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Engineering Mechanics (EM)

   

Thin Film Mechanics

Instability and Wrinkling Analysis of Stretchable Silicon System

The research team at the University of Illinois at Urbana-Champaign (UIUC), Northwestern University in Chicago (NWU) and the Institute of High Performance Computing (IHPC) has made a breakthrough in developing full-scale stretching in fully functional circuits (Figure 1). Researchers at IHPC using a systematic computational study was able to offer significant insights into the mechanics of stretchable silicon system with micron-sized wavy geometries, and this has provided new design principles that guided experimentalists at UIUC to turn fragile and brittle silicon chips into bendable and foldable platforms for novel flexible microelectronics. Their results have been highlighted in SCIENCE (Vol. 320, 2008).

Using this technique, the silicon and the circuits are made as small as possible — roughly one-50th of the diameter of a human hair. Also the materials had to be design in such a way that the silicon would experience minimal strain when the circuit is bent. Then the ultrathin film with circuit is bonded to a slab of stretched, rubber-like polymer substrate. When the stretched polymer substrate snaps back to its original size and shape, it causes the thin film buckles, but does not break, forming wrinkling waves that are ready for stretching out again. This fully stretchable platform, together with logic gates, ring oscillators and differential amplifiers, can be used to build highly flexible high-performance microelectronic devices.

In the fabrication process, a very important step is that the thin film to form highly regular, stretchable wavy structure. Achieving high degrees of mechanical flexibility, or foldability is sustained by those wavy shapes. Base on non-linear mechanics theory to model the wrinkling process, researchers at IHPC build up the high quality computational models for simulating the key fabrication steps as shown Figure 2. Since the complexities of the wrinkling process, sophisticated computational methods and super-fine finite element meshes were applied to perform in the computational simulations. Therefore the high-accuracy computational results are obtained for the wavy patterns as shown in Figure 3. These simulations further provided new understandings of the wavy wrinkling patterns formation and revealed the underlying mechanics principles in governing the formation of this hierarchical structure.

This novel flexible structure has huge potential to become a universal platform for developing the next generation high-performance flexible electronic devices with a wide range of applications. They range from consumer electronics such as display monitors that are bendable to medical technology such as surgical gloves with sensors that could read chemical levels in the blood without impairing the sense of touch, prosthetic limbs to use pressure or temperature cues to change its shape, wireless medical sensors, electronic eye camera, conformable skin sensors and structural health monitoring devices, etc.

With the rapid development of computer capacity, computational modelling and simulation play an ever increasing role in scientific research. Nevertheless, this stimulation process can be made even more effective through a synergetic collaboration among theorists and experimentalists, which often can result in new and cutting-edge knowledge.

Currently IHPC researchers are working diligently to unveil new design principles for developing flexible platform with even larger stretch-ability and compressibility, and strive to provide new design strategies to guide their collaborators to achieve more fascinating structures.

Figure 1, Stretchable Silicon System Figure 2, Finite element simulation and modeling procedure

Figure 3, Comparison of three-dimensional finite element simulation results with the perspective scanning electron micrograph of a wavy sample

 

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This page is last updated at: 22-FEB-2011