What’s the manufacturing process technology of LED chips
LED has a wide range of applications, but the high price of the chip itself and the need to improve the luminous efficiency has always plagued the popularization of LED lighting technology. To improve the luminous efficiency, it is necessary to effectively increase the light extraction efficiency. The luminous color and luminous efficiency of LEDs are related to the materials and the manufacturing process technology used to make LEDs. Different materials used to make LEDs can generate photons with different energies, thereby controlling the wavelength of the light emitted by the LED, that is, the spectrum of color.
So what’s the main manufacturing process technology of LED chips on the market now?
Transparent substrate technology
InGaAlP LEDs are usually prepared by epitaxially growing InGaAlP light-emitting regions and GaP window regions on a GaAs substrate. Compared with InGaAlP, GaAs material has a much smaller forbidden bandwidth. Therefore, when short-wavelength light enters the GaAs substrate from the light-emitting area and the window surface, it will be fully absorbed, which becomes the main reason for the low light-emitting efficiency of the device. A Bragg reflection area is grown between the substrate and the confinement layer, which can reflect the light perpendicularly directed to the substrate back to the light-emitting area or the window, which partially improves the light-emitting characteristics of the device. A more effective method is to remove the GaAs substrate and replace it with a fully transparent GaP crystal. Because the substrate absorption area is removed from the chip, the quantum efficiency is increased from 4% to 25-30%. In order to further reduce the absorption of the electrode area, some manufacturers made this transparent substrate type InGaAlP device into a truncated inverted cone shape, which greatly improved the quantum efficiency.
Metal film reflection technology
The transparent substrate process first originated from companies such as HP and Lumileds in the United States. The metal film reflection method was mainly researched and developed by Japanese and Taiwanese manufacturers. This process not only avoids patents on transparent substrates but is also more conducive to mass production. The effect can be said to be similar to the transparent substrate method. This process is usually called the MB process. First, the GaAs substrate is removed, and then an Al metal film is vapor-deposited on the surface and the surface of the Si substrate at the same time, and then welded together under a certain temperature and pressure. In this way, the light irradiated from the light-emitting layer to the substrate is reflected by the Al metal film layer to the surface of the chip, thereby increasing the light-emitting efficiency of the device by more than 2.5 times.
Surface microstructure technology
The surface microstructure process is another effective technology to improve the light-emitting efficiency of the device. The basic point of this technology is to etch a large number of small structures on the surface of the chip with the size of the light wavelength. Each structure is in the shape of a truncated tetrahedron, which not only expands the light-emitting area, and the direction of light refraction at the surface of the chip is changed, so that the light transmission efficiency is significantly improved. Measurements indicate that for a device with a window layer thickness of 20 µm, the light extraction efficiency can increase by 30%. When the thickness of the window layer is reduced to 10 µm, the light extraction efficiency will be improved by 60%. For LED devices with wavelengths of 585-625nm, after the texture structure is made, the luminous efficiency can reach 30lm/w, which is close to the level of transparent substrate devices.
Flip chip technology
The GaN-based LED structure layer is grown on the sapphire substrate by MOCVD technology, and the light emitted from the P/N junction light-emitting area is emitted through the upper P-type area. Due to the poor conductivity of P-type GaN, in order to obtain good current expansion, it is necessary to form a metal electrode layer composed of Ni-Au on the surface of the P area by vapor deposition technology. The P area leads are drawn through this layer of the metal film. In order to obtain good current expansion, the Ni-Au metal electrode layer should not be too thin. For this reason, the luminous efficiency of the device will be greatly affected, and usually, both current expansion and luminous efficiency must be considered at the same time. However, under any circumstances, the presence of metal thin films will always make the light transmittance worse. In addition, the presence of wire solder joints also affects the light extraction efficiency of the device. Using GaN LED flip-chip structure can fundamentally eliminate the above problems.
Chip bonding technology
Optoelectronic devices have certain requirements on the performance of the required materials, and usually require a large bandwidth difference and a large change in the refractive index of the material. Unfortunately, there is generally no natural material of this kind. Homoepitaxial growth technology generally cannot form the required bandwidth difference and refractive index difference. However, the usual heteroepitaxial technology, such as epitaxial GaAs and InP on silicon wafers, not only has a higher cost but also combines the location of the interface. The error density is also very high, and it is difficult to form high-quality optoelectronic integrated devices. Because low-temperature bonding technology can greatly reduce the thermal mismatch between different materials, and reduce stress and dislocations, high-quality devices can be formed. With the gradual understanding of the bonding mechanism and the gradual maturity of the bonding process technology, chips of a variety of different materials can already be bonded to each other, which may form some special-purpose materials and devices. For example, a silicide layer is formed on a silicon wafer and then bonded to form a new structure. Due to the high conductivity of silicide, it can replace the buried layer in bipolar devices, thereby reducing the RC constant.
Laser Stripping Technology (LLO)
Laser lift-off technology (LLO) uses laser energy to decompose the GaN buffer layer at the GaN/sapphire interface, so as to realize the separation of the LED epitaxial wafer from the sapphire substrate. The technical advantage is that the epitaxial wafer is transferred to a heat sink with high thermal conductivity, which can improve the current expansion in a large-size chip. The n-side is the light-emitting surface: the light-emitting area is increased, and the electrode is small, which facilitates the preparation of microstructures, and reduces etching, grinding, and scribing. More importantly, the sapphire substrate can be reused.
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