Stretchable micro-LED display, consisting of an interconnected mesh of printed micro LEDs bonded to a rubber substrate.

Developed by a team of international researchers, the new process creates tiny, ultrathin inorganic light-emitting diodes (LEDs) that shine brighter and last longer than conventional LEDs.

John Rogers, professor of Materials Science and Engineering at the University of Illinois, teamed up with experts at Northwestern University, the Institute of High Performance Computing in Singapore, and Tsinghua University in Beijing to create the new process, as described in a news story published Thursday by the University of Illinois and in the journal Science.

Micro_LED display printed on a thin sheet of plastic, wrapped around a finger Micro_LED display printed on a thin sheet of plastic, wrapped around a finger

Conventional Inorganic LEDs are brighter and long-lasting, but they’re costly, thick, and difficult to manufacture. Organic LEDs (OLED) are cheaper and easier to make, thinner, and can be applied to flexible surfaces. The new process combines the best of both worlds.

The new process for creating ultrathin, ultrasmall inorganic light-emitting diodes (LEDs) and assembling them into large arrays offers new classes of lighting and display systems with interesting properties, such as see-through construction and mechanical flexibility, that 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.

“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 Rogers. “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.”

The technology could pave the way for TV screens that you roll up and brake light indicators that fit the contour of your car.

The Fabrication Methodology

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. As part of the growth process, a sacrificial layer of material is embedded beneath the LEDs. 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. In this fashion, large light panels and displays can be crafted from small LEDs made in dense arrays on a single, comparatively small wafer. And, because the LEDs can be placed far apart and still provide sufficient light output, the panels and displays can be nearly transparent. The thin device geometries allow the use of thin-film processing methods, rather than wire bonding, for interconnects.

In addition to solid-state lighting, instrument panels and display systems, flexible and even stretchable sheets. One especially promising use for flexible LED sheets lies in the medical field. “Wrapping a stretchable sheet of tiny LEDs around the human body offers interesting opportunities in biomedicine and biotechnology,” said Rogers, “including applications in health monitoring, diagnostics, and imaging.”