Chip-style photolithography brings tiny OLED pixels into focus

Organic light-emitting diodes have long promised a future of extraordinarily sharp displays, yet the materials at their heart have stubbornly resisted the very manufacturing techniques that made modern microchips possible. Now researchers at ETH Zurich and the University of Helsinki have engineered a family of luminescent polymers that double as photoresists, allowing OLED pixels to be patterned with the same photolithographic precision used across the semiconductor industry. The work, published in the journal Nature, opens a route to screens measured in fractions of a centimetre and to microscopic light sources for medicine and biology.

The demand for ever smaller and sharper displays is easy to see in the marketplace. Augmented reality glasses and the electronic viewfinders of professional cameras now rely on screens with diagonals of less than one centimetre, where thousands of pixels must fit into a space smaller than a grain of rice. As pixels shrink, conventional light-emitting diodes run into a fundamental limitation: they are built from crystalline semiconductors, and the smaller such crystals are cut, the dimmer they become. Organic light-emitting materials, which consist of carbon-based compounds rather than crystals, do not suffer from this penalty, which makes them ideal candidates for tiny yet bright pixels.

The obstacle has always been chemistry. Photolithography, the workhorse process of the chip industry, uses solvents and other aggressive chemicals to define structures with nanometre-scale accuracy. Organic light-emitting molecules are delicate, and exposure to these conditions chemically attacks and degrades them. Until now, that incompatibility has kept OLED fabrication separate from the high-precision patterning workflows that semiconductor manufacturers have refined over decades.

The ETH Zurich team, led by Yinyin Bao, now a professor at the University of Helsinki, together with ETH Professor Chih-Jen Shih, solved the problem by designing a new class of long-chain molecules that can be manufactured directly into OLED pixels using light exposure. The key insight was to make the light-emitting polymers themselves serve as photoresists, the light-sensitive materials that form the basis of photolithographic patterning. When exposed to ultraviolet light, the polymers cross-link and become insoluble, allowing fine-scale geometric structures to be created while withstanding the harsh chemical conditions of the process.

The molecular architecture follows a core-shell principle that elegantly separates two conflicting functions. At the centre of each molecular complex sits the colour-emitting molecule, the component that must be protected at all costs. Surrounding it are arms arranged in a star-like pattern whose outer ends react to ultraviolet light and cross-link with the arms of neighbouring stars, rendering the material insoluble wherever the light strikes. The inner portion of each arm plays an equally important role: it keeps the reactive cross-linking groups at a distance from the luminescent core, shielding the sensitive emitter from the chemistry happening at the periphery.

We separate the two functions spatially, Shih explains. The light-emitting molecule is protected inside, whilst the reactive cross-linking groups are on the outside. This allows the photoresist to react during lithography without causing significant damage to the light-emitting core. A co-author from RMIT University in Melbourne further examined this protective effect using computer simulations, confirming how the molecular design preserves the emitter during processing.

To demonstrate the precision achievable with the new materials, the researchers produced a test image of a macaw parrot measuring 300 by 430 micrometres and consisting of 250 by 350 pixels. The image was rendered in fluorescent colours and represents the highest-resolution multicolour fluorescent image produced by photolithography to date. In this demonstration the pixels were not driven electrically but were excited by external light, causing them to fluoresce, meaning the parrot is a static image rather than a functioning display. Even so, it establishes a benchmark for how finely different coloured luminescent materials can be patterned with the method.

The team also showed that the approach can produce electrically powered light-emitting diodes. Working in collaboration with the group of Hua Wang, a professor of electronics at ETH Zurich, they fabricated a glowing ETH logo measuring 1 by 2.4 millimetres, built from tiny light-emitting diodes patterned with the new photoresist polymers. This demonstration bridges the gap between passive fluorescent patterning and the active, electrically driven pixels needed for real screens, proving that the materials survive not only lithography but also the demands of device operation.

Significant engineering challenges remain before the technology can deliver a commercial display. The researchers aim to further reduce the pixel size of the light-emitting diodes, and a functioning screen will also require backplane electronics that allow each pixel to be controlled independently of its neighbours. Those circuit requirements are familiar territory for the semiconductor industry, which is precisely the point: by making OLED materials compatible with chip manufacturing processes, the new polymers allow display fabrication to piggyback on decades of accumulated lithographic expertise rather than requiring entirely new production infrastructure.

Shih sees applications extending well beyond small screens. Tiny, precisely patterned light sources could serve medical technology and devices for biological and neuroscientific research, where light must be delivered with high precision exactly where it is needed. Possible uses include research equipment that examines individual biological cells in a targeted manner or stimulates nerve cells in a Petri dish with light. Very small and precisely controllable light sources produced directly on microchips could also benefit microscopy and sensor systems, giving the new luminescent photoresists a role that reaches from the display industry to the laboratory bench.

Subject of Research: Luminescent polymers compatible with photolithography for manufacturing tiny OLED pixels

Article Title: Tiny OLED pixels can be manufactured like microchips

Article References: Tiny OLED pixels can be manufactured like microchips. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: OLED, photolithography, photoresist, ETH Zurich, University of Helsinki, luminescent polymers, microdisplays, semiconductor manufacturing, core-shell molecules, Nature, augmented reality, medical technology