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While displays based on organic light-emitting diodes (OLEDs) are gaining increasing market share worldwide, microLED displays are already heralding a new market-changing innovation. How quickly they catch on depends primarily on laser-based manufacturing processes.

Their pixels each consist of a red, a green and a blue LED with an edge length of a few micrometers (µm). Depending on the size of the display, hundreds of thousands to hundreds of millions of tiny LEDs must therefore be arranged with the utmost precision in RGB triplets on a backplane. These inorganic light-emitting diodes are currently around 50 x 50 µm in size; the industry is aiming for 5 x 5 µm LEDs. At this point at the latest, it becomes clear that light is the only tool that can be used for efficient production and quality monitoring and also for error corrections and repairs: µLED displays are a photonic technology based on a photonic production chain. The effort is worth it, because the new display technology sets new standards in terms of brightness and image resolution as well as durability and energy efficiency. Whether smartwatches, microdisplays in glasses for augmented, mixed or virtual reality applications, smartphones, tablets, TV sets or large screens in public spaces, µLED displays are seen as a promise for the future with significant advantages over established technologies and huge market potential.

Photonic production chain

Whether the young display technology actually prevailed over mass-produced and mature LCD, LED and OLED displays depends on the efficiency of the production processes. First of all, the red, green and blue LED wafers are grown on separate substrates. They then have to be separated from these wafers, detached and transferred to the glass display backplane. This involves arranging them pixel by pixel as RGB triplets and aligning them with sub-µm precision. Mind you, we are talking about hundreds of thousands to hundreds of millions of µLEDs per screen. To ensure the necessary efficiency, the transfer of the LEDs from their growth wafers to the backplane must be parallelized.

This is precisely where a laser-based manufacturing solution from Coherent for parallelized mass transfer comes into play. At its core is a LIFT (Laser Induced Forward Transfer) process: Pulses from a UV excimer laser hit a gallium nitride (GaN) layer on the back of the LEDs and vaporize it. This not only detaches the tiny diodes from their substrate, but the energy of the laser pulse and the resulting vapor pressure are also sufficient to transfer them to the display backplane positioned in the immediate vicinity. They are bonded there, in other words, fixed and electrically connected.

Flat-top laser beam ensures precise transfer

The transfer process is tricky. If the LEDs start to spin, the required sub-µm-precise alignment of the RGB triplets fails and pixel errors occur. Although Coherent’s laser-based production system is able to make corrections and repairs, the first priority is of course to align the LEDs as precisely as possible. Which substrates, µLED sizes and spacing are used in the respective production process is less important for the system. Reliability and efficiency depend primarily on the LIFT process itself. In display factories, this must be robust, fast and reliable and must position and align the red, green and blue µLEDs precisely even if the distances between the growth wafers and backplane are greater than under ideal conditions in the laboratory. A distance of less than 50 µm between the carrier and target substrate is considered ideal. However, in tests, the Coherent developers were able to prove that it is also possible at a distance of 80 µm to align even 5 x 5 µm LEDs with sub-µm accuracy with their UV transfer system.

One key to this is the stamp-like, flat beam shape with which the pulses from the excimer laser hit the back of the µLEDs. The uniform flat-top profile ensures that the diodes are transferred to the target substrate vertically and without spinning. An acute or oblique incidence of the laser beam would result in directional changes and would stand in the way of the precise arrangement and alignment of the active light chips. In addition to the flat-top beam shape, the short wavelength of the UV pulses also ensures precision in the LIFT process.

Parallelization for fast transfer processes

The question remains as to how the transfer for the mass production of µLED displays can be parallelized, and how isolated pixel errors can be corrected or repaired. The developers have addressed both requirements in the UVtransfer system. In order to detach the tiny red, green and blue LEDs from the sapphire substrates on which they grow with the aforementioned GaN interlayer at high cycle rates—and to transfer them to their target substrate via LIFT—they rely on high-energy ultraviolet laser pulses with nanosecond pulse durations. This approach has proven successful in minimizing thermal stress during laser lift-off (LLO) of sensitive functional layers. Due to the short pulse duration, the energy of the laser does not penetrate deep into the material, which in this case ensures selective vaporization of the GaN layer. At the same time, the high pulse energy is the key to parallelization because the laser beam is split into hundreds or even thousands of individual beams through multiplexing. These are directed onto the µLEDs by appropriate photomasks so that each pulse detaches hundreds or even thousands of LEDs from the sapphire substrate and transfers them precisely to their intended positions, either on an intermediate substrate or directly on the target substrate. The patterns of these photomasks ensure that all µ-diodes are gradually transferred to their RGB triads. A closed control loop ensures that the beams always hit the µLEDs over the entire surface.

Automated correction

If errors do occur, for example because individual µLEDs are defective or do not detach, the Coherent system is also used for repairs. With several hundred million LEDs in large displays, the probability of errors is so high that the repair is also based on automated solutions. These are able to locate defects, remove the affected µLEDs using the LLO process and replace the resulting empty spaces with new microdiodes. The system saves the coordinates of all voids during removal, generates a map and replaces missing diodes using the LIFT method, which uses a single, unsplit UV beam in the selective repair process.

To visualize the contribution of lasers to the production of µLED displays, watch the following video.