Despite the rapid development of solid-state lighting, display backlighting is still a substantial market for leds. For more than a decade, screens have been displayed by these devices initially placed in traditional packages, and more recently in chip-level packages, and they are now the backlight for LCDS.

One of the most successful cases of LED packaging is as a light source in large video billboards, such as in stadiums, shopping malls, etc. Depending on the size and resolution of the display, discrete packaged leds containing red, green, and blue chips form a single pixel, typically with a pitch of 1 mm to 40 mm.

Until now, leds have not been used as direct light-emitting elements in small-pitch displays, that is, pixels. This phenomenon is caused by a number of issues, including cost and manufacturing feasibility. However, the idea of producing displays using microleds and sub-millimeter pixel pitch dates back to the beginnings of leds.

Interest in developing MicroLED-based displays has surged over the past five years, especially after Apple's acquisition of Luxvue in 2014. In October, Facebook acquired Oculus, an immersive virtual reality technology company; In May, Sharp acquired another MicroLED startup, eLux, and more recently, Google invested in Swedish MicroLED manufacturer Glo.

Given these acquisitions, microLED is proving to be more than just a lab. So why are these big brands so interested in this technology? Because microleds can use independent red, green, and blue sub-pixels as independent, controllable light sources, they can form displays with high contrast, high speed, and wide viewing angles.

In fact, MicroLED displays are much stronger than their OLED counterparts, because MicroLED has a wider color gamut, brings higher brightness, lower power consumption, longer service life, greater durability and better environmental stability. In addition, as Apple's recent patent filing shows, Microleds can integrate sensors and circuits to enable thin displays with embedded sensing functions, such as fingerprint recognition and gesture control.

Although microleds are still not on the market, they are not just ideas on paper. At the "International CES" in January 2012, SONY exhibited a 55-inch MicroLED display of 1920×1080 pixels, containing 6.2 million sub-pixels, each of which is a MicroLED chip that can be independently controlled, and received strong media attention. However, SONY has not given a timetable for commercialization, and so far, no microLED TV has entered the market. MicroLED is an inherently complex technology.

Today, there is no universally accepted definition of MicroLED. However, in general, MicroLED is considered to be an LED chip with a total surface of less than 2500 mm2. This is equivalent to a 50mm×50mm square, or a round chip with a diameter of 55mm. According to this definition, microleds are already on the market today: SONY made another appearance in 2016, in the form of small-pitch large LED video walls, and traditional LED packages were replaced by microleds.

The technology for manufacturing MicroLED displays involves all aspects: machining LED substrates into arrays of microleds ready for picking up and transferring to receiving substrates for integration into non-uniformly integrated systems: displays. The display is also integrated with LED, pixel drive transistor, optical devices, etc. The epitaxial chip can hold hundreds of millions of MicroLED chips.

There are two main options for implementing a MicroLED display. One is to pick up and transfer microleds individually or in groups to a thin-film transistor drive matrix, similar to those used in OLED displays; The other is to combine a complete monolithic array of hundreds of thousands of microleds using CMOS driver circuits.

If you take the first of these two approaches, assembling a 4K display requires picking up, placing, and individually connecting 25 million MicroLED chips (assuming no pixel redundancy) to the transistor backplane. Manipulating such a small device with conventional pickup and drop equipment can process at a rate of about 25,000 units per hour. That's too slow, and it will take a month to assemble a single display.

To solve this problem, dozens of companies like Apple and X-Celeprint have developed large-scale parallel grasping technologies. They can process tens of thousands to millions of microleds simultaneously. However, when the MicroLED size is only 10μm, machining and placing with sufficient precision is very challenging.

There are also some problems related to LED chips to overcome. When its size is very small, its performance is affected by side-wall effects associated with surface and internal defects, such as open bonding, contamination, and structural damage. These defects lead to accelerated recombination of non-radiative carriers. The sidewall effect can extend to a distance similar to the carrier diffusion length (usually 1mm to 10mm) : this is not important in traditional leds because of its edge of hundreds of microns, but it is very deadly in microleds. In these devices, it can limit the efficiency of the entire volume of the chip.

Because of these flaws, microleds typically have a peak efficiency of less than 10%, and when the device size is less than 5mm, it can have a peak efficiency of less than 1%, which is far lower than the current best conventional blue light-emitting "macro" leds, which can now produce an external quantum peak efficiency of more than 70%.

To make matters worse, microleds typically have to operate at very low current densities. They are usually driven in areas below 1-10 A cm-2 peak efficiency, as leds are very bright even at this low efficiency. If a phone with MicroLED is operating at its maximum efficiency, its display will provide brightness up to tens of thousands of nits, a level higher than brighter phones on the market today. The screen will be so bright that even intrepid users won't dare look at it.

When leds operate at very low current densities, they are very inefficient, making the technology unable to live up to its promise of cutting energy consumption. Therefore, solving this problem became a priority for MicroLED. Ways to improve efficiency include introducing new chip designs and improving manufacturing techniques. Both methods reduce sidewall defects and keep the carrier away from the edge of the chip.

Developers of MicroLEDs also face challenges related to color conversion, light extraction and beam forming.

Another requirement of modern displays is the elimination of bad spots or defective pixels. It is not possible to achieve a 100% combined return on epitaxy, chip manufacturing and transfer, so MicroLED display manufacturers must develop effective defect management strategies, which can include pixel redundancy and individual pixel repair, depending on the characteristics and cost of the display.

At present, MicroLED is the easiest field to achieve

Microleds can be deployed in any display application from the smallest to the largest. In many cases, they will be better than the final combination of LCD and OLED displays. However, production feasibility and economic costs limit its use. However, detailed analysis shows that smartwatches and other wearable products, such as micro displays for AR/MR Applications, are best able to show the performance of MicroLED displays.

Among them, the implementation of MicroLED on smartwatches is the most likely, because smartwatches have a relatively small number of pixels and a medium range of pixel density, so the chip and assembly cost efficiency is high, and it is also the closest to the current state of MicroLED technology development. They have potentially differentiating features, including the ability to extend battery life, reduce power consumption, and higher brightness to provide good readability in outdoor environments.

If these displays start to appear in large numbers, various sensors could be introduced within the front plane of the display, which could read fingerprints and provide gesture recognition, for example.

Another major opportunity for microleds is head-mounted displays for augmented reality (AR) and mixed reality (MR). In virtual reality, users wear a completely enclosed head-mounted display to isolate them from outside vision; AR and MR Applications overlay computer-generated images onto the real world.

MicroLED displays are made by cutting wafers into tiny devices and transferring them to the transistor backboard in a parallel pick-and-place technique

One of the requirements for these applications is that the overlay image be bright enough to compete with ambient light, especially in outdoor applications.

To meet these conditions, the display must be placed in an unobtrusive position, using composite projection or waveguide optics with an optical efficiency of less than 10% to project the image onto the eye. These requirements determine the display's brightness range from 10,000 to 50,000 Nits, which is 10 to 50 times brighter than the best phones on the market.

Today, microleds are the only candidates that have the potential to provide these levels of brightness while maintaining reasonable power consumption and compactness. Encouragingly, the same reasoning can be applied to head-up displays in cars and other environments, which can be considered a form of AR.

The market MicroLED is trying to make an impact in is smartphones. Currently, OLED displays already offer very good performance at a very competitive cost. If microleds are involved, the size of the sub-pixels must be reduced to a few micrometers, which makes it harder to provide acceptable efficiency.

Success on television is even more likely. In this case, the downside is that the pixel density is relatively low, with a pitch of about 100 mm in a 4K, 55-inch TV. Low density hinders the efficiency of transferring technology because thousands of chips need to be moved per cycle, compared to hundreds of thousands for smartphones or smartwatches. To thrive in this market, alternative high-efficiency assembly technologies need to be developed.