New materials for next generation electronics: Researchers discover stretchable silicon
After the great successes of mechanics in helping to understand failure and performance o microchips about a decade ago, a recent study underlines the importance of mechanics in electronic devices, this time for the next generation microchips that will help to sustain Moore’s law for the years to come.
A team of researchers from the University of Illinois in Urbana-Champaign (UIUC) around John Rogers (Materials Science and Engrg.) and Young Huang (Mechanical and Industrial Engrg.) have created a fully stretchable form of single-crystal silicon with micron-sized, wave-like geometries that can be used to build high-performance electronic devices on rubber substrates.
The key aspect of this new material is that it provides necessary properties to function as electronic device that can be subject to large strains. The researchers believe that this new stretchable silicon offers different capabilities than can be achieved with standard silicon chips. Applications of this material include sensors and drive electronics for integration into artificial muscles or biological tissues, structural monitors wrapped around aircraft wings, conformable skins for integrated robotic sensors, and portable electronics. The snapshot shows the wavy silicon material deposited on an elastic substrate. To create their stretchable silicon, the researchers begin by fabricating devices in the geometry of ultra-thin ribbons on a silicon wafer using procedures similar to those used in conventional electronics. Then they use specialized etching techniques to undercut the devices. The resulting ribbons of silicon are about 100 nanometers thick. In the next step, a flat rubber substrate is stretched and placed on top of the ribbons (Figure upper right). Peeling the rubber away lifts the ribbons off the wafer and leaves them adhered to the rubber surface. Releasing the stress in the rubber causes the silicon ribbons and the rubber to buckle into a series of well-defined waves that resemble an accordion (Figure below)
“The resulting system of wavy integrated device elements on rubber represents a new form of stretchable, high-performance electronics,” said Young Huang, the Shao Lee Soo Professor of Mechanical and Industrial Engineering. “The amplitude and frequency of the waves change, in a physical mechanism similar to an accordion bellows, as the system is stretched or compressed.” A nonlinear continuum mechanics analysis helped to understand these responses of the wavy silicon, for example the dependence of the wavelengths on the silicon properties and thickness, analytically. The mechanics analysis linked the maximum strain in Si ribbon to the applied strain, and therefore provided simple criteria for the development of stretchable silicon such as the maximum stretchability and compressibility.
The scientists have already fabricated wavy diodes and transistors and compared their performance with the traditional devices. Not only did the wavy devices perform as well as the rigid devices, they could be repeatedly stretched and compressed without damage, and without significantly altering their electrical properties.
Besides the unique mechanical characteristics of wavy devices, the coupling of strain to electronic and optical properties might provide opportunities to design device structures that exploit mechanically tunable, periodic variations in strain to achieve unusual responses. In addition to Rogers and Huang, co-authors of the paper were postdoctoral researcher Dahl-Young Khang and research scientist Hanqing Jiang, who will join the Arizona University as an assistant professor. The Defense Advanced Research Projects Agency Department of Energy, and the NSF-funded Nano-CEMMS Center at the University of Illinois funded the work, which was published in January, 2006 “A stretchable form of single crystal silicon for high performance electronics on rubber substrates,” Science, v 311, pp 208-212).