Micro-LED display technology is being heralded as the next-generation display technology in consumer electronics industry. While numerous companies have introduced prototypes and application demonstrations utilizing Micro-LED, it still has not reached maturity yet. Micro-LEDs are defined primarily by size of light emitting LED chips emitted; LED chips smaller than 50mm being considered Micro-LEDs.
Dr. Ji Honglei and his team of optical system engineers at TCL Electronics R&D Center recently published a research article entitled, “Development Status and Technological Challenges of Micro-LED Displays,” in the core journal Liquid Display (ESCI).
This article presents an introduction to Micro-LED display technology’s history, definition and technological challenges with particular attention paid to engineering challenges within this area of research. Furthermore, future development directions of this emerging technology are explored as well as their historical roots. Micro-LED displays still present a number of technical obstacles when it comes to manufacturing chips, mass transfer and creating full color displays; nonetheless they boast exceptional characteristics including high resolution, fast response times, low energy consumption and long lifespans that make them suitable for virtual/augmented reality and electronic billboard displays. Due to this potential application in both academia and industry sectors, research efforts in both have increased significantly.
What Are Micro-LED displays?
Micro-LED displays employ submicron-sized inorganic LED devices as emitting pixels for active matrix displays, constituting an active matrix display technology. Micro-LED, organic light-emitting diode (OLED), and quantum dot light-emitting diode (QLED) all belong to active emission display technologies; however Micro-LED stands apart due to using inorganic GaN chips which offer excellent light emission performance and longevity; due to this outstanding performance and potential application it has seen widespread research within academia since its debut.
As Micro-LED display technology develops further, so has industrial interest. Apple, Samsung, Sony, LG and BOE all joined Apple in developing Micro-LED displays and many startups such as Ostendo Luxvue PlayNitride have emerged specializing in Micro-LED technology since 2014. Since Apple acquired Luxvue back in 2014 and achieved significant breakthroughs post 2018, domestic terminal manufacturers and chip suppliers also joined. Although prospects for micro-LED displays seem brightening up significantly more clearly there still many technical hurdles remaining that need addressing at this stage.
At present, there is no industry-standard definition for Micro-LED Display Technology; scholars and experts may offer different interpretations depending on application scenarios or research environments. Calculating Micro-LED should take into account factors like viewing distance and eye’s maximum resolution. VR/AR applications requiring an 1800 PPI resolution generally call for Micro-LED chip sizes of between 3-5mm for viewing distances of 5cm; chip sizes for 10-12 inch tablet displays with 300 PPI would require 20-30mm, and for large-screen 75in displays featuring 43PPI micro-LED sizes could often exceed 200mm.
Development and Advantages of Micro-LED Display Technology:
Micro-LED display technology represents one of the greatest advancements in LED field, behind blue GaN materials and white LED lighting. Micro-LED technology’s development history indicates its innovations mainly emerged through integrated processes; consequently it is likely that future Micro-LED developments will focus on miniaturization, integration, array transfer and full color displays as key objectives of development .
Micro-LED displays feature a simple structure which reduces light loss within and reduces thickness; micro-LED technologies offer significant performance advantages over their competition:
Micro-LED Displays Offer Superior Image Quality:
Micro-LED displays do not face the limitations posed by light masks and filters, enabling them to quickly achieve 2000-4000cd/m2 levels with ease and provide fantastic contrast levels as well as HDR effects in HDR movies.
Micro-LED display technologies boast high energy efficiencies: LCDs consume 90 times less power compared to Micro-LED, while their electrical-to-optical conversion efficiency allows Micro-LED to consume only half as much.
Long Lifespan:
Micro-LED displays use inorganic semiconductor materials for light emission, guaranteeing stability and long material lifespan.
Micro-LED technology still is not fully developed and continues to face several technical challenges from different corners, including:
Chip Technology:
Micro-LED wafers currently do not meet mass production specifications when it comes to wavelength consistency, light emission efficiency rapidly declines with shrinking chip sizes, plasma etching can damage chip sidewalls and compromise their light emission characteristics and reliability, further complicating their production.
Backplane Technology for Micro-LED Displays in Consumer Electronics:
Both printed circuit boards (PCB) and glass substrates have unique benefits when used as backplane materials for micro-LED displays, but both present unique challenges for mass transfer performance. PCBs may experience expansion/contraction issues that negatively affect mass transfer results while glass substrates offer stability but require precise processing due to non-isotropic size variations.
Mass Transfer Technology:
Transferring chips to driver circuit backplanes is an integral component of driver circuit assembly, with current methods including pick-and-place, laser transfer, fluid self-assembly and roll-to-roll printing being among them. Each of these methods present similar challenges related to precision (transfer accuracy within +-1mm and high yield rates are demanded from all).
Full-Colorization Technology:
Realizing full-color displays using Micro-LED displays requires several techniques that each present their own set of challenges, from RGB triplets, UV/blue Micro-LEDs with light conversion materials, lens synthesis techniques and special structure approaches – each approach comes with issues regarding efficiency, reliability and uniformity that must be overcome in order to be successful.
Bonding Technology for MicroLEDs:
Bonding techniques used in microLED production range from solder paste, metal eutectic bonding and microtube technologies, with challenges including short circuiting between chip electrodes due to small sizes; heat transfer limitations between chips and driver substrates and thermal expansion coefficient differences being among others factors to contend with.
Driver Technology:
For every pixel on a Micro-LED display to function effectively, its own Micro-IC driver driver requires being installed into each individual pixel and using duty cycling to regulate brightness and color levels, leading to challenges like low brightness at lower gray levels and color instability from insufficient driving current.
Micro-LED displays face not only technical constraints but also specific application issues such as:
Anti-Ambient Light Interference: Micro-LED displays lack front-facing light filters like those found on LCD technology, rendering them less capable of controlling ambient light interference.
Large Viewing Angle Color Shift: Color shift issues result from variations in light field distribution of red, green and blue chips; cross-illumination between neighboring pixels may also contribute.
Acquiring Uniformity in Monochromatic and Gray Fields: Striking an even tone across multi-tile Micro-LED displays can be challenging due to variations in chip wavelengths and driving voltage, leading to unpredictable monochromatic and gray field performance.
Power Consumption: Due to their pixel-level control, Micro-LED displays require many driver chips. This increases power consumption due to low voltage/high current driving that results in reduced efficiency and greater line losses.
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Conclusion:
Micro-LED display technology provides an effective complement to mainstream display technologies and fills its shortcomings and application gaps effectively. By taking advantage of its ability to be integrated onto ultra-large displays, this light control solution meets the demands of large displays. Offering superior brightness, color gamut and contrast performance through its pixel-level light control technology, its brightness meets outdoor, semi-outdoor and theater scene needs. Ultra-small displays are used primarily in virtual and augmented reality (VR/AR) technology applications. Utilizing their extremely small grain size, ultra-small displays are capable of meeting thousands of pixel density requirements with their ultra-small grain size.
Micro-LED displays’ inherent self-illumination and material stability give them distinct advantages in terms of response time, operating temperature range and storage requirements, meeting real time reliability needs in aircraft cockpit displays. Technological breakthroughs in drivers and backplanes, using transparent plastic films as substrates, enable display films of any size to be attached to other carriers for widespread displays. Utilizing Micro-LED display technology with its nanosecond response times it becomes possible to realize true naked-eye 3D displays.
Due to its miniaturized and integrated characteristics, Micro-LED display technology is still evolving; hence its higher requirements in materials like chips, backplanes and luminescent media. Terminal displays still present a host of difficulties when used in applications, and micro-LED displays have just started entering consumer electronics; it will take some time before these technologies are widely popularized and adopted.
However, its characteristics will soon be taken advantage of in various fields and new functions based on Micro-LED display technology are being continually explored; perhaps these will bring revolution to future display fields.