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October 17th, 2008

Foldable and stretchable circuits: teaching silicon new tricks

Published on October 17th, 2008 | By: pwray@ceramics.org
Circuit diagram (top frame) and optical images of a stretchable, "wavy" silicon ring oscillator circuit on a rubber substrate, in the "as fabricated" flat state (top micrograph) and in moderate and high states of biaxial compression (middle and bottom micrographs, respectively). Credit: Rogers.

Circuit diagram (top frame) and optical images of a stretchable, "wavy" silicon ring oscillator circuit on a rubber substrate, in the "as fabricated" flat state (top micrograph) and in moderate and high states of biaxial compression (middle and bottom micrographs, respectively). Credit: Rogers.

Researchers led by John Rogers, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign, have developed a new form of flexible, stretchable silicon integrated circuit. Not only can these new silicon circuits wrap around complex shapes, but they can do so without sacrificing electrical performance while stretching, compressing and folding is taking place, the researchers say. “The notion that silicon cannot be used in such applications because it is intrinsically brittle and rigid has been tossed out the window,” says Rogers, whose findings have been published in Science Magazine and posted on its  ScienceXpress website.”Through carefully optimized mechanical layouts and structural configurations, we can now use silicon in integrated circuits that are fully foldable and stretchable,” Rogers says. The development could lead to new types of sensors that can be integrated into artificial muscles, wearable health-monitoring systems or electrical devices that can wrap around aircraft wings and fuselages to monitor structural properties.

‘Wavy’ electronics

Rogers and his UI research team had previously reported the development of a one-dimensional, stretchable form of single-crystal silicon with micron-sized, wave-like geometries in 2005. He said then that the configuration allowed reversible stretching in one direction without significantly altering electrical properties, but only at the level of individual material elements and devices. Now Rogers and collaborators at the UI, Northwestern University, and the Institute of High Performance Computing in Singapore are reporting the extension of this earlier “wavy” development to two dimensions capable of yielding functional integrated circuit systems. Rogers reports constructing integrated circuits consisting of transistors, oscillators, logic gates and amplifiers and notes that these circuits exhibited extreme levels of bendability and stretchability, demonstrating electronic properties comparable to those of similar circuits built on conventional silicon wafers. “We’ve gone way beyond just isolated material elements and individual devices to complete, fully integrated circuits in a manner that is applicable to systems with nearly arbitrary levels of complexity,” Rogers says.

Methodology

To create fully stretchable integrated circuits, the researchers apply a sacrificial layer of polymer to a rigid carrier substrate, Rogers says. On top of the sacrificial layer, they deposit a very thin plastic coating that supported the integrated circuit. He notes that 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. The researchers’ next step, according to Rogers, is to wash away the sacrificial polymer layer and bond the plastic coating and integrated circuit to a piece of pre-strained silicone rubber. Lastly, he says, they relieve the strain and – as the rubber springs back to its initial shape – apply compressive stresses to the circuit sheet. These stresses spontaneously lead to a complex pattern of buckling, creating a geometry that then allowed the circuit to be folded or stretched in different directions, giving it the ability to conform to complex shapes or to accommodate to the mechanical deformations that occur during use. “The wavy concept now incorporates optimized mechanical designs and diverse sets of materials, all integrated together in systems that involve spatially varying thicknesses and material types,” he explains, adding that the “overall buckling process yields wavy shapes that vary from place to place on the integrated circuit, in a complex but theoretically predictable fashion.”

Rogers stresses that attaining high degrees of mechanical flexibility or foldability is important to sustaining the wavy shapes. “The more robust the circuits are under bending, the more easily they will adopt the wavy shapes which, in turn, allow overall system stretchability,” he says. “For this purpose, we use ultra thin circuit sheets designed to locate the most fragile materials in a neutral plane, minimizing their exposure to mechanical strains during bending.” “We’re opening an engineering design space for electronics and optoelectronics that goes well beyond what planar configurations on semiconductor wafers can offer,” Rogers states, indicating that NSF and DOE are funding his research.


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