Starting with commercially available 2 inches and 4 inches epitaxial wafers with a 650μm thick sapphire substrate, the researchers grew a whole-wafer LED epitaxial structure consisting of a stack of 2μm of undoped GaN, 4μm of Si doped n-GaN, an active region made of GaN/InGaN multiple quantum wells (MQW), an AlGaN electron blocking layer and a 200nm Mg doped p-GaN at the top. Then, an Ag-based P metallization layer is deposited on top completed by a SiO2 hard mask.
Only then, the pixels were shaped through a single lithography step to etch the hard mask, the top metallization layer and then the GaN mesas, defining the μLEDs' self-aligned geometry.
The pixels' sidewalls were passivated and a common cathode was created through a damascene process. That included the electrodeposition of copper to fill the mesa grid and a chemical mechanical planarization step to reach the top pixel SiO2 hard mask through which each μLED were individually contacted via another damascene metallization step.
In a paper titled “Processing and Characterization of High-Resolution GaN/InGaN LED Arrays at 10-Micron Pitch for Micro-Display Applications”, presented at SPIE Photonics West in San Francisco, the researchers reported the fabrication of circular μLEDs with diameters of 5μm, 6μm, 7μm and 8μm at a pixel pitch of 10μm. They demonstrated the viability of their manufacturing process by creating several wafer-level micro-displays in the shape of 873x500 pixel arrays (each with a 8.7x5mm footprint).
What the experiment validates is that by filling the whole volume between the micro-LEDs, the common cathode spreads the electrical current between the pixels, providing good thermal dissipation and preventing voltage drops within the micro-LED matrix.