French research institute Leti has developed a new technique for fabricating GaN microLED displays that overcomes existing limitations to commercialisation.
With no upper size limit, the new method could be used to produce displays for applications ranging from smart watches to TVs.
While they promise exceptional image quality and better energy efficiency than existing liquid crystal display (LCD) and organic light-emitting diode (OLED) technologies, microLED displays currently face significant barriers on the road to commercialisation. GaN microLED chips must be separated from the sapphire crystals they’re deposited on, and assembled onto a display backplane. A 4K display requires nearly 24 million microLEDs, which would take six weeks to transfer using pick-and-place methods.
The new approach developed at Leti fabricates elementary units of all-in-one red, green, blue (RGB) microLEDs on a CMOS driving circuit, and transfers the devices to a simple receiving substrate. The units are fabricated with a full semiconductor, wafer-scale approach.
'This new process, in the proof-of-concept stage, paves the way to commercial, high-performance microLED displays,' said François Templier, CEA-Leti’s strategic marketing manager for photonic devices. 'The CMOS-based approach provides higher brightness and higher resolution microLEDs and is a game changer for very large TVs.'
Another challenge when producing microLEDs is improving the performance of the driving electronics, which require more power to deliver brighter images and more speed to support continuously increasing demands for high display resolution. Faster electronics are required to power millions of pixels in a fixed-frame time in microLED displays, but existing driving display technology, known as thin-film transistor (TFT) active matrix, cannot provide the necessary current and speed.
CEA-Leti’s approach fabricates CMOS-driven, high-performance GaN microLED displays with a simplified transfer process that eliminates the use of the TFT backplane. RGB microLEDs are stacked directly onto a micro-CMOS circuit, and each unit is transferred onto a simple receiving substrate. Then, the RGB microLEDs and the backplane are fabricated on a single semiconductor line.
In addition to increasing power and driving speed – and improving display performance – this process avoids several costly steps needed with current technology to make electrical and mechanical contacts between microLEDs and the receiving substrates.
