Recently, because Apple will use AM OLED screen, the OLED concept is a fire. The next-generation Micro-LED, which is considered to be an OLED display, seems to be no longer low-key, and has won glory at the International Symposium on Solid State Lighting in Taiwan. Compared to OLED, Micro-LED technology is still relatively new to many people, but the focus is getting higher and higher. Today, Xiaobian explained the Micro-LED concept, advantages and disadvantages, current status and technical bottlenecks. What is wrong? Welcome to discuss the message.
What is Micro-LED?
Micro LED technology, namely LED miniaturization and matrixing technology. Refers to a high-density and small-sized LED array integrated on one chip. For example, each pixel of the LED display can be addressed and driven separately. It can be regarded as a miniature version of the outdoor LED display, and the pixel distance is from millimeter. The grade is reduced to the micron level.
The Micro LED display, on the other hand, uses the normal CMOS integrated circuit manufacturing process to make the LED display driver circuit, and then uses the MOCVD machine to fabricate the LED array on the integrated circuit, thereby realizing the micro display, that is, the LED display. A reduced version of the screen.
Patent application for LuxVue Active Matrix Micro LED display (Source: LuxVue)
Prominent advantage
The advantages of Micro LED are obvious. It inherits the characteristics of high efficiency, high brightness, high reliability and fast response time of inorganic LED, and has the characteristics of self-illumination without backlight. It is more energy-saving, simple in mechanism and small in size. Thin and other advantages.
In addition, a major feature of the Micro LED is its high resolution. Because of the ultra-small performance, the resolution of the performance is particularly high; it is said that if the Apple iPhone 6S uses micro LED, the resolution can easily reach 1500ppi or more, which is 3.75 times higher than the original Retina display of 400PPi.
Compared with OLED, the color is easier to accurately debug, has a longer luminous life and higher brightness, and has better material stability, long life, and no image imprinting. Therefore, another display technology with lightness and power saving advantages after OLED is common to OLEDs in that it needs to be driven by a TFT backplane, so the TFT technology grades are IGZO, LTPS, and Oxide.
Process flow of quantum dot full-color luminescence based on micro LED display (Source: OSA Publishing)
Disadvantages
1. Cost and disadvantages of large-area applications. Relying on a single crystal silicon substrate as the driving circuit, and from the previous patent published by Apple, there is a step of transferring the LED from the sapphire substrate to the silicon substrate, which means that at least two sets of substrates are required to make a screen. Independent process. This leads to an increase in costs, especially in large-area applications, where there are significant challenges to yield and cost. (For a single crystal silicon substrate, one or two inches is already a large area, with reference to the price of full-frame and larger medium-format CMOS sensor products.) Of course, LuxVue converts the drive circuit substrate to quartz or It is feasible to reduce the cost of large-area applications, but it also takes time. Compared to AMOLED's mature LTPS+OLED solution, there is no cost advantage.
Image source: LuxVue
2, the luminous efficiency advantage is threatened or even overtaken by PHOLED. The efficiency of Phosphorescent OLED (PHOLED) is obvious. UDC's red and green PHOLED materials have also been commercialized on the Samsung Galaxy S4 and subsequent models. The panel power consumption has been level with the high PPI TFT-LCD. Or a slight advantage. Once the lifetime of blue-light PHOLED materials is solved and commercialized, inorganic LEDs will not be as efficient in terms of efficiency.
3. Brightness and longevity are threatened by QLED. QLED research is very hot now. From the data provided by QD Vision, both efficiency and longevity are very promising, and there are many large companies engaged in this research. Of course, QLED is also a strong competitor of OLED.
4. It is difficult to make a curl and flexible display. The flexible display of OLED and QLED has a good prospect, and there have been many Prototype displays, but it is more difficult for LuxVue to make curl and flexibility. If you want to make a product like iWatch, the screen does not have a certain curvature is less aesthetic.
status quo
Speaking of the development status of micro LED, as the current CEO of Nouvoyance is also the founder of P-array in Samsung OLED panel, Candice Brown-Elliott said that before Apple acquired LuxVue, only a few people knew and engaged in the field, and now there are many people. Start discussing this technology.
Two Micro-LED technology experts also said last year that the technology level is still difficult to apply and produce a variety of practical screen panels, and it is unlikely to see this screen technology in iPhone, iPad or iMac products in the near future. But for smaller displays, Micro-LEDs are still a viable option, such as Apple Watch and other small screen applications.
VerLASE's MicroLED array for near-eye display (NED) (Source: VerLASE)
In fact, since LuxVue was acquired by Apple, VerLASE has announced that it has obtained a breakthrough patent for color conversion technology. This technology enables full-color MicroLED arrays to be used in near-eye displays, and has not been reported since. Recently, LEDinside received news from the recent solid-state lighting seminar in Taiwan. Leti, Texas Tech University and PlayNitride presented their micro LED developments at the seminar.
Leti introduced the iLED matrix, which has a Blu-ray EQE of 9.5%, a brightness of 107 Cd/m2, a green EQE of 5.9%, a brightness of 108 Cd/m2, and a full-color display using quantum dots. The Pitch has only 10 um, and the future target is to do To 1 um. Leti's short-range plan cuts in from smart lighting, enters HUD and HMD market in 2-3 years, and grabs VR/AR heat. The remote target is to cut into large-size display applications within 10 years.
The PixeLEDTM display technology, also based on GaN, announced by Taiwan's Play Nitride, is currently transferred to the panel through transfer technology, with a transfer yield of 99%!
It can be seen that many companies in Micro LED technology are following up and the speed of development is also accelerating. However, as far as Apple itself is concerned, this technology belongs to Apple's laboratory technology, and Apple itself has bet on many emerging industries, so it is still to be observed whether mass production will be introduced in the future.
Bottleneck of development
In fact, the core technology of Micro LED is the transfer of nano-scale LEDs, rather than the technology of making LEDs. Due to lattice matching, LED micro devices must first be grown by molecular beam epitaxy on sapphire substrates. To make a display, it is necessary to transfer the LED light-emitting device to the glass substrate. Since the size of the sapphire substrate on which the LED micro device is fabricated is basically the size of the silicon wafer, and the display is a glass substrate having a much larger size, it is necessary to carry out multiple transports.
The technical difficulty of multiple transfer of micro devices is particularly high, and it is more difficult to use in products that pursue high-precision displays. It is also seen from the list of patents published after Apple's acquisition of Luxvue that most of them use the electrical method to complete the transshipment process, so this is the key core technology of Luxvue.
Li Yunli, CEO of Taiwan's Minchuang, also said recently: "The key to success of Micro LED is two: one is the willingness of Apple and Samsung brands; the other is wafer handling technology, which carries millions of ultra-small LED chips at a time. get over."
In fact, Micro LED also faces the third problem, namely, full color, yield, and wavelength consistency. The monochromatic Micro LED array can be realized by flip-chip package and driver IC bonding, but the RGB array needs to be divided into red, blue and green crystals, and hundreds of thousands of LED dies need to be embedded. Granular light efficiency, wavelength consistency, and higher yield requirements, while the cost of binning is also a technical bottleneck that hinders mass production.
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