Leti solves stacking issue for micro-LED displays

May 24, 2017 // By Julien Happich
Moving away from the traditional 3D heterogeneous stacking approach where a full microLED array is first fabricated out of a GaN epiwafer before being flipped and assembled to a separately fabricated CMOS active backplane (with inherent alignment and pixel pitch limitations), researchers from CEA-LETI disclosed their new strategy and a first working prototype at SID's Display Week exhibition.

As lead author François Templier presented during a talk titled "A Novel Process for Fabricating High-Resolution and Very Small Pixel-pitch GaN LED Microdisplays", the novel strategy consists in growing a GaN LED epilayer on a substrate, transferring and bonding this un-patterned epilayer onto a CMOS active-matrix, and after removing the first substrate, patterning the micro-LED array out of the epilayer so as to directly fabricate the LEDs on top the CMOS wafer.


(right) Scanning Electron Microscope image of a 2μm size, 3μm pitch blue microLED array and (left) one of the microLEDs switched on.

In this approach, the microLEDs can be fabricated with the latest semiconductor manufacturing techniques and the best feature sizes, without the limitations of mechanical wafer to wafer alignment which would also limit pixel pitch (since each individual pixel would have to be aligned with a contact pad).

What's more, because GaN epilayers can be grown on 200mm silicon wafers, they can optimally match the size of CMOS active-matrix wafers, so the devices can be fully fabricated on a CMOS line for high yield and mass production.

The researchers demonstrated the feasibility of their approach by fabricating various GaN/Si micro-LED arrays of different pitches, down to 3μm for individual LEDs only 2μm wide. Turn on voltage was about 2V with current levels up to 1mA.  Applications for low cost, mass-produced high-brightness, ultra-high-resolution GaN-based microdisplays include wearable devices (AR/VR), advanced Head-Up Display (HUD) systems and compact projectors.

At the show, CEA-LETI also showcased a blue monochrome GaN-based WVGA resolution (873x500 pixels) prototype microdisplay with a pixel pitch of 10µm (based on previous hybridization strategies with inherent pixel pitch limitations).

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