Researchers at ETH Zurich, the Swiss Federal Institute of Technology, have developed a light-sensitive material that can be turned into tiny, colored light-emitting pixels directly on the surface of a chip. Until now, organic light-emitting diodes (OLEDs) couldn’t survive the harsh chemistry of photolithography, the process used to make microchips. The new material can.
There’s growing demand for pixels this small as screens keep shrinking and getting sharper. In augmented reality glasses or a camera’s electronic viewfinder, the display can measure less than a centimeter across. And the smaller the pixel, the harder it is to make. OLEDs are a better fit here than conventional LEDs. They’re built from carbon-based compounds rather than crystals, so they stay bright even at microscopic sizes. Conventional LEDs, by contrast, get dimmer the smaller their crystals are cut.
The obstacle was photolithography itself, which the semiconductor industry uses to pattern its finest structures. The process relies on solvents and other aggressive chemicals that break down organic light-emitting molecules. Big display makers keep pushing OLED technology in their own directions. In August, LG Display unveiled FLiPP, a method that does away with metal masks, and in September Samsung Display launched a 10,000-nit panel for smartphones.
The ETH team created light-emitting polymers in several colors that double as a photoresist, the light-sensitive layer at the heart of photolithography. Each polymer is built around a molecule with a core-shell design. At its center sits a compound that emits a specific color, with long chains radiating outward like the arms of a star. Under UV light, the outer ends of those arms cross-link with the arms of neighboring molecules, making the exposed area insoluble. That’s exactly the property photolithography depends on.
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The inner part of each arm keeps the reactive ends away from the light-emitting core, shielding it during processing. A co-author at RMIT University in Melbourne, Australia, studied this protective effect using computer simulations. “We separate the two functions spatially,” explained ETH professor Chih-Jen Shih. “The light-emitting molecule is protected inside, while the reactive cross-linking groups are on the outside.”
Shih led the research together with Yinyin Bao, who worked at ETH and is now a professor at the University of Helsinki. The team published its findings in the journal Nature on September 16.
To show off the method’s precision, the researchers made a color image of a macaw measuring 300 by 430 micrometers. It’s made up of 250 by 350 pixels. This isn’t a display yet: the pixels aren’t driven by electric current but fluoresce under external light. Still, no multicolor fluorescent image produced with photolithography has ever reached this resolution. The team also built electrically powered LEDs, using them to form a glowing ETH logo measuring 1 by 2.4 millimeters. That work was done with the group of ETH electronics professor Hua Wang.
Next, the researchers want to shrink the LED pixels even further. A working screen will also need electronics that can control each pixel independently. Shih believes the technology could be useful beyond tiny displays, in medical devices as well as biological and neuroscience research. These miniature OLEDs could target individual cells or stimulate neurons in a Petri dish with light. Because such light sources can be built directly on microchips, they could also find uses in microscopy and sensors.
