Studying the force:
Turner's research could improve development of microdevices
magine reading this article on an electronic screen that could be rolled up and put into a pocket. Someday, the electronics to power this kind of screen may be produced by a process that relies on a very simple tool: a stamp.
Reliable flexible displays are only one of a variety of new microelectronic and micromechanical devices that may become possible thanks to fundamental research by Assistant Professor Kevin Turner. Turner is studying the underlying physics and mechanics of adhesion during a process called microtransfer printing. He will use his research to improve microtransfer printing manufacturing processes, which eventually could be used to produce a host of innovative technologies, such as advanced optoelectronic devices, high efficiency solar cells, and new types of microelectromechanical systems.
His work has garnered a prestigious National Science Foundation CAREER award, which recognizes faculty members at the beginning of their academic careers who have developed creative projects that effectively integrate advanced research and education. Turner’s award comes with a five-year, $430,000 grant.
Microtransfer printing is essentially a process that “prints” with solid materials rather than ink. A silicone stamp is designed with a smooth side that is used to pick up micro- or nanostructures from the substrate on which they are originally fabricated. The stamp is used to transfer these structures—which may be fully processed integrated circuits or building blocks for more complex devices—and places them down on another substrate or functional device.
Traditional silicon-based microelectronic devices are constructed on thick wafers, which produce rigid devices. To create a flexible device, such as a flexible display or processor, very thin layers of single-crystal silicon can be peeled from a thick substrate and placed onto a compliant substrate. Even though silicon is a stiff, brittle material, it can be made extremely flexible by making it less than 1-micron thick.
However, a key challenge is that there are few techniques available to move large-area thin layers, which are floppy and fragile. Microtransfer printing has emerged as a potential option for thin layer transfer since it can be done quickly and used to create a large number of devices.
Microtransfer printing relies on surface adhesion that occurs due to the presence of van der Waals forces. At room temperature, the smooth surface of the silicone stamp bonds directly to micro- or nanostructures via these forces, allowing the structures to be picked up. In nature, van der Waals forces allow gecko lizards to adhere their feet to surfaces in order to scale walls and scamper across ceilings.
Turner will use a combination of modeling and experiments to investigate the fundamental behavior of van der Waals-based adhesion in microtransfer printing processes. Based on this fundamental study, he will explore using surface texture and geometric structures on the surfaces of the silicone stamps to control adhesion. He also will identify optimal stamp designs for the pick up and release of micro- and nanostructures, will research new types of composite stamps based on materials other than silicone, and will examine how different loading techniques can be used to further control adhesion.
“If we measure the forces that govern microtransfer processes and develop computational models that capture the fundamental interfacial behavior, then we can examine higher-level manufacturing questions,” Turner says. “We then can use that knowledge to design more effective manufacturing processes and techniques.”
In addition to his research, Turner will develop advanced graduate courses in adhesion and contact mechanics, as well as an undergraduate elective in the design and manufacturing of nano- and microsystems. He also will host local K-12 teachers in his lab during the summer and will work with the teachers to develop lesson plans about nanotechnology for elementary and high school students.