Since the world's first GaN-based blue LED was introduced in 1993, the development of LED manufacturing technology has been remarkable. Currently, commercially available GaN-based LEDs are all fabricated on sapphire substrates or SiC substrates. However, due to high hardness, poor conductivity and thermal conductivity, sapphire brings a lot of inconvenience to the processing and application of later devices. SiC also has the disadvantages of high hardness and high cost, and the Si substrate with relatively cheap price has excellent The advantages of thermal conductivity and mature device processing technology, so Si substrate GaN-based LED manufacturing technology has received widespread attention in the industry.
At present, Japan Nichia Corporation monopolizes the patented technology of GaN-based LEDs on sapphire substrates. CREE Corporation of the United States monopolizes the patented technology of GaN-based LEDs on SiC substrates. Therefore, the development of GaN-based LED production technology on other substrates has become a hot spot in the world. Nanchang University cooperated with Xiamen Hualian Electronics Co., Ltd. to undertake the National 863 Program “Power-based GaN-based LED manufacturing technology based on Si substrate”. After nearly three years of research and development, it has passed the acceptance of the Ministry of Science and Technology.
1 Si substrate LED CHIP manufacturing
1.1 Technical route
GaN is grown on a Si substrate to produce an LED blue chip.
Process flow: growing AlN buffer layer on Si substrate→ growing n-type GaN→ growing InGaN/GaN multi-quantum well light-emitting layer→ growing p-type AIGaN layer→ growing p-type GaN layer→ bonding with Ag reflective layer and forming p-type Ohmic contact electrode → stripping the substrate and removing the buffer layer → making an ohmic contact electrode of an n-type doped Si layer → alloy → passivation → dicing → testing → packaging.
1.2 Main manufacturing processes
A GaN-based MQW structure was grown on a 50 mm si (111) substrate using a Thomas Swan CCS low voltage MOCVD system. Use trimethylgallium (TMGa) as Ga source, trimethylaluminum (TMAI) as Al source, trimethylindium (TMIn) as In source, ammonia (NH3) as N source, silane (SiH4) and ferrocene Magnesium (CP2Mg) is used as an n-type and p-type dopant, respectively. First, an AlN buffer layer is epitaxially grown on a Si (111) substrate, and then an n-type GaN layer, an InGaN/GaN multiple quantum well light-emitting layer, a p-type AlGaN layer, and a p-type GaN layer are sequentially grown, and then an Ag mirror is formed on the p-plane. And forming a p-type ohmic contact, and then transferring the epitaxial layer to the conductive substrate by a thermocompression bonding method, then etching the Si substrate with a Si etching solution to remove the n-type GaN layer, and roughening the n-type surface with an alkali etching solution After that, an n-type ohmic contact is formed, thus completing the fabrication of the vertical structure LED chip. The structure is shown in Figure 1.
As seen from the structure diagram, the Si substrate chip is a flip-chip structure, and from the bottom to the top are a back Au electrode, a Si substrate, a bonding metal, a metal mirror (p ohmic electrode), a GaN epitaxial layer, a roughened surface, and Au electrode. The current distribution of the structure chip is vertical, the substrate has high thermal conductivity and high reliability; the back surface of the luminescent layer is a metal mirror, the surface has a roughened structure, and the light extraction efficiency is high.
1.3 Key technologies and innovation
The use of Si as a GaN light-emitting diode substrate has greatly reduced the manufacturing cost of LEDs and solved the patent monopoly problem. However, it is more difficult to grow GaN on Si substrates than sapphire and SiC. The thermal mismatch and lattice mismatch are larger, and the difference in thermal expansion coefficient between Si and GaN will also cause cracking of the GaN film. The difference in lattice constant will cause high dislocation density in the GaN epitaxial layer; It is also possible that a 0.5 V hetero barrier between Si and GaN causes an increase in the turn-on voltage and a poor crystal integrity, resulting in a low p-type doping efficiency, resulting in an increase in series resistance, and Si absorption of visible light reduces the outside of the LED. Quantum efficiency. Therefore, in view of the above problems, in-depth study and adoption of luminescent layer dislocation density control technology, chemical stripping substrate transfer technology, high reliability and high reflective characteristics of p-type GaN ohmic electrode preparation technology and bonding technology, high light extraction efficiency Material surface roughening technology, substrate patterning technology, and optimized vertical structure chip design technology have solved many technical problems in a large number of experiments and explorations, and finally successfully produced a light output of 1 mm × 1 mm and 350 mA. A blue light-emitting chip with a power of more than 380 mW, an emission wavelength of 451 nm, and an operating voltage of 3.2 V completes the specifications specified in the project. The key technologies and technological innovations adopted are as follows.
(1) Using a variety of on-line control techniques to reduce edge dislocations and screw dislocations in epitaxial materials, improve thermal mismatch and lattice mismatch between Si and GaN, and solve the problem of GaN single crystal films. For the cracking problem, a crack-free GaN epitaxial film having a thickness of more than 4 μm was obtained.
(2) By introducing AIN, AlGaN multi-layer buffer layer, the stress of the epitaxial GaN material on the Si substrate is greatly alleviated, the crystal quality is improved, and the luminous efficiency is improved.
(3) By optimizing the Si concentration structure in the n-GaN layer and the interface growth conditions between the quantum wells/barriers, the reverse leakage current of the chip is reduced and the antistatic performance of the chip is improved.
(4) By adjusting the p-type layer magnesium concentration structure, the operating voltage of the device is reduced; by optimizing the thickness of the p-type GaN, the light extraction efficiency of the chip is improved.
(5) By optimizing the epitaxial layer structure and doping profile, the series resistance is reduced, the operating voltage is reduced, the heat generation rate is reduced, the working efficiency of the LED is improved, and the reliability of the device is improved.
(6) Using a multi-layer metal structure, taking into account ohmic contact, reflective characteristics, bonding characteristics and reliability, optimizing the welding technology, solving the problem that the silver mirror and p-GaN are not firmly adhered and the contact resistance is large.
(7) A variety of soldering metals are preferred, the soldering conditions are optimized, and a strong bond between the GaN thin film and the conductive Si substrate is successfully obtained, and the crack problem generated in the process is solved.
(8) The surface roughening by the combination of the wet method and the dry method reduces the light loss caused by the internal total reflection and the waveguide effect, improves the external quantum efficiency of the LED, and enables the device to obtain a higher light extraction efficiency.
(9) Solved the problem that the GaN surface roughening depth is insufficient and the roughening is uneven, solves the problem that the rough surface cleaning is not clean and optimizes the metal structure of the N electrode, and obtains on the roughened N-polar n-GaN surface. Low resistance and stable ohmic contact.
2 Si substrate LED packaging technology
2.1 Technical route
The blue light LED is used to excite the YAG/silicate/nitrogen oxide multi-primary system phosphor, and emits yellow, green and red light, and synthesizes the white light.
Process flow: assembly of blue LED chip on metal bracket/ceramic bracket (conductive adhesive bonding process) → bonding (gold wire ball bonding process) → fluorescent glue coating (automatic graphic dispensing / automatic spraying process) → Si plastic packaging (Mold filling process) → cutting ribs → testing → packaging.
2.2 Main packaging process
The power-based GaN-based LED package of the Si substrate is in the form of a lug-like bracket package, which has a Lange type, a rectangular shape, and a double-wing type. The manufacturing process is as follows: using a 194 alloy metal bracket with a high thermal conductivity, the LED chip is first bonded to the bottom of the reflector of the metal bracket, and then the metal lead is connected to the LED chip and the metal bracket electrode through a bonding process to complete the electrical connection. Finally, the chip and electrode leads are covered with an organic encapsulating material such as Si glue to form package protection and optical channels. This package is optimized for light extraction efficiency, heat dissipation, and increased operating current density. Its main features include: low thermal resistance (less than 10 °C / W), high reliability, flexible gel filled with stable internal packing, in the range of -40 ~ 120 °C, no internal stress caused by sudden temperature changes, so that The gold wire is disconnected from the support and prevents the organic packaging material from turning yellow, and the lead frame is not contaminated by oxidation; the optimized package structure design makes the optical efficiency and the external quantum efficiency excellent, and its structure is shown in FIG.
2.3 Key technologies and innovation
The thermal characteristics of the power LED directly affect the operating temperature, luminous efficiency, illuminating wavelength, and lifetime of the LED. The power GaN-based LED chip design of the existing Si substrate adopts a vertical structure to improve the light extraction efficiency of the chip. The thermal characteristics of the chip are improved, and the photoelectric conversion efficiency of the device is increased by increasing the chip area and increasing the operating current, thereby obtaining a high luminous flux, thereby bringing new problems to the packaging design and manufacturing technology of the power LED. . The focus of power LED packaging is to solve the problem of light decay with effective heat dissipation and non-degrading packaging materials. In order to meet the packaging technology requirements, in a large number of experiments and explorations, analysis and solving related technical problems, the key technologies and innovations adopted have the following points.
(1) By designing a new ceramic package structure, total reflection is reduced, and the device achieves high light extraction efficiency and suitable optical spatial distribution.
(2) The electrothermal isolation package structure and the optimized heat sink design are adopted to suit the packaging requirements of the thin film chip.
(3) The metal bracket with high thermal conductivity is used, and the thermal conductive adhesive is used to bond the chip to obtain a good heat dissipation channel with low thermal resistance, so that the light decay of the product is ≤ 5% (1 000 h).
(4) Adopting high-efficiency and high-precision fluorescent glue ratio and spraying process to ensure the controllable and consistent light color parameters of the product.
(5) Multi-layer composite package, which reduces the package stress, implements the SSB bonding process and the multi-stage curing process, and improves the reliability of the product.
(6) Assemble the protection diode to increase the ESD static protection of the product to 8 000 V.
3 product test results
3.1 Si substrate LED chip
By optimizing the surface of the Si substrate and the buffer layer structure, epitaxial materials that can be used for high power chips have been successfully grown. The Pt electrode is used as a mirror to successfully realize the film transfer of the high power chip. The use of silver as a mirror greatly improves the reflection efficiency, and improves the light output power by improving the design of the mirror and introducing a roughening technique. The cleaning process and wafer soldering process of the p-type GaN surface before the vapor deposition of the Ag mirror are improved, the ohmic contact of the silver mirror is improved, the buffer structure is introduced before the quantum well, the luminous efficiency of the chip is improved, and the quantum well/barrier interface growth process is optimized. The luminous efficiency is further improved. By improving the soldering technology, the chip cracking problem during the substrate transfer process is reduced, the yield of the chip preparation is greatly improved, and the reliability is improved. Through the application and improvement of the above-mentioned multiple technologies, a blue light-emitting chip with a light output of more than 380 mW at 350 mA was successfully prepared, with an emission wavelength of 451 nm and an operating voltage of 3.2 V. . Table 1 shows the test results of the photoelectric performance parameters of the chip.
Note: The test conditions are 350 mA DC and Ta=25 °C constant temperature.
3.2 Si substrate LED package
According to the optical structure of the LED and the performance of the chip and packaging material, an optical design model and software simulation method were established to optimize the optical structure design of the package. Through the improvement of the packaging process technology, the total reflection of light is reduced, and the light extraction efficiency of the product is improved. The dispensing process of the conductive adhesive is improved, and the structure and precision of the mounting equipment are improved. The thermal isolation package structure and the optimized heat sink design are used to reduce the thermal resistance of the device and improve the heat dissipation performance of the product. The plasma cleaning process improves the bonding and reliability of the LED package interface. In view of the different requirements of the light color characteristics of the light source, the factors affecting the color of warm white, daylight white and cool white LED are studied: chip parameters, phosphor performance, formulation, dosage, and improved by the fluorescent coating process. The control capability of the power LED color parameter produces a product that is aligned with the illumination gamut specification. The test results of blue and white LED package are shown in Table 2. In the table: φ is the luminous flux; K is the luminous efficacy; P is the optical power; R is the thermal resistance; μ is the light decay; I is the saturation current.
4 Conclusion
The GaN-based LED manufacturing technology of Si substrate is the third LED manufacturing technology route in the world. It is one of the three original LED technologies. Compared with the previous two technical routes, it has four advantages: First, it has original technology property rights. The products can be sold to the international market and are not subject to international patent restrictions. Second, it has excellent performance, good antistatic performance, long life and high current density. Third, the device packaging process is simple, the chip is the upper and lower electrodes, and the single-lead vertical structure requires only a single-electrode lead during device packaging, which simplifies the packaging process and saves packaging cost. Fourth, since the Si substrate is much cheaper than the sapphire and SiC used in the first two technical routes, and the production efficiency is higher in the future, the cost is low. After three years of scientific and technological research, the project applied for 12 invention patents and 7 utility model patents. This technology successfully broke through the patent barriers of semiconductor light-emitting chips (LEDs) in the United States and Japan for many years, breaking the current monopoly of Japan's Nichia Corporation. The sapphire substrate and the US CREE company monopolize the SiC substrate semiconductor lighting technology, forming a three-pronged situation of sapphire, SiC, Si substrate semiconductor lighting technology solutions. Therefore, the introduction of LED products using Si substrate GaN is of great significance for promoting the development of the semiconductor LED lighting industry with intellectual property rights in China.
Labels: LED CHIP, LED CHIPS, LED PACKAGE, LED LAMP, LED LAMPS.
