At Display Week 2019 in San Jose, California, research institute Leti described a new technology for fabricating gallium nitride (GaN) microLED displays on a CMOS process that significantly reduces transfer steps and eliminates the size limit for applications ranging from smartwatches to large televisions.

The new approach fabricates elementary units of all-in-one red, green, and blue (RGB) microLEDs on a complementary metal-oxide-semiconductor (CMOS) driving circuit and transfers the devices to a simple receiving substrate. The units can then be fabricated with a full-semiconductor, wafer-scale approach.

While microLED displays promise exceptional image quality and better energy efficiency than existing liquid crystal display (LCD) and organic light-emitting diode (OLED) technologies, there are currently significant barriers to commercialization, according to Leti. One of the biggest challenges is improving the driving electronics performance, which requires more power to deliver brighter images and more speed to support continuously increasing demands for ever higher resolution. Faster electronics are required to power millions of pixels in a fixed-frame time in microLED displays, but existing thin-film transistor (TFT) active matrix display technology cannot provide the necessary current and speed.

Leti’s new approach fabricates high-performance GaN microLED displays with a simplified transfer process that eliminates the use of the TFT backplane. The principle is to fabricate a series of elementary units and transfer them onto a receiving substrate containing only lines and columns. Each elementary unit consists of all-in-one RGB microLEDs on a CMOS driving circuit. Each unit is then transferred on one pixel of the receiving substrate. The receiving part, rather than being a TFT backplane, is a simple, low-cost receiving substrate with only lines and columns. For each pixel, only one transfer is needed instead of three or four. Overall, this approach combines the excellent driving electronics of CMOS and a simple transfer process (only one transfer per pixel).

Hence, 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.

micro led transfer

Leti’s new approach for facbricating microLED displays with CMOS (top) and elementary unit consisting of all-in-one RGB LEDs on CMOS driving circuit (bottom). (Source: Leti)

Leti said that there are several advantages with using CMOS — such as device performance, high levels of integration, and short connections from the fact that microLEDs are stacked directly onto the circuit. It also provides the best fill ratio — namely, the smallest surface taken by the microLED plus the electronics with respect to the total pixel area. This is ideal for transparent displays, allowing the largest transparent area possible. Another advantage of this new approach is that the elementary unit can embed other functions than just light emission — for example, image sensing and many other functions can be added to the unit as long as they can be fabricated using CMOS technology.

“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. He added that the CMOS-based approach provides higher-brightness and higher-resolution microLEDs and is a game-changer for very large TVs.

CEA-Leti presented a paper on the breakthrough titled “A New Approach for Fabricating High-Performance MicroLED Displays” during Display Week 2019.