Wafer scale RBG MicroLEDs simplify transfer process

May 15, 2019 //By Julien Happich
At Display Week, researchers from CEA-Leti presented several papers relating to the manufacture of RBG MicroLED pixels and their transfer to large area displays.

The first paper, “A New Approach for Fabricating High-Performance MicroLED Displays” demonstrates how to remove several processing steps altogether when manufacturing full colour microLED-based displays. Instead of successively singulating blue, green and red microLEDs from their native wafers in order to transfer them onto a TFT backplane or to stack them with CMOS drivers themselves singulated and placed on a backplane (four transfer steps or more), the researchers first bonded a blue LED epiwafer onto a full wafer of CMOS driving circuits. They then fabricated the InGaN blue LEDs directly on the bonded epiwafer and then created the RGB pixels at wafer level, depositing adequate conversion phosphors on the blue microLEDs to obtain the green and red emitters.

Fabricating microLED displays with integrated CMOS driving makes for a simpler transfer step. (bottom) Elementary unit consisting of all-in one RGB LEDs on CMOS driving circuit.

More complex approaches consist in singulating blue, red and
green LEDs from their wafers as well as CMOS driving circuits
to transfer them separately.

The result is a full RGB microLED wafer from which they can singulate tens of thousands of RGB microLEDs each already stacked and connected with the underlying CMOS driving circuit. What’s more, the full connectivity provided at wafer level makes it much easier to test the individual RGB-LED-on-CMOS dies just after their fabrication and before their transfer.

These all-in-one RGB microLEDs can then be transferred in one single step onto a simpler backplane merely consisting of a passive grid of conductive lines and columns, eliminating the need for a TFT backplane and its inherent driving and size limitations (low carrier mobilities compared to CMOS and large feature sizes).

“This new process, in the proof-of-concept stage, paves the way to commercial, high-performance microLED displays,” explained François Templier, CEA-Leti’s strategic marketing manager for photonic devices and first author of the paper.

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