Technical difficulties and application prospects of Micro-LED

What is Micro-LED?

Micro LED technology, namely LED miniaturization and matrix technology, refers to a high-density micro-sized LED array integrated on a chip. For example, each pixel of an LED display can be addressed and driven individually to the micron level.

The Micro LED display is a LED display driving circuit made by the normal CMOS integrated circuit manufacturing process at the bottom layer, and then the MOCVD machine is used to make an LED array on the integrated circuit, thereby achieving a miniature display screen.

(Read more: A Basic introduction of LED)

Patent applied for LuxVue active matrix Micro LED display (Image source: LuxVue)

Advantage

In theory, Micro-LED and OLED are basically the same. They both use self-luminous technology and display colors independently. They can also achieve 4K and above picture quality performance, even better than OLED. Most commonly, OLEDs have excellent energy consumption due to the physical characteristics of self-luminescence, and the same is true for Micro-LEDs. Micro-LED can be more energy-efficient and more efficient than OLEDs. It is said that its energy consumption is still 50% of the OLED screen; in terms of life, thanks to Micro-LED is an inorganic material and its structure is simple, so its life is more durable than OLED (organic).

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-luminous and no backlight, more energy-saving, simple structure, small size, thin and other advantages.

Furthermore, because Micro-LED is composed of tiny LED arrays, in addition to the advantages of low energy consumption and long life, it can also achieve high brightness, high resolution and ultra-high pixel count. Micro-LEDs have better efficiency, so the area of the light source can be small, and the interval can be pulled apart because the light source is small, which not only makes the performance of true contrast better, but also leaves great flexibility for PPI to go up. This can be seen in the 5000-ppi Micro-LED chip published by Ostendo in the United States. The pixel density is ten times that of today’s VR flagship. It is said that if the Apple iPhone 6S uses micro LED, the resolution can easily reach more than 1500ppi, which is 3.75 times higher than the original Retina 400PPi display. In addition, each pixel of the Micro-LED can be individually addressed and emit light separately. In addition to the high brightness unique to the LED itself, the Micro-LED can achieve ultra-high contrast and can present a very gorgeous picture. Micro-LED can also achieve seamless splicing and have good adaptability to any size and shape requirements.

Compared with OLED, its color is easier to accurately debug. It has longer luminous life, higher brightness, better material stability, long life and so on. Therefore, it is another thin, light and power-saving display technology after OLED. The commonality with OLED is that it also needs to be driven by a TFT backplane, so the TFT technology level is IGZO, LTPS, Oxide.

Processing process of quantum dot full-color light emission based on micro LED display (Image source: OSA Publishing)

Disadvantages

1. Costs and disadvantages of large-scale applications. Relying on a single crystal silicon substrate for the driving circuit, and from the previous patents issued by Apple. There are steps to transfer LEDs from a sapphire substrate to a silicon substrate, which means that at least two sets of substrates and Independent crafts. This will lead to an increase in cost, especially for large-scale applications, which will face huge challenges in yield and cost.

(For single crystal silicon substrates, one or two inches is already a very large area, refer to the price of full-frame and larger medium-format CMOS sensor products.) Of course, from a technical point of view, LuxVue converts the driving circuit substrate to quartz or Glass is feasible to reduce the cost of large-scale applications, but it also takes time. Compared with the mature LTPS + OLED scheme of AMOLED, there is no cost advantage.

Image source: LuxVue

2. The luminous efficiency advantage is threatened or even surpassed by PHOLED. Phosphorescent OLED (PHOLED) efficiency improvement is obvious to all. UDC’s red and green PHOLED materials have also begun commercial use on Samsung Galaxy S4 and subsequent models of panels, panel power consumption has been equal to the high PPI TFT-LCD. Once the life of the blue PHOLED material is resolved and commercialized, the efficiency of inorganic LEDs will not be cheap.

3. Brightness and lifetime are threatened by QLED. From the data provided by QD Vision, it is very promising in terms of both efficiency and life, and there are many large companies engaged in this research. Of course QLED is also a strong competitor to 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 flexible. If you want to make products like iWatch, the screen does not have a certain curvature, which is not aesthetically pleasing.

Development

Speaking of the development status of micro LED, just as Candice Brown-Elliott, the current CEO of Nouvoyance and the founder of P-array pixels in Samsung OLED panels, said that before Apple acquired LuxVue, few people knew and were engaged in this field, and now many people have Start discussing this technology.

And two Micro-LED technology experts also said last year that this technology level is still difficult to apply 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. But for smaller displays, Micro-LED is still a viable option for applications such as Apple Watch with small screens.

VerLASE’s MicroLED array on near-eye displays (NED) (Image source: VerLASE)

In fact, since LuxVue was received by Apple, I have seen VerLASE announce the acquisition of a breakthrough color conversion technology patent. This technology can make full-color MicroLED arrays suitable for near-eye displays, and there have been no related reports since. Recently, LEDinside got the news from the recent Taiwan Solid State Lighting Symposium. Leti, Texas Tech University and PlayNitride all presented their micro LED research and development results at the seminar.

Leti launched the iLED matrix, with a blue EQE of 9.5% and a brightness of 107 Cd / m2; a green EQE of 5.9% and a brightness of 108 Cd / m2. It uses quantum dots to achieve full-color display. Pitch is only 10 um to 1 um. Leti plans to cut in from smart lighting in the near future, and enter the HUD and HMD markets in the middle to 2-3 years to grab VR / AR fever. The long-term goal is to cut into large-scale display applications within 10 years.

Taiwan Play Nitride announced the same GaN-based PixeLEDTM display technology, the company is currently transferring to the panel through transfer technology, the transfer yield can reach 99%!

It can be seen that Micro LED technology has been followed by many companies, and the development speed is also accelerating. But as far as Apple is concerned, this technology belongs to the Apple Lab stage technology, and Apple itself has bet on many emerging industries, so it remains to be seen whether it will be mass-produced in the future.

Bottleneck of development

In fact, the core technology of Micro LED is the transfer of nano-level LEDs, not the technology itself of making LEDs. Due to lattice matching, LED micro-devices must first be grown by molecular beam epitaxy on a sapphire substrate. To make a display, LED light-emitting micro-devices must be transferred to a glass substrate. Since the size of the sapphire substrate used to make the LED micro-device is basically the size of the silicon wafer, and the display used to make the display is a much larger glass substrate, it will inevitably require multiple transfers.

It is particularly difficult for the multiple transfer technology of micro-devices, and it is even more difficult to use it for products that pursue high-precision displays. According to the list of patents obtained after Apple’s acquisition of Luxvue, it can be seen that most of them use electricity to complete the transfer process, so this is the key core technology of Luxvue.

Taiwan’s CEO Li Yunli also recently said: “The key to the success of Micro LED is two: one is the willingness of Apple and Samsung brands; the other is the chip moving technology, which can handle millions of ultra-small LED chips at a time. get over.”

In fact, Micro LED also faces a third problem, namely full color, yield, and consistency of light emission wavelength. Single-color Micro LED arrays can be realized by flip-chip packaging and driver IC bonding, but RGB arrays need to transfer red, blue, and green three-color die in stages, and hundreds of thousands of LED die need to be embedded. Particle light efficiency, wavelength consistency, and yield requirements are higher, and the cost of binning is also a technical bottleneck that hinders mass production.

Matt Williams is the founder and CEO of Add USA Story - A platform for global startup, looking to find happiness in the world of startups, investments, health, technology, business and more.

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