Stretchable micro-LED display, consisting of an interconnected mesh of printed micro-LEDs bonded to a rubber substrate. (D. Stevenson and C. Conway, Beckman Institute, University of Illinois)

A new LED display process developed by an international team of researchers offers properties like see-through construction and mechanical flexibility - which would be impossible to achieve with existing technologies.

Applications for the arrays, which can be printed onto flat or flexible substrates ranging from glass to plastic and rubber, include general illumination, high-resolution home theater displays, wearable health monitors, and biomedical imaging devices.

Researchers from the University of Illinois, Northwestern University, the Institute of High Performance Computing in Singapore, and Tsinghua University in Beijing collaborated on the technology.

“Our goal is to marry some of the advantages of inorganic LED technology with the scalability, ease of processing, and resolution of organic LEDs,” said John Rogers, the Flory-Founder Chair Professor of Materials Science and Engineering at the University of Illinois.

Compared to organic LEDs, inorganic LEDs are brighter, more robust, and longer-lived. However, organic LEDs are also attractive because they can be formed on flexible substrates in dense, interconnected arrays. The new technology combines features of both.

Array of micro-LEDs (black squares) printed onto a sheet of plastic, wrapped on a curved support. (D. Stevenson and C. Conway, Beckman Institute, University of Illinois)

“By printing large arrays of ultrathin, ultrasmall inorganic LEDs and interconnecting them using thin-film processing, we can create general lighting and high-resolution display systems that otherwise could not be built with the conventional ways that inorganic LEDs are made, manipulated, and assembled,” Rogers said.

To overcome requirements on device size and thickness associated with conventional wafer dicing, packaging, and wire bonding methods, the researchers developed epitaxial growth techniques for creating LEDs with sizes up to 100 times smaller than usual. They also developed printing processes for assembling these devices into arrays on stiff, flexible, and stretchable substrates.

A sacrificial layer of material is embedded beneath the LEDs as part of the growth process. When fabrication is complete, a wet chemical etchent removes this layer, leaving the LEDs undercut from the wafer, but still tethered at anchor points. To create an array, a rubber stamp contacts the wafer surface at selected points, lifts off the LEDs at those points, and transfers them to the desired substrate.

“The stamping process provides a much faster alternative to the standard robotic ‘pick and place’ process that manipulates inorganic LEDs one at a time,” Rogers said. “The new approach can lift large numbers of small, thin LEDs from the wafer in one step, and then print them onto a substrate in another step.”

By shifting position and repeating the stamping process, LEDs can be transferred to other locations on the same substrate. Large light panels and displays can be crafted from small LEDs made in dense arrays on a single, smaller wafer. Because the LEDs can be placed far apart and still provide sufficient light output, the panels and displays can be nearly transparent.

Instrument panels, display systems, and flexible (even stretchable) sheets of printed LEDs can be achieved. “Wrapping a stretchable sheet of tiny LEDs around the human body offers interesting opportunities in biomedicine and biotechnology,” Rogers said, “including applications in health monitoring, diagnostics, and imaging.”

(University of Illinois)