MicroLED Display: Road to Commercialization

(Reprinted from Display Daily)

MicroLED display technology is making steady progress towards commercialization. The technology still faces several complex  manufacturing challenges, which will take time to resolve. Display Week 2021, gave a glimpse of microLED technology developments by various companies to resolve the manufacturing issues.
MicroLED has the potential to outperform both LCD and OLED technologies in terms of color reproducibility, brightness, reliability and low power consumption. To compete with LCD (Quantum Dot & miniLED) and OLED in the consumer market, microLED products need to have zero defects, high performance, high uniformity, high volume scalability and cost competitive manufacturing. The success of commercialization and mass production will depend on the ability to scale up for volume production with competitive price performance.

Increasing Investments
According to Yole’s presentation at DisplayWeek 2021,
“more than 8900 MicroLED display patents have been already filed by 480 organizations. More than $5.1 Billion has already been spent on MicroLED. Start-ups raised more than $250 million in the last 18 months. Display makers dominate activity followed by start-ups. Chinese companies have been dominating activity (45%) and Korea came back strongly in 2020 (22%) driven by Samsung and LG. Samsung was most active in 2020, followed by CSOT, BOE, Visionox, Playnitride, Konka and Facebook. More than 50 prototypes of products have already been produced by more than 30 companies. Samsung has introduced a 4K 110-inch MicroLED TV (2021) but it costs $155K. Vuzix promised to ship the first AR device based on JBD (Jade Bird Display) MicroLED later this year”.

Complex Manufacturing
MicroLED is a self-emitting display consisting of arrays of microscopic LEDs each forming a pixel. It has a simple structure but a complex manufacturing process. The microLED supply chain consists of the following steps.

• GaN epiwafer LED creation
• Processing of thin film LEDs
• Mass transfer of the chips to the display backplane
• Inspection and repair processes

Display integration of MicroLED can be heterogeneous or monolithic integration. In the heterogeneous type, makers use pick and place process to make microLED arrays, and chips are transferred from an epiwafer or carriers to the display backplane (LTPS or Oxide TFT on a glass or flexible PI substrate). The monolithic integration process involves microLED array and backplane hybridization.

Chip Manufacturing
Companies are developing solutions to reduce costs and remove roadblocks for MicroLED commercialization. Saphlux has developed NPQD technology that integrates an RGB pixel within one chip. This process can reduce total system costs. According to DSCC's microLED presentation, at Display Week 2021,
“Triad chip (RGB sub-pixels built into a single chip)(offered by Seoul Viosys),

• Pros: (a) reduced cost of mass transfer (b) larger chips: easier to handle during transfer/ repair.
• Cons: (a) with native colors: increase complexity and cost of the LED manufacturing (b) with color convertors: QD material costs.

Smartpixel (microLED bonded on Microdriver IC)

• Pros: removes the need for TFT backplane
• Cons: cost of CMOS wafer and hybridization”.

Color Conversion: The mass transfer process that requires bonding RGB (Red Green Blue) MicroLED to the display backplane accurately and efficiently is very challenging. Also different colors of LED need slightly different voltage and drive currents. Color conversion by Quantum Dots (QDs) (QDCC) or by phosphor provides another alternative. Using single-color (blue) MicroLED chips and color converting them with QD layers can help in the manufacturing process.
Nanosys’s presentation at DisplayWeek 2021 showed
“a breakthrough of new heavy metal free green QDCC emitter that delivers product ready brightness, color accuracy and reliability. The microLED size needs to be below 7 micron to be cost competitive in future displays."
According to the company its color-converted red provides 3.5x better performance at an economical cost, and is crucial for emerging display applications. AR requires ultrahigh efficiency and brightness. QDCC microLEDs at 1.8 microns can deliver the resolution and energy efficiency required for the future of AR.
Nanosys has acquired the microLED display company “glō” recently to accelerate MicroLED development & adoption.

Inspection, Repair & Yield
Manufacturing and yield problems can become issue in epitaxy, chip, mass transfer, bonding, driving technology, backplane, inspection and repair. The integration of in-process test, redundancy and repair are used to improve yield. Also as display resolutions gets higher, pixels are getting smaller: the trend is moving from 50 micron to 5 micron to 1 micron. This requires development of inspection tools that can achieve sub-micron resolutions. According to an InZiv presentation, it has built high-resolution inspection tools that can inspect any size microLED and analyze defects to improve processes and accelerate time to market. InZiv makes high-resolution tools for OLED and QLED also.
MicroLED companies also need to use mass testing and repair methods to reduce the cost and time of assembly. Companies such as Toray are working on equipment for microLED wafer inspection, repair and transfer. Laser repair equipment can remove defective microLEDs. As a presentation by DSCC on microLED session explained
“redundancy is an alternative way to reduce number of defective pixels. However, redundancy is expensive as it greatly increases LED consumption and the number of transfers. If the speed of the repair process is high enough, redundancy should not be required.“

Uniformity: microLED chips need wavelength uniformity and thickness uniformity for display applications. Solutions from MOCVD suppliers such as Aixtron, Veeco and others are helping to resolve issues caused be poorer uniformity. According to an Instrument Systems (Konica Minolta Group) presentation, the uniformity analysis of displays requires accurate color and luminance measurements. A combination of a highly accurate spectroradiometer and a fast RGB camera can provide a fast and accurate solution.

Mass transfer & Bonding
Mass transfer is critical for cost competitiveness with TFT-LCD and OLED.

• X Display Company (XDC) presented that mass transfer throughput and yield can be improved by using elastomer stamps. The company showed that for a 5.1" display, by using their PixelEngine and microICs, they are able to achieve functional sub-pixel yield of: red100%, green 99.996% and blue 99.998%. By using PixelEngine devices (microIC, RGB, All-In-One) they are able to increase mass transfer throughput. The company said elastomer stamp mass transfer is scalable using a conformable stamp with tunable short-range adhesion forces that achieved 99.99% transfer yields in R&D.
• Recent news from Electronics and Telecommunications Research Institute (ETRI) Korea showed that a new material, SITRAB adhesive, can be used to combine the transfer and bonding process into one process (applied homogenized laser to the SITRAB film attached to the MicroLED for a few seconds to develop the core process of simultaneous transfer and bonding). ETRI said this technology can reduce the high investment costs for transfer and bonding equipment and can also help to perform repair of defective pixels.
• Coherent recently announced its new UVtransfer integrated laser system that performs three processes in MicroLED fabrication: Laser Lift-Off (LLO), laser-Induced Forward Transfer (LIFT), and repair/trimming.
• Vuereal presented its Micro-Solid printing technology that can overcome the limitation to throughput and yield challenges.
• A collaboration of PlayNitride, Unimicron Corp (PCB backplane), and Macroblock (driver ICs) have shown the use of mass transfer of MicroLED onto a printed circuit board backplane, which can reduce production costs.
• Compound Photonics has developed its IntelliPix end to end microdisplay platform for AR/MR. The company presented that it is well positioned to extend IntelliPix’s benefits and pixel array design expertise to direct view displays.

Supply Chain Development
There are many different options and processes to resolve microLED manufacturing issues for commercialization. A supply chain ecosystem is proving difficult to develop for mass production because of the many underlying issues.

• There is no standard manufacturing solution for commercialization. Every firm has its own idea.
• Start-up companies are mostly fabless with licensing capabilities, and they need partnerships for manufacturing. Large established companies are reluctant to depend on a single start-up for their supply.
• Suppliers with LTPS-TFT capacity for backplane or other manufacturing capabilities need to invest capital for transfer, inspection/testing, and create a path to mass market.
• Panel suppliers and set suppliers need to be confident about profit margins for on-going business to allow them to invest capital.

More partnership and collaborations among panel makers, set makers and MicroLED chipmakers are required. Major display suppliers - Samsung, LGD, AUO, Innolux, BOE, Tianma, CEC Panda, Visionox and others are all increasing their activities in microLED display, which will help to establish supply chains for volume production.

Application Market
There are already many microLED demonstrations in different applications, such as large-size TV, automotive transparent display, flexible display, wearable devices and as AR/HUD display source.

• PlayNitride: has developed a selective mass addressable repair technology for lower cost. The company showed a 1.39" circular display for wearables and a 7.56" transparent display for photoframes as well as an 89" 5K ultra wide curved displays at DisplayWeek 2021.
• Royole: A microLED with ultra low aperture ratio and simple or non-encapsulation requirements, can lead to high stretchability, fine pitch and high transmittance. Using 90x 150 micron LEDs, a 2.7" 42 PPI RGB stretchable microLED panel was also demonstrated.
• Vuereal: its Micro-Solid Printing Technology has allowed the development of custom displays for niche markets such as high-brightness, high-transparency (>80%) display, wearable, and automotive. Samples will be available in Q4 2021.
• Jade Bird Display: announced the mass product of 0.13" monochrome microLED microdisplay panels and optical engines and assemblies. These products enable ultra-compact AR glass designs.
• AUO: received the best technology demonstration award at DisplayWeek for its integrated vehicle cockpit with microLED display after showing a 12" auto display, a 10.4" transparent display and a 1.4" round MicroLED display panel.

Companies are initially targeting digital signage, public display, AR/VR, wearable, and automotive (HUD) applications.

In Conclusion
High-speed transfer, assembly technologies, yield and defect management need to improve and supply chains need to be established before large volume commercialization in consumer products can be done. In the meantime, suppliers will focus on markets with higher priced, lower volume, value-based products. (SD)
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Sweta Dash is the founding president of Dash-Insights, a market research and consulting company specializing in the display industry. For more information, contact [email protected] or visit www.dash-insight.com