What is micro led?
Micro LED technology, namely led miniaturization and matrix technology. It refers to the high-density and small-size LED array integrated on one chip. For example, each pixel of LED display screen can be addressed and driven to light independently. It can be regarded as a miniature version of outdoor LED display screen, reducing the pixel distance from millimeter level to micron level.
The micro LED display is that the bottom layer uses the normal CMOS integrated circuit manufacturing process to make the LED display driving circuit, and then uses the MOCVD machine to make the LED array on the integrated circuit, so as to realize the micro display, that is, the reduced version of the LED display.
Patent application for luxvue active matrix micro LED display
Micro LED has obvious advantages. It inherits the characteristics of inorganic led, such as high efficiency, high brightness, high reliability and fast reaction time. It also has the characteristics of self illumination without backlight, and has the advantages of energy saving, simple mechanism, small volume and thin.
In addition, micro LED has a major feature of ultra-high resolution. Because it is very tiny, the performance resolution is particularly high; It is said that if Apple’s iPhone 6S uses micro led, the resolution can easily reach more than 1500ppi, which is 3.75 times higher than the 400ppi displayed by the original retina
Compared with OLED, its color is easier to debug accurately, has longer luminous life and higher brightness, and has the advantages of better material stability, long service life and no image imprint. Therefore, it is another display technology with light weight and power saving advantages after OLED. Its commonality with OLED is that it also needs to be driven by TFT backplane. Therefore, the TFT technology level is igzo, LTPS and oxide.
1. Cost and disadvantages of large-scale application. It relies on monocrystalline silicon substrate as the driving circuit, and from the patent published by apple, it has the step of transferring led from sapphire substrate to silicon substrate, which means that making a screen requires at least two sets of substrates and independent processes. This will lead to an increase in cost, especially in large-area applications, which will face great challenges in yield and cost. (for monocrystalline silicon substrate, an inch or two is already a large area. Refer to the price of full frame and larger medium frame CMOS sensor products) of course, from a technical point of view, luxvue is feasible to convert the driving circuit substrate into quartz or glass to reduce the cost of large-area application, but it also takes time. Compared with AMOLED’s mature LTPS + OLED solution, it has no cost advantage.
2. The luminous efficiency advantage is threatened or even surpassed by PHOLED. The efficiency improvement of phosphorescent OLED (PHOLED) is obvious to all. The red and green PHOLED materials of UDC have also been commercially available on the panels of Samsung Galaxy S4 and subsequent models, and the panel power consumption has been leveled or slightly superior to that of TFT-LCD with high PPI. Once the life problem of blue PHOLED material is solved and commercial, inorganic LED will not be cheap in efficiency.
3. Brightness and life are threatened by qled. Qled research is very hot now. According to the data provided by QD vision, both efficiency and service life 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 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 difficult for luxvue to make curl and flexibility. If you want to manufacture products such as Iwatch, the screen is not aesthetic without a certain curvature.
Speaking of the development status of micro led, as Candice Brown Elliott, the current CEO of nouvoyance and the founder of p-arranged pixels in Samsung OLED panel, said, before Apple acquired luxvue, few people knew and engaged in this field, and now many people have begun to discuss this technology.
Two micro LED technology experts also said last year that it is difficult to apply this technology to produce various practical screen panels, and it is unlikely to see this screen technology in iPhone, iPad or IMAC products in the near future. However, for smaller displays, micro LED is still a feasible option for small screen applications such as apple watch.
Application of microled array of verlase in near eye display (NED)
In fact, since luxvue was received by apple, it has been seen that verlase announced to obtain a breakthrough color conversion technology patent, which can make the full-color microled array suitable for near eye displays. There has been no relevant report since then. Recently, ledinside got news from the recent Taiwan solid-state lighting seminar. Leti, Texas Tech University and playnitride all showed their micro LED R & D achievements at the seminar.
Leti has launched the LED matrix, with blue EQE of 9.5% and brightness of 107 CD / m2; The green EQE is 5.9%, and the brightness can reach 108 CD / m2. The full-color display is realized by using quantum dots. The pitch is only 10 um, and the future goal is 1 um. Leti’s short-range plan is to start from smart lighting, enter the HUD and HMD markets in the medium-term 2-3 years, catch up with the VR / AR heat, and the long-range goal is to start large-scale display applications within 10 years.
The pixeledtm display technology, also based on gallium nitride, released by play nitride in Taiwan, is currently transferred to the panel through transfer technology, and the transfer yield can reach 99%!
It can be seen that many enterprises have followed up micro LED technology, and the development speed is also accelerating. However, in terms of Apple itself, this technology belongs to the stage of Apple laboratory technology, and apple itself has also bet on many emerging industries, so it remains to be seen whether to introduce mass production in the future.
Bottleneck of development
In fact, the core technology of micro LED is the transport of nano led, not the technology of making LED itself. Due to lattice matching, led micro devices must be grown on sapphire substrates by molecular beam epitaxy. To make a display, we must transfer the LED light-emitting micro device to the glass substrate. Because the size of sapphire substrate for making LED micro devices is basically the size of silicon wafer, while the manufacturing of display is a much larger glass substrate, it must be transported for many times.
The technical difficulty of multiple transfer of micro devices is particularly high, and it is even more difficult to be used in the pursuit of high-precision display products. It can be seen from the list of patents obtained after Apple’s acquisition of luxvue that most of them complete the transfer process by electrical means, so this is the key core technology of luxvue
Li Yunli, executive director of Taiwan’s Chuang Chuang, also said recently: “there are two keys to the success of micro led: one is the willingness of brand factories such as apple and Samsung; the other is chip moving technology. There is a threshold to overcome when handling millions of ultra-small LED chips at one time.”
In fact, micro led also faces the third problem, namely, full-color, yield and luminous wavelength consistency. Monochrome micro LED arrays can be realized through flip chip packaging and driving IC bonding. However, RGB arrays need to transfer red, blue and green grains in several times, and hundreds of thousands of LED grains need to be embedded. They have higher requirements for the light efficiency, wavelength consistency and yield of LED grains. At the same time, the cost of sub bin is also a technical bottleneck hindering mass production.