and aircraft wings to monitor structural properties.
The stretchable circuits can operate during stretching, compressing, folding and other types of extreme mechanical deformations, without a reduction in electrical performance. There use can also be extended in healthcare for heart monitoring systems and for use as thin sheets of electronics that wrap around the brain, monitoring electrical activity for future seizures in people with epilepsy. The researchers are reportedly working on latex surgical gloves with integrated electronics that could add sensing functionality or, in some cases, provide tactile feedback for training surgical students.
Although silicon is usually brittle, it can bend when thin enough. When the silicon atoms (naturally spaced closer together than the silicon germanium atoms) contact to the top layer of silicon germanium, they "strain" to bond to it. "If you strain the silicon lattice, then you can improve the electron mobility and performance in your device," says John Rogers, professor of material science at the University of Illinois, Urbana.
To create their fully stretchable integrated circuits, the researchers begin by applying a sacrificial layer of polymer to a rigid carrier substrate. On top of the sacrificial layer they deposit a very thin plastic coating, which will support the integrated circuit. The circuit components are then crafted using conventional techniques for planar device fabrication, along with printing methods for integrating aligned arrays of nanoribbons of single-crystal silicon as the semiconductor. The combined thickness of the circuit elements and the plastic coating is about 50 times smaller than the diameter of a human hair.
Next, the sacrificial polymer layer is washed away, and the plastic coating and integrated circuit are bonded to a piece of prestrained silicone rubber. Lastly, the strain is relieved, and as the rubber springs back to its initial shape, it applies compressive stresses to the circuit sheet. Those stresses spontaneously lead to a complex pattern of buckling, to create a geometry that then allows the circuit to be folded, or stretched, in different directions to conform to a variety of complex shapes or to accommodate mechanical deformations during use.
The researchers constructed integrated circuits consisting of transistors, oscillators, logic gates and amplifiers. The circuits exhibited extreme levels of bendability and stretchability, with electronic properties comparable to those of similar circuits built on conventional silicon wafers.
The new design and construction strategies represent general and scalable routes to high-performance, foldable and stretchable electronic devices that can incorporate established, inorganic electronic materials whose fragile, brittle mechanical properties would otherwise preclude their use, the researchers report.
The work was funded by the National Science Foundation and the U.S. Department of Energy.
References: Northwestern University, University of Illinois, Technology Review
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