MicroLED’s show great potential for future display applications. They will enhance the user experience and extend the range of applications for both super-large direct view displays as well as super-bright micro displays as demanded for AR/VR applications.
In the early stage of the product life cycle, technologies move fast, and many challenges are not solved today with the consequence that microLED manufacturing standards are not yet fixed.
Shrinking die sizes along with the ability to transfer large quantities of dies simultaneously from the growth wafers onto temporary carriers and final backplane substrates are still a big challenge.
Deep-UV (DUV) excimer lasers, with their large available pulse energies and micron precision, are suitable candidates for facilitating these transfer steps in mass production.
Overcoming fragility
In contrast to miniLED´s, where the sapphire growth wafermight stay with the LED, microLED´s need to be released from the wafer and are only a few micrometres in thickness. As a result they are very fragile, meaning non-mechanical processing techniques are required to lift them off the wafer.
In order to produce a cost-competitive microLED display, the die sizes must be reduced to a few microns and the street widthsmust shrink correspondingly to achieve the maximum number of microLED´s on one wafer. On the other hand, the RGB pixel pitch on the substrate must be enlarged so that a selectivity that enables this is required while transferring the dies from the growth wafer to the substrate. Assembling each subpixel from different wafers and the pitch enlargement requires that with each shot only selected – e.g. every sixth or tenth – microLEDs are transferred. The selectivity in this case is incorporated in the mask defining pitch enlargement and selectivity adequate for the specific transfer step.
Figure 1: DUV excimer laser-based mask imaging system: a homogenised top-hat beam enables mass transfer
Coherent has developed and designed a high-resolution mask-based imaging system (see figure 1) offering micron precision and a large field size to transfer multiple dies at the same time. Mask imaging can achieve a resolution of approximately 2µm L/S. The resolution of a DUV optical system in combination with a high-energy laser is a future-proven concept that fulfills the requirements today and future years.
Other transfer technologies, for example stamps, are not able to combine resolution and throughput scalability at the same time and therefore are reaching their limits for mass production concepts. For the mask-based system the throughput is determined by the density/pitch of the microLED´s, the field size on the wafer, and the repetition rate of the laser, enabling the mass transfer of a hundred million microLEDs per hour.
Potential for mass-production
If a 4K TV needs about 24 million RGB pixels, the processing time of such a TV needs to be in the range of minutes in the future. The three colours must be transferred separately in a three-step process. In Figure 2 a processed receiver substrate is shown after the individual colour transfer, in this case it´s 40 x 40µm² GaN dies with a 10µm street width on the epi-wafer.
Figure 2: RGB mass transfer with 80µm distance between the donor and receiver substrate; die sizes 40 x 40µm²
Another advantage of the laser-based mass transfer process is that there are several supported ways to transfer a microLED to the substrate. Today one approach is to use a laser lift-off step to release the microLED´s from the growth wafer to a temporary carrier, followed by a selective transfer step to increase the pitch to the display resolution. Here, DUV lasers are the best choice for both laser lift-off and transfer. Another approach with reduced process steps is to use the laser-based mass transfer direct from the growth wafer with the advantage of selectivity and less steps, which minimises the risk of damages and inaccuracies (as shown in figure 3).
Figure 3: Laser mass transfer directly from the growth wafer versus transfer from a temporary carrier
Looking forward to industrial mass production, the laser-based mass transfer process shows the greatest potential due to precision and throughput scalability. Starting at R&D system level with a transfer of approximately two hundred thousand microLED´s per minute, there are concepts for a few million microLED´s per minute by increasing the field size at the substrate and consequently the laser energy.
With a few million microLED´s transferred per minute, the total production time of a full display reaches the level of throughput required to reduce the costs to a more reasonable level. Very large microLED displays today will be manufactured by a tile approach and depending on the size of the display, a certain number of smaller panels will be stitched and connected to one display.
The combination of high optical resolution, beam shaping, and high accuracy mechanical concepts are future-proven solutions for next generation of microLED display manufacturing, enabled by UV high energy lasers.
Oliver Haupt is director of strategic marketing for the Flat Panel Display Segment at Coherent
Jan Brune is manager of the Application Lab at Coherent LaserSystems in Göttingen, Germany