With the gradual application of LEDs in the field of lighting, the market has higher and higher requirements for the luminous efficacy of white LEDs. At the same time, the price has also become a key factor for the rapid promotion of LEDs. The market urgently needs cost-effective LED products. GaN-based vertical structure LEDs have good heat dissipation capability and can withstand large current injection. Such a vertical structure LED chip can be equivalent to several formal structure chips, and the converted cost is only a fraction of the formal structure. Therefore, GaN-based vertical structure LEDs are the market trend and an inevitable trend in the development of semiconductor lighting.
At present, the world has entered the “energy-saving era”, and all countries are actively looking for emerging industries that are energy-saving and environmentally friendly. The energy consumed by lighting accounts for more than 20% of the total energy consumption. Therefore, reducing lighting electricity consumption is an important way to save electricity. LED energy-saving lamps are a new generation of solid cold light sources. They have the characteristics of low energy consumption, long life, easy control, safety and environmental protection. They are ideal energy-saving and environmentally friendly products and are suitable for various lighting places. However, at present, LEDs have not yet entered general lighting on a large scale. One of the main reasons is that the cost performance of LEDs (lm/$) is too low, and the market needs a solution to quickly improve the cost performance of LEDs. There are two main ways to improve the cost performance of LEDs, one is to improve the luminous efficiency of LEDs, and the other is to reduce the production cost of LEDs. However, the speed of efficiency improvement and cost reduction still falls short of the market’s expectations for the cost-effectiveness of LEDs. However, the vertical structure LED can ensure a certain luminous efficiency and use a large current to drive, such a vertical structure LED chip can be equivalent to several formal structure chips, and the converted cost is only a fraction of the formal structure. Therefore, the vertical structure LED will inevitably accelerate the process of LED application in the field of general lighting, which is the market direction and the inevitable trend of semiconductor lighting development.
2. Advantages of GaN-based vertical structure LEDs
In terms of LED structure, GaN-based LEDs can be divided into front-loading structure, flip-chip structure and vertical structure. At present, the more mature III-nitrides mostly use sapphire material as the substrate. Due to the insulating properties of the sapphire substrate, the common GaN-based LED adopts a front-mounted structure. The schematic diagram of the front-mounted structure is shown in Figure 1. The light emitted by the active region exits through the P-type GaN region and the transparent electrode. The structure is simple and the manufacturing process is relatively mature. However, the front-mounted structure LED has two obvious shortcomings. First, the p and n electrodes of the front-mounted structure LED are on the same side of the LED, and the current must flow laterally through the n-GaN layer, resulting in current crowding, high local heat generation, and limiting the drive current; secondly , due to the poor thermal conductivity of the sapphire substrate (35W/(m?K)), which seriously hinders the dissipation of heat.
In order to solve the problem of heat dissipation, the American Lumileds Lighting Company invented the flip-chip (Flipchip) technology. The schematic diagram of its structure is shown in Figure 2. In this method, a large-size LED chip suitable for eutectic welding is first prepared, and a silicon base plate of corresponding size is prepared at the same time, and the gold conductive layer and the lead-out conductive layer of the eutectic welding electrode are made on it. (Ultrasonic gold ball solder joints). Then, use eutectic bonding equipment to bond the large-size LED chips to the silicon substrate. The on-chip structure has greatly improved the heat dissipation effect, but the usual GaN-based on-chip LED is still a lateral structure, and the phenomenon of current crowding still exists, which still limits the further improvement of the drive current.
The vertical structure can effectively solve the two problems of the front-mounted structure LED (the schematic diagram is shown in Figure 3). The vertical structure GaN-based LED uses a substrate with high thermal conductivity (Si, Ge, Cu and other substrates) to replace the sapphire substrate. The heat dissipation efficiency is greatly improved; the two electrodes of the LED chip with a vertical structure are on both sides of the LED epitaxial layer, and the patterned n-electrode makes the current almost all vertically flow through the LED epitaxial layer, and the current flowing laterally through the LED epitaxial layer. It can avoid the current crowding problem of the front-mounted structure, improve the luminous efficiency, and also solve the shading problem of the P pole and increase the light-emitting area of the LED.
3. Manufacturing method of vertical structure LED
The biggest difference between the GaN-based vertical structure LED process and the front-mounted structure LED process is that the vertical structure LED needs to introduce substrate transfer technology. The so-called substrate transfer technology refers to replacing the original growth substrate with a new substrate with high thermal conductivity and high electrical conductivity. The specific steps are divided into two steps. First, the new substrate and the epitaxial wafer are bonded together by the method of wafer bonding or electroplating (as shown in Figure 4), and then the methods such as laser lift-off, grinding and wet etching are used to bond the new substrate and the epitaxial wafer together. The original growth substrate is removed.
4. Development status of GaN-based vertical structure LEDs
At present, several major LED manufacturers in the world, such as Cree of the United States, Osram of Germany, Philips Lumileds of the United States, and SemiLEDs of the United States, all have their own GaN-based vertical structure LED products. In addition, Japan, South Korea, Taiwan and major domestic LED manufacturers are actively developing GaN-based vertical structure LED chip technology.
4.1 Development status of GaN-based vertical structures abroad
Cree Company of the United States is one of the professional companies in the world that uses SiC as the substrate material to manufacture epitaxial wafers and chips for blue light-emitting diodes. The problem of heat dissipation and improving the efficiency of light extraction. In August 2006, the company launched EZBright high-power LED chip, the schematic diagram of the chip structure is shown in Figure 6. Under the current of 350mA, the optical power of the chip can reach 370mW. Cree’s power LED chip product EZ series adopts thin film chip technology and has reached the industry-leading light efficiency level. According to a report in May 2011, Cree’s white light LED device research and development level has reached 231 lm/W, which is a power type white light. LED has the best results ever reported.
At the beginning of 2007, Lumileds introduced thin film flip-chip (TFFC) LED products. In fact, the thin-film flip-chip structure is also a type of vertical structure LED. Thanks to technological advancements, this product can perform optimally in any environment. The Luxeon K2 using TFFC technology is an LED specifically designed, binned and tested to operate at 1000mA. The thermal resistance of the packaged LED is only 5.5°C/W. The reduction in thermal resistance allows the LED to operate at higher currents, reducing the investment and cost of thermal management design. The binned and tested product (minimum light output 160lm, 1A drive current) easily exceeds 220lm at higher drive currents. Figure 7 shows the schematic diagram of the structure of the TFFC LED and the photo of the luminescence.
German company Osram compared the light extraction efficiency of four structures of GaN-based LEDs (as shown in Figure 8), in which the “ThinGaN” chip is actually a vertical structure LED. The company uses wafer bonding and lift-off technology to transfer the LED light-emitting layer to a new substrate (GaAs substrate or Ge substrate). As can be seen from Figure 8, the light extraction efficiency of the “ThinGaN” chip is up to 75%, which shows that the thin-film structure LED has great advantages in light extraction. In terms of products, the company started selling the latest version of the white LED “OSTAR Lighting” that emits 1000lm of luminous flux in 2007. When the input power of this product is 27W (working current 700mA), the luminous flux of 1000lm can be obtained, and the luminous efficiency at this time is about 37lm/W. When the working current is reduced to 350mA, the luminous efficiency can be increased to 75lm/W. The luminous flux of the conventional OSTAR Lighting is about 600lm at 27W input power. In addition, the company also launched the “Diamond DRAGON” white LED, which can obtain a luminous flux of 250lm when the operating current is 1.4A, and the product life is more than 50,000 hours.
American SemiLEDs company was established in 2004. After Osram and Cree, it uses substrate transfer technology to commercialize the production of GaN-based vertical structure LEDs. The company first introduced a GaN-based LED on a metal Cu substrate, the schematic diagram of which is shown in Figure 9.
4.2 Development Status of GaN-based Vertical Structures in China
Peking University in China was the first to carry out the research and development of GaN-based vertical structures. Since then, research institutions and major LED companies have successively carried out research and development in this area. Tongfang Optoelectronics began to engage in the research and development of vertical structure LEDs in 2008. After more than two years of hard work, it has accumulated a complete set of Cu substrate plating, leveling and sapphire stripping technology (see photo in Figure 9). The vertical structure LED chips produced at present can achieve an efficiency of more than 100lm/W after packaging white light. After the LED industry experienced the “337 investigation”, patents will also become the focus of competition. At present, Tongfang Optoelectronics has more than 10 vertical structure chip manufacturing patents, three of which have been authorized by the State Intellectual Property Office, two have been authorized by the Taiwan Patent Office, and one has been authorized by the United States Patent Office, laying a solid foundation for the development and production of vertical structures. Good foundation.
The vertical structure LED obtains the advantage of high lumen output power by virtue of its suitability to work under large driving current, so as to obtain high cost performance. Therefore, GaN-based vertical structure LEDs are the market trend and an inevitable trend in the development of semiconductor lighting, and will gradually become a mainstream product. At present, several major international factories have achieved mass production, and although domestic factories have all carried out research and development work, none of them has reached the real mass production level. The vertical structure chips currently produced by Tongfang Optoelectronics have a luminous efficiency of more than 100lm/W after packaging.