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A Wide Variety Of Methods Are Being Explored To Effect Mass Transfer Of Thousands Of MicroLEDs Simultaneously
Perhaps the most mature transfer option, according to Yole uses a polymer stamp to move thousands of LEDs, such as in X-Celeprint’s Micro Transfer Printing process. Each stamp is custom fabricated from injection-molded polydimethylsiloxane (PDMS) and consists of glass backing (for rigidity), a smooth PDMS layer and PDMS “posts” that are lithography-defined and etched for high accuracy in placement. An adhesive ink grabs an array of MicroLEDs, and printing is performed using laser or other means. X Display Co., the sister display company to X-Celeprint, recently delivered its first 300 mm transfer tool, and it licenses many of its technologies for MicroLED manufacturing. Stamp transfer processes are proving reliable, scale-able and capable of high throughput, but further process optimization is needed to achieve six 9s yield. Laser-assisted mass transfer is offered by 3D-Micromac AG and Coherent.
Perhaps the most mature transfer option, according to Yole uses a polymer stamp to move thousands of LEDs, such as in X-Celeprint’s Micro Transfer Printing process. Each stamp is custom fabricated from injection-molded polydimethylsiloxane (PDMS) and consists of glass backing (for rigidity), a smooth PDMS layer and PDMS “posts” that are lithography-defined and etched for high accuracy in placement. An adhesive ink grabs an array of MicroLEDs, and printing is performed using laser or other means. X Display Co., the sister display company to X-Celeprint, recently delivered its first 300 mm transfer tool, and it licenses many of its technologies for MicroLED manufacturing. Stamp transfer processes are proving reliable, scale-able and capable of high throughput, but further process optimization is needed to achieve six 9s yield. Laser-assisted mass transfer is offered by 3D-Micromac AG and Coherent.
Laser lift-off works by ablating a microscopic layer of GaN, which forms an expanding nitrogen gas layer to enable lift-off. Laser-assisted lift-off is commonly used to remove the sapphire substrate from the processed MicroLED wafer. At the microdevice or small field sizes, multiple high-energy laser pulses can be used to transfer groups of MicroLEDs with high accuracy (±1.5 µm). While known for good selectivity and reliability, laser-assisted transfer methods are currently limited to small areas and require further development to speed throughput.
ELux Display pioneered a novel fluidic self-assembly process (see figure 4), which uses active-matrix substrates from a conventional LCD fab. Using a substrate with wells only slightly larger than the MicroLEDs, liquid containing pretested MicroLEDs is applied to the surface, and oscillation motion encourages the LEDs to settle in the wells in a highly accurate and unform manner. Fluidic assembly randomizes the MicroLEDs in liquid, which prevents the mosaic patterns caused by epi wafer nonuniformity.
ELux Display pioneered a novel fluidic self-assembly process (see figure 4), which uses active-matrix substrates from a conventional LCD fab. Using a substrate with wells only slightly larger than the MicroLEDs, liquid containing pretested MicroLEDs is applied to the surface, and oscillation motion encourages the LEDs to settle in the wells in a highly accurate and unform manner. Fluidic assembly randomizes the MicroLEDs in liquid, which prevents the mosaic patterns caused by epi wafer nonuniformity.
The MicroLEDs are fabricated from conventional blue LED wafers as flip chips with anode and cathode electrodes arranged as concentric rings on the surface. Prior to assembly, the MicroLEDs are tested using microPL mapping for shorted die or low EQE devices and optical inspection for process defects or contamination. ELux’s advantage, relative to deterministic systems using lasers or other methods, is that fluid assembly is designed to harvest only known-good-die. As a result, defects never enter the display manufacturing process.
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