Microchips Get Bent Out of Shape

Researchers have been busily developing bendable electronics for potential use in a range of medical device applications, from sensors that map and treat neurological disorders to balloon catheter components that perform diagnostic and surgical functions. However, while flexible electronic components commonly consist of circuits embedded in an elastic material that are connected using stretchable interconnects, a new method for manufacturing such components starts with standard microchips and relies on conventional fabrication techniques. This process could eventually facilitate the mass production of flexible electronics.

The new fabrication method combines reliable stretchable interconnections with commercial, off-the-shelf chips, explains Jan Vanfleteren, an electrical engineer at the Interuniversity Micro Electronics Centre at the University of Ghent (Belgium). The first step in the fabrication process involves the thinning of a conventional, unpackaged integrated circuit (IC), according to Vanfleteren. Next, the die is packaged by embedding it between two thin layers of polyimide and providing metallic conductors. After the circuit has been structured to make it stretchable, the component is embedded in polydimethylsiloxane, an elastomer material.

Because thinned dies often suffer from warpage as a result of residual stress caused by the surface films, the researchers must ensure that the dies lie flatly on the base polyimide film to prevent voids between the die and the adhesive. This care is required to avoid breaking the die and to minimize dimensional errors during subsequent photolithography steps.

"Any material becomes flexible if it is sufficiently thin," Vanfleteren says. "Although conventional, unpackaged ICs are brittle, thinned versions measuring 20 to 30 µm in thickness are flexible." This phenomenon, Vanfleteren adds, is explained by the fact that when a material is bent, strains are introduced along the material's cross-section, which varies linearly with the thickness. Decreasing the material's thickness therefore decreases the strain magnitudes and improves its mechanical flexibility.

"It is not specifically the chip, but rather the whole technology platform combining ultrathin bendable chips and compliant, elastic electrical interconnections, that makes our technology of interest for medical implantable devices or wearable sensing systems," Vanfleteren states. "These applications typically benefit from compact and lightweight systems that fully conform or dynamically adapt to irregularly shaped surfaces, such as the human body."

The Ghent researchers have found that the introduction of a polyimide support around the interconnections increases the components' reliability greatly. Hence, the new stretchable electrical interconnections have been shown to offer stable resistance when stretched up to 100%, although the electronics will not commonly undergo such severe stretching in target applications, Vanfleteren remarks. The components have also withstood cyclic stretching at lower strain levels, such as 500,000 cycles at 10%.

In theory, any IC can be thinned and embedded using the technology developed at the University of Ghent, Vanfleteren comments. This technology therefore opens the door to producing bendable electronic components for a variety of applications in which conformability and miniaturization are key requirements. "For example, to achieve further miniaturization, we are currently investigating the 3-D stacking of several thin dies in a polyimide package," Vanfleteren adds. "This design could be of interest for manufacturers of such implantable devices as cochlear implants or such wearable devices as hearing aids."