A Brief Discussion on LED Process Technology

LEDs have a wide range of applications, but the high price of the chip itself and the need to improve the luminous efficiency have always plagued the promotion and popularization of LED lighting technology. To improve the luminous efficiency, it is necessary to effectively increase the extraction efficiency. The luminous color and luminous efficiency of LEDs are related to the materials and processes used to make LEDs. Different materials used to make LEDs can produce photons with different energies, thereby controlling the wavelength of the light emitted by the LED, that is, the spectrum or color.

1. Transparent substrate technology
InGaAlP LED is usually prepared by epitaxially growing InGaAlP light-emitting area and GaP window area on GaAs substrate. Compared with InGaAlP, GaAs material has a much smaller bandgap width. Therefore, when short-wavelength light enters the GaAs substrate from the light-emitting area and the window surface, it will be completely absorbed, which becomes the main reason for the low light output efficiency of the device. A Bragg reflection area is grown between the substrate and the confinement layer, which can reflect the light perpendicular to the substrate back to the light-emitting area or the window, partially improving the light output characteristics of the device. A more effective method is to remove the GaAs substrate first and replace it with a fully transparent GaP crystal. Since the substrate absorption area is removed in the chip, the quantum efficiency is increased from 4% to 25-30%. In order to further reduce the absorption of the electrode area, some people make this transparent substrate type InGaAlP device into a truncated inverted cone shape, which greatly improves the quantum efficiency.

2. Metal film reflection technology
The transparent substrate process first originated from HP, Lumileds and other companies in the United States. The metal film reflection method is mainly researched and developed by Japanese and Taiwanese manufacturers. This process not only avoids the transparent substrate patent, but also is more conducive to large-scale production. Its 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 the Al metal film is evaporated on its surface and the Si substrate surface at the same time, and then fused 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 chip surface, thereby increasing the luminous efficiency of the device by more than 2.5 times.

3. Surface microstructure technology
The surface microstructure process is another effective technology to improve the light extraction efficiency of the device. The basic point of this technology is to etch a large number of small structures with a size of the wavelength of light on the surface of the chip. Each structure is in the shape of a truncated tetrahedron. This not only expands the light extraction area, but also changes the refraction direction of light on the surface of the chip, thereby significantly improving the light transmission efficiency. Measurements show that for devices with a window layer thickness of 20μm, the light extraction efficiency can increase by 30%. When the window layer thickness is reduced to 10μm, the light extraction efficiency will be improved by 60%. For LED devices with a wavelength of 585-625nm, after making a texture structure, the luminous efficiency can reach 30lm/w, which is close to the level of transparent substrate devices.

4. Flip-chip technology
The GaN-based LED structure layer is grown on the sapphire substrate through MOCVD technology, and the light emitted from the P/N junction light-emitting area is emitted through the P-type area above. 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 through evaporation technology. The P area lead is led out through this layer of metal film. In order to obtain good current expansion, the Ni-Au metal electrode layer cannot be too thin. For this reason, the luminous efficiency of the device will be greatly affected, and usually both current expansion and light output efficiency must be taken into account at the same time. But no matter what the situation, the presence of the metal film will always make the light transmission performance worse. In addition, the presence of the lead solder joints also affects the light output efficiency of the device. The use of the GaN LED flip-chip structure can fundamentally eliminate the above problems.

5. Chip bonding technology
Optoelectronic devices have certain performance requirements for the required materials, usually requiring 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. The required bandwidth difference and refractive index difference cannot be formed by homogeneous epitaxial growth technology. The usual heterogeneous epitaxial technology, such as epitaxial growth of GaAs and InP on silicon wafers, is not only costly, but also has a very high bit dislocation density at the bonding interface, making it difficult to form high-quality optoelectronic integrated devices. Since low-temperature bonding technology can greatly reduce the thermal mismatch problem between different materials, reduce stress and dislocation, it can form high-quality devices. With the gradual understanding of the bonding mechanism and the gradual maturity of the bonding process technology, chips of various different materials can be bonded to each other, which may form some special-purpose materials and devices. For example, a new structure can be formed by forming a silicide layer on a silicon wafer and then bonding it. Since silicide has a high conductivity, it can replace the buried layer in bipolar devices, thereby reducing the RC constant.

6. Laser Lift Off (LLO)
Laser lift off (LLO) uses laser energy to decompose the GaN buffer layer at the GaN/sapphire interface, thereby separating 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 large-size chips. The n-side is the light-emitting surface: the light-emitting area is increased, the electrode blocks less light, it is easy to prepare microstructures, and it reduces etching, grinding, and scribing. More importantly, the sapphire substrate can be reused.