Breakthrough OLED and MicroLED Display technologies advancing to meet the need from commercial to military applications

Rajesh Uppal April 26, 2021 Military, Nanotech, Photonics Comments Off on Breakthrough OLED and MicroLED Display technologies advancing to meet the need from commercial to military applications 1,245 Views

Flat-panel display technology is exploding on several fronts as more screens are required for more devices. In total, the worldwide display market is expected to grow from $150 billion in 2019 to $228 billion by 2028, according to Touch Display Research. Today, the display market is dominated by two technologies—liquid crystal displays (LCDs) and organic-light emitting diodes (OLEDs).

An LCD consists of thin-film transistors (TFTs), which determine the resolution of the display. LCDs are cheap products built in giant fabs. Today, the largest LCD glass size being built is based on a Gen 10.5 technology. OLED is a next-generation display technology that is replacing LCD displays in several markets, such as small displays for mobile applications, TVs and microdisplays. Previously most of the screens were LCDs (Liquid Crystal Displays), though they were a huge step up from cathode-ray displays; they had some setbacks compared to OLED displays.

An organic light-emitting diode (OLED or organic LED), is a light-emitting diode (LED) made from thin films of organic light emitting materials, like carbon and hydrogen, that emit light when electricity is applied. OLEDs have a much simpler structure compared to LCDs and are more efficient than LCDs. OLED displays are comparatively thinner, have high contrast, better colors, and have faster refresh rates. Each pixel produces light individually, eliminating the need for a backlight and thus making it potentially thinner. This is one of the biggest reasons for its bendability.

This organic layer is situated between two electrodes; typically, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, portable systems such as smartphones, handheld game consoles and PDAs. A major area of research is the development of white OLED devices for use in solid-state lighting applications. OLEDs are gaining steam in smartphones. In 2018, 27% of all cell phones used OLED displays, but that figure is expected to reach 54% by 2023, according to DSCC.

Although LCDs and rapidly maturing OLED displays have been the high-end electronic display of choice, they are power hungry and do not offer high level of brightness; therefore, the manufacturers’ war for innovation supremacy continues. The next-generation, self-emissive display technology will be more energy efficient, while the smart lighting components will better control lighting, intensity, color, direction, shape by integrating pixelated, non-organic LEDs.

MicroLEDs represent an emerging flat-panel display technology that employs arrays of microscopic LEDs to comprise the individual pixel elements. Compared to conventional LCD displays, microLED displays can deliver superior contrast, faster response times and reduced energy consumption. A microdisplay using microLEDs might consist of 100,000 pixels at a size of 10mm x 10mm.

Gadgets are getting slim and sleek along with offering exceptional viewing experience with extraordinary contrast ratio. From commercial applications to military applications, the need for micro displays has arisen and technology has been advanced to cater to the need. MicroLEDs can be incorporated in two product types—microdisplays as well as mid- to large-sized displays. A mid- to large-size display includes smartphones, watches and TVs. Microdisplays include products that resemble the Google Glass, as well as other augmented reality (AR) glasses.

An exponential growth in wearable technology – from fitness trackers and smartwatches to Google Glass and Microsoft’s HoloLens – is highly likely to accelerate the mass market of GaN micro-LEDs. ‘Greater efficiency’ is a major advantage of GaN micro-LED – enabling reduced power consumption and making it a viable technology for battery-powered products. Newer consumer trends are deemed to shape the future of GaN micro-LED market. For instance, a widespread adoption of wearable display or near-eye display (NED) devices for virtual reality experience and emerging trends of ‘auto-infotainment’ bringing a rapid evolution in conventional automotive display units will signify ample sales opportunities to be exploited.

But microLEDs are difficult to make and expensive. That’s why mainstream commercial displays using microLEDs are not available today and won’t appear for several years. In R&D for several years, microLEDs are used to make displays for TVs, watches and head-mounted systems like the Google Glass. Numerous startups, display manufacturers and consumer brands now are developing their own microLED displays, devices and process in various sizes, ranging from medium-to-large sizes to ultra-small dimensions. Dozens of companies are working on micro-light emitting diodes (microLEDs), including Apple, Facebook, Samsung and TSMC . Additionally, TV vendors, display makers, startups and equipment vendors are also jumping on the bandwagon.

OLED display technology

The OLED is basically sandwiched between layers that transport electrons between an anode and a cathode. The anode is on the top surface and needs to be transparent as the light from OLED can pass through it. ITO (Indium Tin Oxide) is the most widely used anode. The OLEDs are built on a glass substrate with super-thin layers of the chemicals mentioned above deposited on it.

The substrate also incorporates a grid of thin-film transistors which make up the active matrix in Samsung’s AMOLED displays. And this whole thing is encapsulated in a thin layer of glass or metal because the compounds in an OLED breakdown in contact with air and are quickly degraded by water. As these require high temperatures like 1800 F, it starts damaging the glass layer when big screens are made. So, scientists found out new methods (Low-Temperature Poly Silicon) where this process can be done at 800F. And now, these temperatures are suitable to build the OLED on plastic. The Samsung Fold, the first foldable smartphone, is built on a plastic called polyimide, which is super heat resistant and can bear more than 2,85,000 folds. They use colorless polyimides for the front screen and orangish polyimides for the backplate. To be folded, the screen needs to be flexible, but ITO, the common anode used in OLEDs, is brittle and can break after a few folds.

The color of the light emitted by an LED depends upon the semiconductor and metal compounds inside it. In OLED screens, the semiconductor materials (like Si, Ge & Ga) are replaced by some special conductive organic compounds like Bis[2(diphenylphosphino)phenyl] ether oxide and Tris-(8-hydroxyquinolinato) aluminum. These are electroluminescent dyes.

PEDOT: PSS (Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)) is a super-thin transparent conducting material. However, it is hydrophilic (water-loving), it absorbs water when moisture is above 50%. It should be appropriately sealed to avoid damage. For this, Samsung SDI has developed optically clear adhesives for foldable smartphones, and these are responsible for holding everything tight.

Stanford’s materials scientists revealed working on a possible next-gen ultra-high-end OLED displays

Stanford’s materials scientists in Oct 2020, revealed working on a possible next-gen ultra-high-end OLED displays that repurposed solar panel technology. The Stanford University report noted that by expanding on existing designs for electrodes of ultra-thin solar panels, Stanford researchers and collaborators in Korea have developed a new architecture for OLED – organic light-emitting diode – displays that could enable televisions, smartphones and virtual or augmented reality devices with resolutions of up to 10,000 pixels per inch (PPI). (For comparison, the resolutions of new smartphones are around 400 to 500 PPI.)

Such high-pixel-density displays will be able to provide stunning images with true-to-life detail – something that will be even more important for headset displays designed to sit just centimeters from our faces.

Brongersma, who is a professor of materials science and engineering and senior author of the Oct. 22 Science paper detailing this research stated that “We’ve taken advantage of the fact that, on the nanoscale, light can flow around objects like water. The field of nanoscale photonics keeps bringing new surprises and now we’re starting to impact real technologies. Our designs worked really well for solar cells and now we have a chance to impact next generation displays.”

In lab tests, the researchers successfully produced miniature proof-of-concept pixels. Compared with color-filtered white-OLEDs (which are used in OLED televisions) these pixels had a higher color purity and a twofold increase in luminescence efficiency – a measure of how bright the screen is compared to how much energy it uses. They also allow for an ultrahigh pixel density of 10,000 pixels-per-inch. The next steps for integrating this work into a full-size display is being pursued by Samsung, and Brongersma eagerly awaits the results, hoping to be among the first people to see the meta-OLED display in action. For more on this, read the full Stanford News report.

Researchers at the National Research Nuclear University MEPhI have created a unique method to virtually design organic light-emitting diodes.

According to MEPhI scientists, the method to select emitting molecules, based on quantum-chemical calculations, will greatly accelerate and facilitate the creation of various third-generation OLEDs, transferring this to the field of computer modelling. “We’ve redefined the principles of emitter’s molecular design by analysing the molecule of 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN), the current champion in terms of efficiency. Having investigated the multi-directional processes characteristic of this molecule and other organic semiconductors, we have identified the key features of their structure that contribute to effective luminescence,” Alexandra Freidzon, assistant professor at NESPI MEPhI, Candidate of chemical sciences, explained.

Light emission in OLED occurs due to the phenomenon of recombination: the collision of carriers of opposite charges, electrons and so-called holes, which are molecules or atoms without an electron. The quantum efficiency of the first-generation OLEDs was under 25%, while with the third-generation OLEDs, which the scientists are currently working on, it’s already possible to use 100% of electron-hole pairs. This is possible due to the use of one of the intra-molecular processes, thermally activated delayed fluorescence (TADF).

“We’re the only ones to have managed to apply the quantum chemical method, which is highly precise in the position of the molecule’s energy levels. This is critically important in the TADF theory, since errors in the level positions qualitatively change the whole picture. Moreover, we’ve managed to bring together all the processes leading to and competing with TADF, as well as evaluate their speeds within a single model, without additional approximations,” Alexandra Freidzon said. According to MEPhI scientists, the computer screening of materials with clear selection criteria will help to drastically reduce the amount of experimental work and accelerate the discovery and launch of efficient new third-generation OLED emitters.

According to the authors of the study, the quantum chemical method they use requires complex calculations, but the data obtained will make it possible to calibrate and improve the quality of simpler and cheaper methods to study organic semiconductors. The study was carried out in cooperation with the Photochemistry Centre of the Russian Academy of Sciences as part of Russian Science Foundation grant No. 19-13-00383. The research team is currently developing AI elements that will help process data sets for more efficient screening.

MicroLEDs technology

MicroLEDs, as the name implies, are just really small LEDs. MicroLEDs are microscopic versions of an LED without a package, and a multitude of them need to be incorporated into a display. One microLED measures less than 100μm (less than 50μm are common), which can be 1/100 the size of a conventional LED. MicroLEDs are self-emissive and don’t require a backlight. In theory, a display using microLEDs provides more color and higher brightness with lower power than today’s displays.

Like LEDs, a microLED can be configured with only one color (i.e. red LED or red microLED). MicroLEDs also can be configured with the three main primary colors—red, green, blue (RGB). The concept behind using them in displays is that each individual LED would be sub-pixel for a pixel. So every pixel would have three LEDs: one that emits red, one that’s green, and one that’s blue.

One of the main driving forces for creating a display purely out of microLEDs is that they could be far more efficient than a traditional LCD display or a LED display. Similar to OLED, this is an emissive technology, where there’s no additional filtering and selection required as you’re emitting pure color directly from the sub-pixel. Nonetheless, microLEDs have higher resolutions and a greater luminance than OLEDs. MicroLEDs have a maximin pixel per inch (PPI) of 5,000 with 105 nits, compared to 3,500PPI and ≤2 x 103 nits for OLEDs, according to LG Display. TVs are a different story. “The main reason is image quality. With microLEDs, you can have better color than OLEDs or LCDs.

When it comes to your own devices, MicroLEDs make for very bright and colorful screens. The colors will be even better than the quantum dot TVs right now. You’re going to have a better experience due to the viewing angle and the fact that these LEDs emit in all directions.

Because it uses organic carbon- and hydrogen-based chemistry, OLED is sensitive to degradation by oxygen, moisture and heat. MicroLED, based on inorganic gallium nitride, doesn’t have this sensitivity, and so offers long lifetime, lower colour shift with age and no screen burn-in. Also, a modular approach allows the construction of displays of various sizes and shapes, including very large displays.

With unprecedented advancements in technologies, GaN micro-LED technology promises to fulfill these projections by combining the efficiency of Gallium nitride (GaN)-based LED direct emission and potential for manufacturing microscopic LED. One demo of a smartwatch-size display by Silicon Valley–based Glo shines at 4,000 nits (candelas per square meter) while consuming less than 1 watt. An equivalent LCD display would burn out in seconds trying to meet half that brightness.

So you get the high viewing angle, high efficiency, high color and high brightness that everyone in the display world wants. Then there’s always the potential that these things can be put on transparent substrates. So it could look like a piece of glass or plastic on your wall when it’s turned off, or it’s semi-transparent, and it’s not until you turn it on that you actually see it. This will be possible because these LEDs are going to be so small with space in between them that it could appear transparent if it’s put on glass, ITO or plastic substrates.

This also means it could be flexible if you put it on plastic. So this could be one of those technologies that start to enable flexible displays of some sort. To be honest, there are probably things that we can’t even predict what might happen with this because it’s so outside of what we’re used to.

Dual micro-OLED VR headset to take entertainment to next level:

The entertainment is getting to the whole new dimension with innovative VR glasses equipped with micro-OLED displays are coming into the market. Luci has unveiled its Luci Immers, a VR headset equipped with dual micro-OLED. The California-based firm is a subsidiary of AT Holding.

Minqin “Crystal” Tang, the Executive Vice President at Luci, stated that the design of headset is sleek and small. Through the combination of a head-mounted display and luxury design, the company aims to revolutionize the entertainment industry. It weighs nearly 140 grams, which is less than 40 percent of other headsets. The dual micro-OLED offers very clear optics along with an excellent contrast ratio. The 70-degree field of view enables viewers to watch the video equivalent to a 1,023-inch HD TV screen. Moreover, the pixel density of the virtual screen is 3,147 pixels per inch (PPI) and contrast ratio of 100,000:1.

The U.S. DOD requirements

In 2017, Kopin announced a new product line, the Brillian LCD microdisplay, which targeted military HMD applications. The display chosen for the Army HMD is a transmissive color filter array (CFA), SXGA (1280 x 1024) LCD microdisplay with a 0.97” (24.6mm) diagonal size and a pixel pitch of 5 μm x RGB x 15 μm. The brightness achieved will depend on the backlight used to illuminate the LCD and Kopin says the device is capable of more than 34,000 nits. This very high brightness is need for two reasons. First, see-through AR HMDs are typically very inefficient and need a very bright source to produce enough light for the image to be visible. Second, the images and graphics in an aviation AR HMD must be bright enough for the pilot to see them against, for example, a white cloud layer beneath him in direct sunlight.

President and CEO of Kopin, Dr. John C. C. Fan, said, “We are very pleased to have the Brillian LCD designed into a color HMD for the US Army helicopters. It has been very challenging to provide color HMDs for pilots because historically color microdisplays could not offer super high brightness with good contrast required for the application. Full-color Brillian displays exhibit brightness levels greater than 34,000 nits (10,000 foot-lamberts) and contrast ratios higher than 300:1. The Brillian display exhibits approximately double the contrast ratio and color transmission of our standard LC microdisplays.” When the next-generation of microLED microdisplays come along and are designed into HMDs, they are likely to have both higher brightness, higher contrast and longer life than even Kopin Brillian devices.

Researchers of the U.S. Air Force have been searching for companies that can build micro OLED displays for upgrading the military electronics. Owing to advantages provided by OLED displays, such as simple structure and less power consumption, over currently deployed LCD displays, the U.S. Air Force researchers have been looking for upgrade. For Air Force applications, an OLED display needs 640 by 480 pixels, with a pixel pitch, not more than 15-microns. In addition, these displays need to be resistant to shock and vibration and should operate within temperature range of -40 to 65 degrees Celsius. The contrast ratios must be more than 10,000:1. The color pixel format required is 2,048-by-2,048-pixel within 1.25-inch package capable of showing a minimum of 256 different gray levels. Researchers have been looking for companies that would provide manufacturing capabilities that would offer economically viable and military-grade micro-OLED displays.

Challenges of making microLEDs

The key challenge existing today for GaN micro-LED manufacturers is the placement of gallium nitride material onto a substrate.

MicroLEDs are difficult to implement. For example, to develop just one HDTV, the system requires 6 million individual microLEDs. So in a fab, 6 million microLEDs must be manufactured and then transferred onto a backplane in the TV without an error. Making microdisplays using microLEDs is also daunting.

You need these LEDs to be very, very small and you need to find a way to generate those three different colors. Almost all the LEDs in display products are blue so the industry knows very well how to make blue LEDs, but it’s far less common to make a red or a green LED.

There are a lot of challenges associated with it. Firstly it takes very complex electronics to control each individual sub-pixel independently, but then you need all the pixels to communicate, which is even more complex, but achievable.

But with this size LED and each individual pixel having three of them, you’re using literally millions of LEDs on a single 65″ TV. So there’s the sheer number of components and they all have to be placed perfectly.

These are very small length scales. They have to be located in groups of three, and they have to be in a high-yield place in those locations, and they all have to work well. Because if you have one or two pixels where one of the colors is defective, you’ll see a few pixels that are dead on the display. If you’ve ever looked at an older TV that has some damaged pixels, your eye immediately picks up on that.

“Unlike OLED, inorganic LEDs can’t be deposited and processed over very large areas. LEDs are grown on 4- to 8-inch wafers and the art of making microLED displays therefore consists in singulating individual emitters and transferring and assembling them onto a backplane substrate,” Virey said.

“For most consumer displays such as TV or smartphones, microLED with die size ranging from 3 to 10μm are required to ensure cost compatibility with the applications. For an 8K display, close to 100 million of those must be assembled without a single error with a 1 to 2μm placement accuracy at a throughput exceeding 100 million units per hour. Transfer and assembly are therefore often seen as the single largest technical challenges to overcome to enable microLED manufacturing.”

That’s not the only problem. “For example, while the external quantum efficiency (EQE) of traditional LED can reach 70% or more, the EQE of small microLED (size <5μm) was until fairly recently limited to 1% to 5%. At those levels, microLED can’t deliver on the key promise of better efficiency than OLED. Fortunately, dramatic progress has been reported by various groups over the last 2 years,” Virey said.

“Cost reduction is critical for commercialization of microLED applications,” said Steve Hiebert, senior director of marketing at KLA. “This cannot be achieved until the yield at each stage of microLED production is improved. The sources of the yield issues are incoming substrate, epi processes, microLED fabrication processes, and transfer processes, although there will be some uniqueness depending on whether the manufacturing method is a mass pick-and-place process or a monolithic fabrication process,”

The initial products are expected to be expensive amid a multitude of challenges with the technology. “MicroLED displays are still in the development phase and no consumer products are available yet,” said Eric Virey, an analyst at Yole Developpement, in a paper.

Manufacturing technologies

In the fab, meanwhile, there are some similarities and differences between making microLEDs and traditional LEDs. Mukund Raghunathan, product marketing manager at KLA said, “To achieve the size shrink of sub-100μm such as 20μm or lower, the fabs may need different process equipment and a much cleaner cleanroom environment. At 20μm, the ability to tolerate micron-level defects is much lower compared to a miniLED, as the microLED transfer and repair process is very costly.”

There are various ways to make a display using microLEDs. The process flow depends on the display type. In simple terms, the first step is to make an assortment of microLEDs on an epitaxial substrate. The devices are diced, tested and then transferred to a backplane using mass-transfer techniques.

The first step is to make the LEDs themselves. A traditional LED is made using a metal-organic chemical vapor deposition (MOCVD) system. In this system, thin layers of GaN materials are epitaxially deposited on a wafer.

MOCVD is also used for microLEDs. In one example of this process, the Hong Kong University of Science and Technology and others demonstrated a flow where an n-GaN layer is grown on top of a sapphire substrate, followed by a multiple quantum well (MQW) layer and a p-GaN layer. Another way is to deposit the layers on 200mm silicon substrates.

Each method is challenging. “Achieving wavelength uniformity and low defect density to reduce production costs are key factors to successfully apply MOCVD in microLED technology,” said Somit Joshi, vice president of marketing for the MOCVD division at Veeco. “The biggest challenge is generating high quality epi consistently across a large wafer population to meet the single bin requirements for wavelength and brightness uniformity. Since microLEDs require a very tight uniformity across a large transfer area, the requirements are much more stringent than LEDs that are individually packaged. Less than 10 dead pixels are allowed in a display to fulfill the general standard of the display industry. Thus, the yield of LED epitaxy must be very high to reduce the possibility of dead pixels.”

There are other challenges. “Sorting and binning are the methods to enhance wavelength uniformity for conventional LEDs,” Joshi said. “But microLEDs are too small to be sorted and binned. Therefore, the uniformity of LED epitaxy is even more critical. The requirement for epitaxy in conventional LEDs is around 8 to 10nm. In comparison, the general requirement of color uniformity for displays is to reach 1 to 2nm across the display depending on the type of display. It is impractical to achieve a wavelength uniformity of 1 to 2nm across the wafer. With the appropriate transfer technology, the uniformity requirement of microLED can be relaxed to 3 to 5nm across the wafer. This 3 to 5nm uniformity requirement can be met using advanced MOCVD tools.”

Once the layers are grown, the p-GaN and MQW layers are etched to insulate the pixels, according to the Hong Kong University of Science and Technology. Then, the last step is to build connecting pads.

From there, the goal is to take each microLED and transfer it to a TFT backplane or another surface. For this, there are various approaches, such as monolithic and pick-and-place.

The companies involved broadly fit into two categories. Some are making monolithic displays, where the gallium nitride pixels are made as a complete array on a chip and a separate silicon backplane controls those pixels. And others are using “pick and place” technology to transfer individual LEDs or multi-microLED pixels into place on a thin-film-transistor (TFT) backplane. The former is suited to microdisplays for applications like augmented reality and head-up displays. The latter is a better fit for larger displays

For tiny microdisplays, the industry uses the monolithic approach. “What we call monolithic is when we fabricate the display on one substrate,” Leti’s Templier said. “In monolithic, you start with an IC driver. It’s on a CMOS wafer. You grow the LED on top of it. Then, you pattern the LED. You fabricate at the wafer scale. You make several LEDs and then singulate them in the end.”

In monolithic approach, a pathway to a high-throughput, high-yield technology that bonds the backplane to the microLED array is key. The United Kingdom’s Plessey Semiconductors demonstrated a throughput-boosting technology recently, by bonding a wafer full of Jasper Display Corp.’s silicon CMOS backplanes to a wafer of its microLED arrays.

New York City’s Lumiode is founded on the idea that such bonding steps aren’t necessary. “When you have to bond two things together, yield is limited by how that bonding happens,” says Vincent Lee, the startup’s CEO. Instead Lumiode has been developing a process that allows it to build a TFT array on top of a premade GaN microLED array. That has involved developing low-temperature manufacturing processes gentle enough not to damage or degrade the microLEDs. Much of the work this year has been translating that process to a low-volume foundry for production, says Lee.

In contrast, the pick-and-place approach is used for mid- to large-sized displays. In one example, the microLEDs are fabricated using three different so-called epiwafers. The blue microLEDs are made on one epiwafer, while the red microLEDs are made on another epiwafer. Another wafer is for the green microLEDs.

Then, each microLED is diced and transferred onto a TFT backplane using a high-speed pick-and-place system. Some systems can pick-and-place 10,000 LEDs at once, which helps speeds up the process.

There are other approaches. For example, using what it calls microtube technology, Leti has fabricated a blue and a green prototype display with 40 x 40 pixels at 210μm pitch on a passive matrix.

“First, we process red, green and blue microLEDs with a 25μm side size on different epitaxial materials. MicroLEDs are integrated with N and P contact metallic pads on top, and singulated at a pitch of 210μm,” said Jeannet Bernard and others at Leti, in a paper. “In parallel, we fabricate an interconnection passive matrix with microtubes on the top side with the same pitch size. Then we transfer the microLEDs by flipping the substrate and hybridizing pads on microtubes. The transfer is finalized by removing the epitaxial substrate from the microLEDs. We repeat this transfer step for the 2 remaining colors and thus we obtain a RGB display with a pixel pitch of 210μm.”

Regardless of the approach, the industry faces some challenges. “Typical microLED displays combine conventional LEDs based on sapphire substrates with thin-film transistor (TFT) logic, or in some cases with CMOS,” said Martin Eibelhuber, deputy head of business development at EV Group. “The key challenge is to integrate the LED wafer with CMOS while providing high mechanical stability together with optimal electrical performance to achieve full functionality. To achieve this, a combination of very high precision alignment and high integration of the process flow is required to enable very small pixel dimensions.”

There are other challenges. “These approaches are not yet cost-effective for the consumer market,” KLA’s Raghunathan said. “For instance, while the mass pick-and-place approach enables selective replacement of a defective pixel, which includes dead or dim pixels, it comes at a cost of additional mass transfer steps to replace the defective pixel. Likewise, in a monolithic process, repair and replacement of a defective pixel increases the cost because it requires additional wafer-level processing–first to remove the defective pixels and then to substitute them with non-defective pixels.”

Mikro Mesa Technology unveils a breakthrough solution of mass transferring 3 um microLED to take a big step toward mass production of microLED displays

Mikro Mesa Technology announced a cutting edge technology to mass transfer 3 um uLED chips which is the smallest one in the industry. Beside the ability to handling tiny chips, this process also reduces bonding temperature to below 200 Celsius and eliminates the using of pressure during chip bonding. The low process temperature also benefits the making of flexible plastic display.The large transfer stamp size(close to 4”) which can tremendously reduce transfer counts and the cycle time. By using the pick up and place method, the mass transfer process is capable to bond different color uLED chips and make a full color display.

According to Mikro Mesa’s information, this transfer technology is able to handle chip size of 2um~5um and makes the display resolution up to 1,800 dpi. Due to the above advantages, the uLED display could be used in a lot of applications including wearable devices, mobile devices, television and augment reality.

Industry Demonstrations

A multitude of companies are trying to solve these and other problems, but the industry requires some new breakthroughs. Several companies already have demonstrated microLED TVs. In 2012, Sony demonstrated the Crystal LED display, a 55-inch prototype microLED TV. It incorporated 6 million microLEDs. Samsung has demonstrated “The Wall,” a 146-inch microLED TV , new 75-inch display as well as a 219-inch version of The Wall using microLEDs.

MicroLEDs are showing up in a variety of emerging display formats. Taiwanese firm AU Optronics recently introduced a prototype 9.4-inch flexible display using blue microLED pixels under red and green color-conversion filters on low-temperature polysilicon (LTPS) plastic substrate. Plessey Semiconductors announced it will help Facebook prototype and develop new technologies for potential usage in the augmented reality/virtual reality space.

Samsung Electronics in 2018 introduced a prototype TV with a chip-on-board (COB) processed RGB microLED display. The company plans to launch the product this year. Samsung Display has also started development of quantum dot nanorod LED (QNED) technology, which applies nano-tube LEDs onto an oxide TFT glass substrate. Furthermore, many consumer brands are expected to release their own microLED displays or devices in the near future.

Many others have also demonstrated the technology with some even racing to develop commercial products. “Samsung is trying to bring a microLED TV to the market this year. Hisense and TCL also demonstrated one as well,” said Jennifer Colegrove, chief executive of Touch Display Research, in a presentation.

At the recent Display Week conference, there were several papers on microLEDs. Among them were: AU Optronics devised a 12.1-inch 169-ppi full-color microLED display using LTPS-TFT backplane.

Kyocera has developed a small-size microLED display by LTPS high integration technology with the target more than 200-ppi, which is on the same level as achieved by LCD and OLED. Sharp reported on a novel microLED display bonded onto a silicon driver, which it calls “Silicon Display.” A 0.38-inch full-color display with a resolution of 1,053-ppi was demonstrated.

“The microLED market is poised for much more rapid growth once the technologies for manufacturing microLED chips—including mass transfer—gain more maturity,” Kang said. “The growing use of microLED display technology will push display makers to evolve away from current LCD and OLED display technologies.”

Micro-LED market

The display panel market is huge, and many leading industry players are working on optimizing the existing display technologies such as LED, LCD, OLED, quantum dot LED, and e-paper. Samsung is pursuing quantum dot LED and OLED, while LG Display is focusing on OLED technology. The market is dominated by Samsung, LG Display, AU Optronics, Innolux, and Japan Display. These players may enter into partnerships or collaborations, or even acquire one of the many startups that are focusing on developing micro-LED technology.

Micro-LED technology, when mature, is expected to penetrate different display applications. The huge display market offers numerous opportunities for the development of micro-LED technology, which is much brighter and low-power-consuming compared with the existing technologies, such as LCD and OLED.

Additionally, a drive of innovations in lighting industry and the emergence of smart lighting concept that not only lower the energy demands but also help shape urban spaces and smart cities are expected to prepare greater grounds for GaN micro-LED market. According to the research firm Allied Market Research, the global micro display market is expected to reach $3 billion by 2022. “A lot of companies consider that AR glasses will be your future cell phone,” Leti’s Templier said. “For the microdisplay, there is one main reason for microLEDs. We want higher brightness for AR. The requirement for the brightness is 100x more than existing technology. You can have microdisplay using OLED. They typically provide 1,000 nits. For AR, you need 100,000.”

The global micro-LED market size is estimated to grow from USD 409 million in 2020 to USD 18,835 million by 2026, at a CAGR of 89.3%. The most significant factor driving the growth of this market is the growing demand for brighter and more power-efficient display panels for smartwatches, mobile devices, and AR/VR devices.

The micro-LED market is witnessing various changes and technological innovations in chip manufacturing, chip transfer, and many other areas. Micro-LED -based products are not yet commercialized, and players are focusing primarily on prototype development and technology protection.

As 0f 2019, more than 1500 patents have been filled by 125 companies. Some players have filed patents, while others have acquired the intellectual property via license agreements, mergers, and acquisitions. Leading players filing patents for micro-LED technology are Apple (LuxVue) (US) and Ostendo Technologies (US). Many R&D labs/institutes, such as CEA-Leti (France), Hong Kong University of Science and Technology (HKUST, Hong Kong), and the University of Illinois (US), have also filed patents related to micro-LED technology.

More than 50% of micro-LED-related patents belong to mass transfer, interconnect, pixel & display architecture, and driver circuits. Around 15% of patents belong to chip structure and chip manufacturing. The remaining ones are related to epitaxy, color conversion, yield management, testing, etc.

The US is expected to be the major contributor to the micro-LED display market in North America because of its well-established economy and the presence of a prominent smartphone, AR/VR, television, and smart wearable suppliers in the country. The rapid adoption of the latest display technologies, along with the growing number of applications, is the main reason attributed to the expected prominent share of North America in the micro-LED display market. Many leading brands, such as HP, Dell, and Apple, are headquartered in the US. These factors are expected to contribute to North America playing a key role in the growth of the micro-LED display market.