In general, from the substrate size, crystal quality, substrate cost further analysis of sapphire and silicon substrates.
Sapphire substrates can be up to 6", while silicon substrates can be up to 8" or even 12", and silicon substrates have an advantage in this respect.
In terms of crystal quality, sapphire is better, while silicon substrate needs continuous optimization to achieve results.
The wavelength uniformity is better controlled by silicon substrate.
In terms of chip process, silicon substrate chip process does not require laser stripping, it can be removed by grinding, so that no damage is done.
Sapphire substrates need to be laser stripped for large damage.
Regarding transfer substrates, silicon and silicon homogeneous transfer substrates have better performance.
So the chip performance in general, the two are similar, where sapphire can do flip-flop and vertical, silicon substrate can only do vertical. In terms of chip reliability, silicon substrate without damage also has some advantages.
So from this point of view, in the future, to achieve a larger size and the combination of subsequent processes, silicon substrate is a good choice.
Micro-LED chips and LED chips have some different characteristics.
LED chips are in the high current lighting interval, the main work hopes to reduce the Droop effect under high current.
micro-LED chips work in the interval below 2A/cm2, mainly to reduce the non-radiative compound to enhance the quality of the material.
It is not the same as the LED chip for lighting. At the same time, the chip process size is getting smaller and smaller, the peak efficiency moves to the direction of high current density, that is, to the right.
And we want it to be biased to the left, so this is also a problem that needs to be overcome and solved.
Currently, flip chip structure and vertical chip structure are the main two different micro-LED chip structures, for high resolution, vertical structure is needed, and low resolution can use flip chip structure.
Because the electrodes of flip chip structure are on the same side, the size of flip chip structure cannot be too small. Flip-chip structure can be directly applied to the mega-transfer process, while the vertical structure requires additional laser stripping process and electrode process after the mega-transfer, which is relatively complex.
For chip cost, there are advantages and disadvantages.
At present, the more common micro-LED chip structure on the market has three kinds.
1, Chip on Wafer, that is, in the entire Wafer shipments, a single integrated way will be more.
2, weakened structure. The structure to do huge amount of transfer with elastic film or roller transfer technology, etc.
3、Freestanding Chip on Carrier, which is completely placed on the temporary substrate, this structure can cooperate with laser transfer, elastic film, roller transfer technology.
The chip has to be bonded to the driver substrate after it is ready.
The two most discussed bonding methods are: mega-transfer technology and monolithic integration technology.
Mega-transfer technology transfers thin-film chips with sizes below 30 microns from the Wafer to the substrate for large area applications.
The monolithic integration technology is either precisely welded to the display substrate, or the entire side of the Wafer is transferred to the substrate, and then the semiconductor process is done to separate them one by one and combine them with the driver circuit on the substrate.
Such a do is completely limited by the Wafer, generally only suitable for small area high PPI display, such as AR/VR applications
Mega-transfer technology is a very hot technology at the moment.
There have been many reports, but the reports only focus on patents, and prototypes are still very rare.
Because of the confidentiality and technical requirements of major companies, few comprehensive performance indicators such as process, accuracy, yield, transfer rate, cost, and detailed equipment information have been made public.
The six main methods, electrostatic force, electromagnetic force, van der Waals force printing, roller type, laser, and self-assembly. Which technology can win is still a question mark. Now only Demo samples are out, but their yields and costs still need time to test.
Regarding electromagnetic force adsorption and electrostatic adsorption technology solutions, the development of the equipment is not yet known because many companies are not publicly available.
Fluid assembly, there are some experimental machines, but also not particularly public.
What you hear more often now is the elastic impression transfer.
Laser transfer is also claimed to be under development by some manufacturers, but not on the market. So these are all unknowns.
Monolithic integration technology started with the alignment bonding technology of a chip done by Texas Tech University, followed by Strathclyde University, and HKUST/SCTU.
Monolithic integration is generally at the stage of monochromatic light, and there are no color samples yet.
To make colorful products you must perform color mixing techniques.
There are two ways that are more commonly used now. One is RGB, which means that three different chips of blue, green and red are transferred to the driver circuit through three giant transfers to achieve color mixing.
Theoretically will get a very good effect of the display, but because of three giant transfer, three different repair, three different material system of the chip process will also increase its cost, which is also a problem to be considered.
Quantum dot color mixing technology uses a monochromatic display, such as blue or violet light, and then mixes the colors with quantum dots.
The advantage is that only one giant transfer is done, low cost and easy to operate.
However, the stability of the material of quantum dots itself, the stability of the color, and the loss of energy, all these need further development.
Quantum dot color mixing technology is also a popular technology at the moment.
There are four different technical solutions, including inkjet printing technology, which directly sprays in with quantum dot black water and uses a cloth-in motor to control the position of each drop of ink, one by one, to the desired position.
Aerosol jet printing, which atomizes quantum dot ink and then sprays it to the specified location.
Photolithography, one is in mesh format, where quantum dots are painted on and then scraped off to get the quantum dot die you want.
Another kind is shown in the picture, photolithography is just like the lithography process of semiconductor, through the yellow light process, exposure traction, and finally get the desired pattern of quantum dots.
The elastic stamp transfer, similar to the giant transfer, picks up the quantum dots and places them on the desired position, thus completing the transfer of quantum dots.
The future line width of quantum dots can be achieved below 5 microns, while the life span is also a test, hopefully four or five years later, it can reach 10,000 hours.
Quantum efficiency is increased to more than 95%. With such a requirement, we hope that new materials will emerge to promote the development of quantum dot technology.
Detection technology is also a huge challenge. Usually LEDs are detected with probes, how can Micro-LEDs be detected? As shown in the figure.
Micro-hyperspectral imaging system detection, non-contact PL detection, contact photoelectric detection, non-contact EL detection.
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