Rubber-Like Silicon Chips?

Scientists at the University of Illinois, Urbana Champaign, and Northwestern University, in Evanston, IL, have developed a new form of stretchable silicon integrated circuit that can wrap around complex shapes such as spheres, body parts and aircraft wings, and can operate during stretching, compressing, folding and other types of extreme mechanical deformations, without a reduction in electrical performance.
“The notion that silicon cannot be used in such applications because it is intrinsically brittle and rigid has been tossed out the window,” said John Rogers, a founder professor, materials science and engineering, University of Illinois.“The wavy concept now incorporates optimised mechanical designs and diverse sets of materials, all integrated together in systems that involve spatially varying thicknesses and material types,” Rogers said. “The overall buckling process yields wavy shapes that vary from place to place on the integrated circuit, in a complex but theoretically predictable fashion.”Achieving high degrees of mechanical flexibility, or foldability, is important to sustaining the wavy shapes, Rogers said. “The more robust the circuits are under bending, the more easily they will adopt the wavy shapes which, in turn, allow overall system stretchability. For this purpose, we use ultrathin circuit sheets designed to locate the most fragile materials in a neutral plane that minimises their exposure to mechanical strains during bending.”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.“We’re opening an engineering design space for electronics and optoelectronics that goes well beyond what planar configurations on semiconductor wafers can offer,” Rogers said.The work was funded by the National Science Foundation and the U.S. Department of Energy.