Stacking microLEDs onto a full-CMOS active matrix, with micro tubes
The presentation titled “High-resolution active-matrix 10-μm pixel-pitch GaN LED microdisplays for augmented reality applications” explains how high-resolution 873×500 pixel GaN/InGaN µLED arrays, designed at a 10µm pitch following a self-aligned process, were successfully connected to a separately designed full-CMOS active-matrix bearing hard microtubes grown on top of its contact areas.
About 2.8µm high and 4µm in diameter and coated with a 240nm gold layer, the micro tubes can be grown using conventional IC process steps on top of fully functional CMOS circuits. Their proprietary composition was chosen to be relatively hard compared to Al-0.5Cu pads, so they can be pressed into softer contact pads to establish a reliable electromechanical contact.
As well as the high-density interconnection pitch they allow, a particular benefit of these micro tubes is to compensate planarity and uniformity defects, without the use of flux but only so-called soft hybridization pads formed on the contacts of the GaN/InGaN µLED array.
After dicing the LED-arrays and the CMOS active matrix chips from their respective wafers, the researchers flipped the GaN chip over the silicon chip for a press-fit assembly (with an insertion force of about 0.1g per connection, or roughly 40kg for the insertion of 450 000 micro-tubes). The microdisplays can then be attached onto a daughter board and connected through wire-bonding.
With this hybridization approach, Leti’s researchers have managed to fabricate very bright active-matrix WVGA (873×500) blue and green micro-displays with a 10µm pixel pitch and respective luminances of 300cd/m2 and 10000 cd/m2. The micro-display also exhibited a good uniformity of luminance with a very low dispersion between pixels, while the active matrix allowed the researchers to drive the individual pixels through sixteen grey levels, at video speed.
Next on their research roadmap, the authors are working on the realization of full-colour microLED GaN microdisplays for wearables, advanced Head-up display (HUD) systems and compact projectors.