Due to the advantages of ultra-high resolution, high color saturation, nanosecond response time and low power consumption, Micro LED has become a new generation of display technology for the layout of Apple, Sony, Facebook, Samsung, LG, OSRAM, Nichia and other international large factories.
As a global leader in GaN-based silicon-based LED technology, Crystal Photonics has recently turned its attention to Micro LEDs.
The cost, yield, and compatibility with the transfer/bonding process must be taken into account in selecting the route of the micro LED chip. The larger the size of epitaxial wafer, not only can reduce the cost of chip, improve the utilization of epitaxial area, but also more easily compatible with IC process to improve the production efficiency and yield of Micro LED.
Currently, only GaN-based LEDs on silicon substrate can be produced in 8-inch quantities, and excellent uniformity of wavelength dispersion less than 1 nm on an 8-inch epitaxial wafer is achieved in a single MOCVD cavity, which is crucial for micro LEDs, said Fu Yi, vice president of Crystal Energy Optoelectronics. Commercial silicon wafers of 12 inches or more are fully mature. With the introduction of high homogeneity MOCVD epitaxial large cavity, there is no essential difficulty in upgrading silicon substrate LED epitaxial to larger wafer size.
Fu Yi introduced that GaN-based LED on silicon substrate used chemical wet stripping process to obtain LED film chip. This wet stripping avoids the damage to LED epitaxial layer, which is very important to ensure the efficiency and yield of micro-LED driven by small current.
As a comparison, the damage of GaN epitaxial layer caused by laser stripping on sapphire substrate is unavoidable, and it can be predicted that the challenge of laser stripping substrate yield will become more and more serious when sapphire substrate can be upgraded to a larger size. Although SiC and GaN substrates do not require laser stripping in thin film chip fabrication, the high price of these two substrates (especially large size substrates) will make it more difficult to compete with OLED in the market.
He pointed out that the current structure design of micro LED thin film chips can be divided into flip-chip (dual-electrode on the same side) and vertical (upper and lower electrode) two kinds. For typical Size MicroLEDs (chip edge length is less than 10 micron), the positive and negative electrode connection can be achieved by using the dual electrode structure on the same side and controlling the single bonding of the backplane. However, the short circuit between the positive and negative electrodes is easy to occur during the bonding process, and the bonding accuracy is also challenged. In contrast, thin film vertical micro LEDs with upper and lower electrodes are more conducive to bonding yield, but a transparent layer of co-negative (or co-positive) electrodes is needed.
In summary, the fabrication of large-size silicon-based LED thin film chips can produce low-cost and high-yield micro-LED chips, regardless of the dual-electrode structure or the upper and lower electrode structure required by the back-end process.
Fu Yi believes that the low cost, large size, non-destructive peel silicon substrate thin film LED technology will strongly promote the development and industrialization of Micro LED.
For the research camp of adding crystal photovoltaics to micro LEDs, Dr. Liu Guoxu, CTO, commented: "GaN-based technology on silicon substrate is a natural choice for manufacturing micro LEDs. Crystal photovoltaics has accumulated more than ten years of technical and mass production experience in the field of GaN-based LED on silicon substrate, such as transferring to the application of micro LEDs and micro LEDs. It will take a big step forward. "