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As a bridge for cooperation between Fujian and Taiwan, Xiamen has laid a solid foundation for the optoelectronic display industry by relying on its unique locational advantages, attracting investment and providing policy services based on "one enterprise, one policy". After years of development, Xiamen has formed a relatively complete industrial chain layout covering LEDs, glass substrates, panels, modules, liquid crystal displays, and complete machines. As an important carrier of Xiamen's optoelectronic display industry, Huli District has not only performed outstandingly in the regional industrial chain layout, but also has achieved considerable results in the technological evolution of enterprises, which is worthy of in-depth observation and research.

In order to more clearly sort out the technical difficulties and future development direction in the field of optoelectronic display, the reporter of The Paper interviewed Professor Huang Kai, Vice Dean of the School of Physical Science and Technology of Xiamen University and Executive Director of Xiamen Future Display Technology Research Institute of Tan Kah Kee Innovation Laboratory. The Xiamen Future Display Technology Research Institute where Professor Huang Kai works is an important link in the industry-university-research cooperation in Xiamen's optoelectronic industry. As an outstanding scientist in this field, Huang Kai and his team are committed to providing technical support for Xiamen's optoelectronic display industry and playing an important role in the communication and coordination of upstream and downstream of the industrial chain.

The Paper: Why has Xiamen's optoelectronic display industry been able to develop? Where did the relevant technology originally come from?

Huang Kai:In early 2000, Xiamen began to focus on the development of the optoelectronics industry. At that time, a lot of energy and resources were invested, including support from various ministries and commissions of the state. Xiamen also introduced a series of policies to support the development of the LED industry. At that time, Japan and Taiwan were the best in LED, but after more than ten years of development, Xiamen has achieved a significant advantage in the global semiconductor lighting field. At its peak, Xiamen's LED lighting products accounted for nearly 40% of the global market. In recent years, optoelectronics and display have begun to merge with each other to a certain extent, and lighting has gradually transformed and closely integrated with display. Optoelectronic LEDs are increasingly used in the display industry, not only the application area is increasing, but also the value chain is constantly improving.

Xiamen's development in the field of optoelectronic display is somewhat accidental and inevitable. Its geographical location is close to Taiwan, and the natural cultural integration has greatly promoted the development of the industry. At the same time, Xiamen's start was not late. The global optoelectronic display industrialization began around 2000. In June 2003, the "National Semiconductor Lighting Project" was launched, and Xiamen responded quickly and invested a lot of resources to support industrial development. At that time, even the director and deputy director of the street office, and even ordinary clerks, talked about LED when they met, and indeed formed an atmosphere where everyone cared about and paid attention to the development of LED. In terms of technology sources, domestic LCD and OLED technologies mainly come from Japan, South Korea and Taiwan. China paid a lot of money to introduce these production lines and sent a group of technical backbones to Japan to study. After studying abroad for one or two years, they brought back advanced technology and experience.

With the support of scientific research in mainland China and Taiwan, China has gradually developed and expanded its optoelectronic display industry. After about 20 years of efforts, China has cultivated a complete optoelectronic industry ecosystem and talent team from scientists to engineers to business operators.

The Paper: What is the trajectory of technological evolution?

Huang Kai:Laptops have been becoming thinner and lighter since around 2010. Before 2010, LCD screens were mainly used. LCDs do not emit light by themselves and require a backlight to realize the display function. Its function is to realize images by selectively transmitting or blocking the backlight. The screen is composed of many pixels, each of which is divided into three sub-pixels: red, green, and blue. In order to display red light, the screen blocks blue and green light. Therefore, in addition to the liquid crystal layer as a "valve", a backlight is also needed to illuminate the screen. Early monitor backlights used small fluorescent lamps, which limited the thickness of the screen. Around 2010, fluorescent tubes were replaced by LED backlights, which greatly reduced the thickness of the screen. The LED chips used for backlights are only a few hundred microns, which is very thin, so LED backlights quickly replaced traditional fluorescent backlights. This change is also a huge boost to the LED industry, with LED backlights accounting for 60% of the total LED output.

Then came OLED (organic light-emitting diode) displays, which do not require backlight. OLED technology is used in shopping mall entrances, conference hall screens, etc. For example, a 4K resolution screen has 4,000 pixels horizontally and 2,000 pixels vertically, for a total of 8 million pixels. Each pixel needs to be able to display three colors: red, green, and blue, so 24 million sub-pixels are required, and there is also a set of driving circuits behind each pixel. Since OLED is a small molecule material and is easily oxidized, the vapor deposition of the light-emitting layer and the back-end packaging are relatively complicated.

Micro-LED is an inorganic material that is more stable than OLED, so it has lower requirements for packaging. Technologies such as LED and LCD complement each other in the development process, and now they are slowly becoming competitive technologies. Although most of the screens of laptops are still LCD, the backlight is almost entirely LED, and there are two backlighting methods: one is that the LED of the edge device is introduced into the optical fiber through the light guide plate, and the other is to use multiple Mini-LED chips as the backlight source. As users' expectations for products continue to increase, technology is also constantly improving, and new solutions continue to emerge.

Xiamen is in a leading position in LED epitaxial chips. The substrate of the epitaxial chip is a single crystal material. The epitaxial material grows layer by layer to form different functional layers. The main components and doping of each functional layer are different. It is necessary to precisely control the inflow of gas at high temperature to decompose and recombine the material on the crystal surface and gradually grow. The control accuracy is at the nanometer level. In general, the development trend of display technology is that pixel density continues to increase and chip size continues to shrink. LCD or OLED and Mini-LED and Micro-LED technologies are competitive or parallel technologies to a certain extent. There is a consensus that, at least in the short and medium term, no one technology will completely replace other technologies. Each technology has its applicable scenarios and advantages. For example, the cost advantage of LCD means that it still has a good market demand.

The Paper: The industry seems to believe that Micro-LED is a universal solution. Is there really such a universal solution?

Huang Kai:Generally speaking, the harder something is to make, the harder it is to destroy. The preparation of Micro-LED requires a temperature of one thousand degrees, and organic compounds usually cannot withstand temperatures above one hundred degrees, so inorganic compounds are needed to be more stable. Micro-LED usually uses chips smaller than 50 microns, and 5-6 chips can be placed in the diameter of a hair. The preparation is very difficult, and the previous mechanical transfer technology can no longer meet the requirements, requiring the use of stamp-type or laser mass transfer technology.

Micro-LED has performed well at the current stage. With its excellent stability and comprehensive performance indicators, it has become a display technology with the best comprehensive indicators. Micro-LED requires high speed, high precision and high yield, and places extremely high demands on processes and equipment. In contrast, the chip size of Mini-LED and conventional LED generally does not reach the micron level, but may be millimeter level, but because it is far enough away from us, the display effect is still good. The traditional display solution is to make the sub-pixel that needs to emit light by blocking the other two sub-pixels, while Micro-LED uses self-luminous technology. Each sub-pixel can emit light independently, thereby achieving higher contrast and color performance, which makes it have great potential in display applications, but it also means greater challenges in manufacturing and process. Directly let the sub-pixel that needs to emit light emit light.

At the application level, currently, large screens over 100 inches can only be realized by Micro-LED. The reason is that Micro-LED can achieve seamless splicing. For example, due to the limitations of the evaporation process, OLED cannot achieve large-area and uniform display. In addition, the driving circuit of OLED also has the problem of loss. For a complete large screen, due to the existence of resistance materials, the current obtained after applying voltage to the left and right ends will be different, which is a difficult problem to overcome when the screen size is too large. Micro-LED can achieve seamless splicing and can splice large screens of any size according to needs. At the same time, the pixel density can be very high, which is a major advantage. The extreme capability of our laboratory can reach about 15,000PPI (15,000 pixels per inch).

In addition, the brightness of Micro-LED is significantly higher than other display technologies. It can reach several million nits, while the maximum brightness of OLED is only about 10,000 nits. This is mainly related to its material properties. Micro-LED uses inorganic materials, which have higher stability and durability. In contrast, the organic materials used in OLED are less stable. When a large current is injected, the organic materials are easy to decompose and the thermal effect is relatively strong, causing the materials to burn out.

The display screens that our lab can currently produce have a pixel density of more than 60 microns. No matter how close you put the screen, you can't see the individual pixels. This is comparable to the display effect of a mobile phone screen. LED displays were originally used mainly for engineering applications, but now they have broken through this limitation and can enter the consumer electronics market. However, there is still a long way to go in terms of industrialization, and the main challenge is that the cost is still very high.

The Paper: How did the change from LED lighting to LED display happen?

Huang Kai:It is precisely because of the development of the LED lighting industry that the future display industry will be extended and gradually extended to the high end of the value chain. Without China, LED may stop at lighting. The Chinese are particularly good at making something very cheap. From the end of the last century to the beginning of 2000, before localization, the price of an LED was 6 US dollars, and now the price of an LED is less than one cent.

The semiconductor industry has such a characteristic that as costs decrease, performance usually steadily improves. An important feature of the semiconductor industry is that as product costs decrease, the market scale will expand rapidly and gradually enter thousands of households. Scaling up can not only reduce costs, but also give rise to new applications. The original high production cost made LED only used for lighting at most, but now LED can be widely used in the display field.

For LEDs, the performance requirements for chips used in display screens are much higher than those for lighting purposes. For example, the display screen requires color uniformity, and it cannot be brighter on one side and redder on the other.

At present, the world's largest LED chip factory is Sanan Optoelectronics, followed by Jingyuan Optoelectronics and Qianzhao Optoelectronics. It is worth noting that two of these three companies are in Xiamen. Xiamen has a relatively strong advantage in this industry and has explored more application scenarios. In addition, as long as technical barriers are formed, these companies can have strong output capabilities. Unlike other industries, high-tech industries do not have obvious regional market restrictions. They face the global market. The key lies in how much of the global market each city can get.

The Paper: From the perspective of chip manufacturing technology, what are the main differences between chips in the display field and chips in integrated circuits?

Huang Kai:Optoelectronic chips use the vapor phase epitaxy process, while integrated circuit chips use the liquid phase epitaxy process. Chips in the display field are classified according to their functions. Chips for optoelectronic devices are called optoelectronic chips, power conversion devices are called power chips, and there are also radio frequency chips, etc. Integrated circuit chips require very high precision in the horizontal direction, and grooves need to be cut on a silicon wafer, and the width of the grooves reaches the nanometer level. LED chips are special in that they require very high precision in the vertical direction. LED chips are carved on a substrate by photolithography, usually using gallium arsenide or sapphire as the substrate material, and the material is grown by high temperature. The substrate is six inches or four inches, rather than by single crystal pulling. The material growth process needs to be constantly adjusted. There is usually a quantum well structure in the middle of the LED. The total number of layers may reach hundreds of layers, and the thickness of each layer needs to be precisely controlled to less than one nanometer. In extreme cases, even a molecular layer is required to be controlled to an accuracy of about 0.2 nanometers.

From the perspective of technical difficulty, we have mastered the technology of LED chips earlier. Compared with integrated circuit chips, each has different technical difficulties. Once an industry has mastered key technologies and is in a leading position, the entire industry chain will gradually balance to ensure that it remains at the forefront. If a problem is stuck and not solved for a long time, it will become more and more vulnerable to constraints and affect equipment, basic raw materials, processes and other aspects.

The size of LED chips is also shrinking. On the same epitaxial wafer, the number of Micro-LED chips is ten times that of Mini-LED, but the cost does not increase much. When the industry chain is truly mature, Micro-LED can actually reduce costs. The main cost comes from the transfer process. Micro-LED transfers the chip to the pixel through mass transfer, then encapsulates it, applies glue on it, covers it with a glass plate and a driver backplane, and then it can be powered on.

The Paper: What is the positioning of the Future Display Technology Research Institute in Xiamen’s entire optoelectronic display industry?

Huang Kai:When planning to establish the institute, the city of Xiamen gave us a very clear mission, which was to support the development of related industries in Xiamen and help expand the existing industrial chain. Xiamen City has a very high vision, so throughout the development process, we have formed a good interaction with companies such as Sanan Optoelectronics and Tianma Microelectronics. The institute is incorporated into the integrated construction of the Tan Kah Kee Laboratory in the form of a relatively independent platform.

We mainly develop a series of technologies, transfer technologies, and incubate and transform them when they are mature. We are the first new R&D institution for Micro-LED in China. Our communication and interaction with enterprises and joint R&D are relatively mature, and our partners are all the largest LED manufacturers in China.

The research institute is not a mass production unit, but many suppliers of auxiliary materials are cooperating with us because we have an experimental line. The new materials developed by various companies can be sent to us for verification. In this way, when selling, companies can quote the data of proofing here. This is also one of the important functions of our platform, which can promote mutual trust between upstream and downstream industries.

Xiamen continues to strengthen and supplement the supply chain in this field. Although the industry has now gathered to a certain scale, the chain between upstream and downstream is not yet complete, and the development and production capacity of related equipment and basic raw materials are still lacking. Manufacturing is already quite good and is considered a leader in China. We hope to play a further role and promote the deeper transformation of scientific and technological achievements, not only to serve the industry, but also to create more value in supplementing the supply chain.

Whether it is independent research and development, or assisting in the introduction of relevant teams or some upstream and downstream companies in the industrial chain, we have to do solid things. As for whether the basic raw materials are produced locally in Xiamen, this requires specific analysis of the specific situation. As a "garden city", whether Xiamen is suitable for the development of the chemical industry needs to be discussed with the local government. We will definitely carry out corresponding research and development, but whether it is suitable for landing in Xiamen after research and development needs further discussion.

The Paper: How does the institute encourage innovation?

Huang Kai:Our R&D team is composed of part-time teachers from Xiamen University and full-time engineers employed by the Institute. These engineers are very important to us and have made great contributions to our current achievements.

The research orientation of universities is often different from that of the industry, and paying too much attention to the evaluation indicators of universities may be detrimental to technology development and industrial services. For example, engineers need to concentrate on exploring a certain process, which is probably impossible to write a paper in a university.

The institute currently has a very good input-output ratio, and some key indicators of the chip are ahead of the international level. For example, the luminous efficiency of two-micron Micro-LEDs has reached 41% for blue light and 34% for green light, while the best level publicly reported internationally is about 12% for blue light and only 8% for green light. This is mainly due to Xiamen University's scientific research strength in this area and our institutional advantages, which enable engineers to concentrate on exploring processes, focusing on adjusting detailed parameters, and improving various indicators.

If profit is the main goal, the company's R&D progress may be a small-step fast approach, that is, if you are slightly ahead today, I will work harder tomorrow and invest just more research resources than my competitors. This approach is usually difficult to achieve breakthrough progress. The reason why we are able to significantly lead our international peers in some technologies is largely because our engineering team can focus on R&D.

We mainly encourage innovation through financial rewards and high respect for engineers. As a relatively pure scientific research unit, we are not afraid of failure and encourage innovation. Our researchers have relatively rich experience and strong basic scientific research capabilities. Many of the ideas they put forward have been tested in the industry. We can form a better research direction by slightly filtering and judging the feasibility of these ideas.