In the past one or two years, with the Micro LED display and other technologies, products, concepts, we tend to focus on the theoretical future development prospects of this technology, the technical difficulties of the existence of a more in-depth understanding of the reports and articles less, the following transfer from the snowball article analysis is more detailed, for reference!
The figure shows the evolution from sapphire and silicon substrate sizes.
Regarding the size of substrate, everyone is pursuing larger sizes.
This is the trend of continuous progress of the whole semiconductor IC process. Large substrate size will bring about an increase in yield and a reduction in overall cost. Especially for the LED industry, the mainstream of sapphire substrate is still 4 inches.
Basically, the 4-inch sapphire process is far behind the mature process of the IC industry, which has been developing silicon substrates for five or six decades.
If you want to upgrade the IC-like process, the size of the substrate should be at least 6 inches, 8 inches or more, to be able to upgrade to be compatible with the IC-like process.
At present, it is still relatively difficult for sapphire.
For silicon substrates, the semiconductor process is compatible, but there are still some technical problems with material growth.
Micro-LED epitaxy has very different requirements compared to existing LED epitaxial technology.
Micro-LED as a display application has more stringent requirements.
The human eye is very sensitive, 2nm wavelength difference, the human eye can capture very clearly, so for the entire production process, the epitaxial uniformity put forward higher requirements.
The general industry believes that the uniformity of epitaxy should be achieved within 2nm without binning and meet the requirements of subsequent giant transfer.
In addition, the warpage rate of the substrate should be strictly controlled, which is considered for the subsequent chip process, when Bow more than 50μm, the subsequent deviation of the lithography process will cause difficulties in line width control, especially the smaller the size of the Micro-LED to do, the more critical this part of the operation, so Bow warpage control, but also for the chip process yields and line width control.
In addition, the entire Wafer surface Particles and Defects need to be strictly controlled, the industry is generally agreed to be less than 0.2cm2. for the 4-inch chip, not more than 15 Defects or Particles. this also puts forward new technical requirements for the MOCVD equipment and process environment.
This figure shows the effect of large size produced by Vecco and Allos collaboration, where Std is less than 1nm.
The limitations of the micro-LED industry are summarized by Yole regarding LED manufacturers, manufacturers in the display field, and manufacturers doing transfer packaging.
These three industries are actually very discrete, LED manufacturers are generally limited to less than 6 inches, mainly for the 4-inch display and lighting technology applications.
At the same time, most of the display manufacturers now use OLED, LCD mature technology, they have no experience in the LED industry Ⅲ-V semiconductor material epitaxy, device preparation, semiconductor process, etc.
Packaging Mass transfer, Pick and Place and other technologies are currently completely blank state.
So there is a split between these major areas.
How to find a way to integrate LED and display field, the emerging transfer, is the key to the future of Micro-LED.
We believe that the silicon substrate Micro-LED epitaxial technology is able to integrate the two industry chains, silicon substrate can also be integrated together with the IC process.
Therefore, the silicon substrate Micro-LED technology may be a key to the future development of Micro-LED display in the future.
Why silicon substrate is not yet the mainstream of the LED community?
Mainly because growing GaN material on top of silicon is much more difficult than growing GaN material on top of sapphire.
There are several reasons for this.
First, GaN grows on top of Si, which is a heterogeneous epitaxy.
The lattice constant adaptation up to 17% is very large.
Second, the growth conditions in more than a thousand degrees, after growth back to room temperature will have a thousand degrees of temperature difference, the material thermal expansion coefficient is different and will cause huge stress, GaN and Si have more than 56% of the thermal expansion coefficient difference, so the material growth is difficult.
Thirdly, Ga atoms in GaN itself will have etching reaction with Si. If the GaN is grown directly on top of Si, it will be reacted off by etching. Therefore, it is now popular internationally to grow AIN on top of Si as a buffer layer, and then grow GaN material on top of the buffer layer.
This requires very fine control of the AIN material and interface, as well as the growth of high-quality GaN material on top of the buffer layer material, and these are the current technical challenges. There are also some international attempts, such as just showing Allos, Vecco and domestic manufacturers are also doing this work and efforts. Our institute also has some unique technologies of its own.
Uniformity of wavelength.
When using two different substrates, sapphire and silicon, all substrates are placed on a graphite disk during growth.
The temperature heat is transferred to the GaN and silicon wafers by heating the graphite disk.
During epitaxial growth, the entire wavelength is very sensitive to temperature, basically a 1°C difference in temperature between the two surfaces will result in a difference in wavelength, for example, a 2 nm difference in blue light.
If the uniformity within the whole Wafer reaches less than 2nm, it means the temperature of the sample surface cannot exceed 1℃, which is very challenging work.
For existing MOCVD equipment, the temperature of the surface is measured with a Pyrometer using a 980nm laser.
For silicon, which is an opaque object, this light can hit directly on top of the silicon and read the temperature of the surface on the wafer in real time for real-time control. For sapphire substrates it is more difficult.
Sapphire is a transparent material, and when the laser hits it, the temperature of the graphite disk is measured. The temperature of the sapphire surface is controlled indirectly by the temperature of the graphite disk.
So in terms of directness and effectiveness of temperature control, silicon is a little better than sapphire, which is helpful for improving the wavelength uniformity.
At the same time epitaxial materials, most manufacturers of GaN materials on silicon, defect density are more than 3 × 108 cm2, compared to sapphire, the defect density is relatively large.
For the future Micro-LED needs to reduce the defect density to 1 × 108 cm2 below, in order to have competition with sapphire.
To reduce the defect density, it is critical to improve the quality of AIN material.
With the existing MOCVD equipment for LED production, there are some inherent limitations.
For example, the design of the reactor hardware, Chamber design will need some new design enhancements.
In terms of the epitaxial material growth process, there are the same problems that need to improve the quality of the epitaxial.
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