Micro LED, 9 challenges for commercialization

Recently, LED Inside, the LED-specialized media, presented the technical problems that are emerging with Micro light emitting diodes (LEDs) which must be solved in new display technology.

Micro LEDs are superior to an organic light emitting diodes (OLEDs) as well as liquid crystal displays (LCDs) in terms of brightness, contrast, response speed, and power consumption.

There are, however, several technical challenges that need to be addressed in order to commercialize them. There are 9 technical challenges mentioned by LED Inside: Epitaxy, chip, mass transfer, full color, bonding, power drive, backplane, inspection and repair technology.

The main obstacle to Micro LED epitaxy technology is the improvement of wavelength and thickness uniformity. Epitaxy refers to a crystal growing with a crystal structure and orientation similar to a substrate. When the wavelength and thickness are uniforms, the wavelength is more concentrated and the backend inspection cost of the epitaxy maker can be greatly reduced. If the LED chip is reduced to less than 100 micrometers (㎛, one-millionth of a meter), power leakage may occur due to non-uniformity caused by the cutting damage around the LED chip, which may affect the overall luminescence characteristics.

Mass transfer manufacturing process, in which LED chip is transferred to the substrate, presents the biggest technical barrier. Damage should be avoided when the chip rises from the temporary substrate to the substrate. The process of picking up an element from a temporary substrate with a stamp and placing it on an actual substrate can be applied to a product of 10 μm or more. However, the disadvantage is that it takes too long. Fluidic assembly technology can increase throughput per hour for products over 20μm but requires three transfer times to achieve full color. Laser transfer technology can be applied to products over 1㎛, but laser equipment is expensive, making the initial investment burdensome.

For full-color technology solutions, the RGB chip color conversion solution has insufficient luminance effects and low yield problems associated with technologies below 20 μm. Instead, quantum dot technology can fill this gap. But some of the challenges facing quantum dot solutions such as coating uniformity and reliability still pose challenges.

Bonding technology is also a roadblock. Because the Micro LED chip is too small, a large amount of metal in the solder paste may cause a short circuit. Because of this, adhesion process technology is another big obstacle for commercializing the Micro LED process. Currently, there are four directions: metal bumps, adhesives, wafer bonding, and micro-tube.

In terms of driver technology, each pixel in an Active Matrix drive array is connected to a circuit and driven separately. This allows the Micro LED to operate at low power and maintain full brightness without loss of noticeable display brightness over the entire illumination time. But Micro-LED drive currents are very small, which complicates circuit design and increases the space density of the drive power module.

Backplane technology has four types: Glass, a flexible substrate, a silicon substrate, and a printed circuit board (PCB). Currently, PCB backplanes are most widely used due to their high compatibility. The splicing can be tailored to various sizes, and the substrate can be selected according to the required backplane.

Inspection and testing techniques also pose challenges. Micro LED application products use a large amount of chips. Moreover, photoluminescence and electroluminescence of Micro LED modules must be detected quickly and accurately. To save time and money, it is necessary to take the mass test method. Rapidly and accurately identifying good products is an important issue in the manufacturing process and is one of the main causes of the current Micro LED test technology difficulty.

Lastly, there is a repair technology. From the current viewpoint of Micro LED recovery solutions, there is: UV irradiation rescue technology, laser welding rescue technology, selective rescue technology, selective laser rescue technology, and redundant circuit design solution.