Ever since 2010 when microLED display technology emerged, analysts, manufacturers and industry experts have been excited by its potential for superior performance.
It is easy to see why: MicroLEDs have excellent performance attributes. They provide lower power consumption, higher pixel density, faster (nanosecond) response time and wider viewing angles than organic LED (OLED) displays or LED-backlit liquid crystal displays (LCDs).
Crucially, in direct sunlight conditions, they provide an order-of-magnitude higher luminance than OLED displays or LCD, which is vital for handheld devices, in addition to being a key enabler for near-eye displays.1
Vuzix next generation microLED smart glasses teaser
MicroLED display-based projectors are built into the latest smart glasses from Vuzix. Video Credit: Radiant Vision Systems
Yet, manufacturers are still working to close the gap between promise and market reality as establishing a cost-effective fabrication process for microLED displays has proven to be challenging.
To produce these nano-scale elements requires numerous steps, with defects potentially occurring at each one. The consumer device marketplace expects near-perfect performance and visual quality from today’s display devices.
Manufacturers must carry out a careful inspection at each step to meet these high expectations, and they must also achieve high yields to ensure production costs remain viable for the mass market.
The timeline for moving microLED displays from research labs to mass production has been slow and is still not complete. Recent advancements and developments have helped to move the industry nearer to finally achieving consumer marketability for microLED devices.
End-to-end MicroLED Fabrication Challenges
Creating MicroLED displays is a multi-phase process that includes some, or all, of the following steps:
- Substrate removal
- Mass transfer
- Epitaxial growth
- Chip fabrication/wafer fabrication
- Photolithography
- Testing
- Repair
- Bonding and interconnection with the control circuit
- Panel assembly
MicroLEDs require new processes and methods even though LED technology has been around for decades and is widely utilized for LCD displays. Every component needs to be optimized for performance and cost and almost every step in the fabrication process has had to be re-invented.
For instance, tiny micro-LEDs have a large relative surface area, which may result in more defects happening during the fabrication process. So, solving engineering and manufacturing challenges is vital, including die size miniaturization while maintaining chip design, high efficiency and chip manufacturing technique improvement.2
Simplified schematic of microLED display layers (left). Image Credit: source via Creative Commons. Plessey 0.26 arrays bonded to CMOS backplane for monolithic microLED displays (right). Image Credit: source.
Their small size (usually <50 μm and some as small as 3 μm) brings new challenges. Most LEDs today are fabricated on 100 mm and 150 mm sapphire wafers, though InGaN-on-silicon is growing in popularity, particularly among large silicon-based foundries.
Historically, LEDs are assembled in a surface-mount technology (SMT) package, wire-bonded in place and encapsulated with epoxy or silicone.
The main difference with microLEDs is they are utilized in bare die form instead of in packages. This difference, in addition to much tighter design tolerances, makes the fabrication of microLEDs a very costly endeavor.3
Developers are working to address challenges like decreased external quantum efficiency (EQE), bonding methods, color conversion, low-efficiency and low-yield in the mass transfer process and enhancing the performance of the backplane and panel.
The mass transfer step has proven especially challenging due to the fact that traditional LED pick-and-place and flip-chip methods do not work for tiny microLEDs.
Techniques like laser-induced transfer, elastomer stamp, electrostatic transfer, fluidic self-assembly (FSA), roll-to-roll (R2R) or roll-to-panel (R2P) transfer and more have all been attempted with mixed success.4
One fabrication method for microLED displays uses integrated CMOS driving to simplify the transfer step. The result is an elementary unit consisting of all-in-one RGB LEDs on a CMOS driving circuit. Image Credit: eenews.
A more complex approach focuses on singulating blue, red and green LEDs from their wafers as well as CMOS driving circuits to transfer each separately. Image Credit: eenews.
Latest Breakthroughs and Advancements
MicroLEDs remain a focus of research and development activity despite all of these challenges, and creative new methods and solutions continue to emerge. The latest sign of progress is Samsung’s recent announcement of a 110-inch microLED television. Experts predict economies of scale will eventually prevail in this market even though it is priced at a hefty $155,000 (USD).5
Samsung’s 110” microLED television offers 4K resolution with 8 million pixels. Image Credit: © Samsung, Source.
Some other recent advancements include:
- Royole unveiled the world’s first stretchable microLED display in 2021. These new stretchable displays are not only foldable and rollable but also capable of 3D freeform shaping, including twisting, pulling, convex and concave deformations.
- Coherent’s development of a new integrated laser system for microLED manufacturing. It carries out three steps in the fabrication process: Laser Lift-Off (LLO), Laser-Induced Forward Transfer (LIFT) and Repair/Trimming. It can potentially lower manufacturing costs by combining these steps.
- Researchers at universities in Shanghai, Taiwan and Chengdu have been looking at solutions for producing full-color microLED displays, including new advancements in transfer printing, growth methods and color conversion.
A flexible new microLED display is stretchable, foldable, and rollable, opening up new possibilities for wearable and embedded displays. Image Credit: © Royole.
MicroLED Inspection Requirements
Variations in color or brightness, defects and other irregularities can quickly hurt brand reputation, deflate buyer satisfaction and erode market share. Fabricators and foundries must meet near-zero tolerance for defects in finished display devices.
If quality issues cannot be addressed and corrected at the component level, high production costs and low yields will impede the viability of microLED display technologies for the mass market.
Quality inspection must also be performed at multiple stages because of the complex, multi-stage process of fabricating microLED displays.
Manufacturers do not want to miss defects at the chip/wafer stage which could end up being built into an assembled device that must be discarded along with all its invested parts and labor. So, inspection at key points throughout the process is vital.
LED fabricators are stepping up their use of inline metrology, automated optical inspection, and testing protocols. Image Credit: KLA.
MicroLED Inspection Solutions for Every Fabrication Stage
Whichever microLED fabrication techniques ultimately prove most successful, the requirement for quality assurance throughout the manufacturing process is persistent.
Radiant already offers an effective suite of measurement and inspection solutions for microLEDs from the chip/wafer level to the panel/assembly level and they have almost 30 years of experience developing solutions for all stages of electronics production.
Radiant’s measurement solutions (including ProMetric® Imaging Photometers and Colorimeters with optional Microscope Lens) are allowing microLED manufacturers to examine display performance and uniformity at various fabrication stages with accurate and repeatable results to ensure the absolute quality of microLED display devices.
Variation in luminance and chromaticity from pixel-to-pixel can lead to uniformity issues as microLED displays are emissive (each microLED pixel is individually powered and emits its own light).
Applying patented demura techniques, Radiant’s high-resolution imaging systems and TrueTest™ software allow manufacturers to inspect and correct each pixel and subpixel element accurately to increase yields.
Radiant’s ProMetric systems provide the high-resolution imaging needed for extremely precise inspection of individual pixels and subpixels, to capture and measure the discrete output of each pixel and to register subpixels correctly, regardless of layout or shape.
Radiant’s Microscope Lens, usually applied for subpixel characterization in R&D and lab environments, allows the full resolution of the imaging system to be applied to an extremely small (zoomed-in) portion of a display or wafer, for much better measurement precision at each light-emitting element (each individual microLED).
Radiant’s measurement systems can be utilized to assess luminance and color uniformity across the entire panel at once after microLEDs are transferred to a backplane.
The benefit of a 2D imaging colorimeter or photometer is the ability to capture a large spatial area in one single image, comparing measured values to detect and qualify any areas of nonuniformity or mura (blemishes).
MicroLED from Jasper Display shown before and after uniformity correction (demura) using Radiant's solution. Image Credit: Radiant Vision Systems
Radiant’s ultra-high-resolution imaging systems (up to 61 MP) can also capture a single image of a display for assessment, measuring pixel-level luminance and color values for all pixels in the display at once
These comprehensive measurements allow extremely efficient pixel uniformity testing and calculation of correction factors for display calibration (demura) in production settings.
References
- Peters, L., “MicroLEDs Moving from Lab to Fab.” Semiconductor Engineering, July 22, 2021.
- “Micro-LED display development unhindered by COVID-19.” Semiconductor Today, July 16, 2021.
- Peters, L., “MicroLEDs Moving from Lab to Fab.” Semiconductor Engineering, July 22, 2021.
- Wu, Y., et al., “Full-Color Realization of Micro-LED Displays.” Nanomaterials, December 10, 2020
- Chen, Z., Yan, S., and Danesh, C., “Topical Review MicroLED technologies and applications: characteristics, fabrication, progress, and challenges.” Journal of Physics D Applied Physics, January 2021.
Acknowledgments
Produced from materials originally authored by Anne Corning from Radiant Vision Systems.
This information has been sourced, reviewed and adapted from materials provided by Radiant Vision Systems.
For more information on this source, please visit Radiant Vision Systems.