MicroLED will trigger the next display revolution? The analysis of the advantages and technical difficulties and can realize the field

Despite the rapid development of solid-state lighting, but the display backlight is still a substantial market LED. In more than 10 years, the screen is displayed by these devices - the first of these devices are placed in the traditional package, recently more on the chip level package, and they are now back light LCD.

The LED package is one of the most successful case is used as the light source of large video billboards, such as stadiums, shopping malls etc.. According to the screen size and resolution, including red, green and blue chip LED discrete package to form a single pixel spacing is typically 1 mm to 40 mm.

As of today, LED has been used as a small space in the display screen directly - pixel light emitting element. This phenomenon is caused by many problems, including cost and manufacturing feasibility. However, the use of microLED and sub mm pixel pitch display ideas can be traced back to the beginnings of LED.

In the past five years, the development of display interest based on microLED increased, especially after the 2014 acquisition of Apple Corp Luxvue. Last October, Facebook acquisition of immersive virtual reality technology company Oculus; and in May of this year, SHARP acquired another microLED startup eLux, Google Micro LED and the recent injection of Swedish manufacturer Glo.

In view of these acquisitions, that microLED is not only stay in the laboratory. So, why are these big brands of this technology so interested? Because microLED can be independent of the red, green and blue sub pixels as a light source independently controllable, can be formed with high contrast, high speed and wide viewing angle display.

In fact, many of the OLED MicroLED display than the opponent better, because microLED has a wider color gamut, higher brightness, lower power consumption, longer service life, better durability and better environmental stability. In addition, such as Apple's recent patent documents show, microLED can be integrated sensor and circuit implementation with embedded display function of the sense of measurement, such as fingerprint recognition and gesture control.

Although microLED still did not enter the market, but they are not just stay in the paper on the idea. In January 2012 the "International CES" on SONY, featuring a 55 inch microLED display 1920 x 1080 pixels, contains 6 million 200 thousand sub pixels, each of which is a microLED chip that can be controlled independently, strong concern by the media. However, SONY also did not give a timetable for the commercialization, so far, not a microLED TV to enter the market.

MicroLED is a very complex technology

Today, microLED is not a generally accepted definition. However, in general, microLED is considered to be the LED chip total surface is less than 2500 mm2. This is equivalent to 50mm * 50mm square or 55mm diameter circular chip. According to this definition, microLED today has emerged on the market: SONY in 2016 appeared again, with small spacing large LED video wall in the form of traditional LED packages are replaced by microLED.

Manufacturing microLED display technology aspects involved: LED substrate processed into ready for picking up and transferred to the microLED array substrate, for integration into heterogeneous integrated system: display. Display and integrated LED, pixel driving transistor and optical device. The epitaxial wafer can accommodate hundreds of millions of microLED chip.

The MicroLED display has two main options. One is the microLED alone or packet picked up and transferred to the thin film transistor matrix, which is similar to the OLED display in use; another is to use the CMOS driver circuit will be hundreds of thousands of MicroLED complete monolithic array combination.

If the two methods in the first, then assembled a 4K display to pickup, placed and connected with 25 million separate microLED chip (assuming no redundant pixels) to the transistor backplane. With the traditional pick and place equipment handling such small devices, processing speed per hour is about 25000 units. This is too slow, a month time required to assemble a single display.

In order to solve this problem, like apple, X-Celeprint and other dozens of companies have developed large-scale parallel crawling technology. They can also be processed to tens of thousands of millions of microLED. However, when the microLED size is only 10 m, with sufficient precision processing and placed very challenging.

There are some related problems to overcome LED chip. When its size is very small, its performance will be with the surface and internal defects (such as damage, pollution and open type bonding structure) related to the influence of side wall effect. These defects lead to nonradiative carrier recombination accelerates. The side wall effect can be extended to similar to the carrier diffusion length (usually 1mm to 10mm): This is not important in traditional LED, because it has hundreds of microns edge, but it is very deadly in microLED. In these devices, it can limit the efficiency of the whole volume of the chip.

Because of these shortcomings, the peak efficiency of microLED is usually less than 10%, when the device size is less than 5mm, its peak efficiency may be less than 1%, which is far less than the best of traditional blue light emission of the 'macro' LED, which now produces more than 70% of the peak external quantum efficiency.

What is worse, microLED usually must be very low current density. They are usually below 1-10 A cm-2 peak efficiency of regional drive, because even in this low efficiency, LED is also very bright. If a microLED mobile phone to run the highest efficiency, the display will provide up to