Addressing the Challenges of MicroLED Display Uniformity

Author:
Anne Corning

MicroLED displays have had a long development timeline since they were first discovered in the early 2000s. They’ve hung out in the “emerging” technology category for a while, having achieved commercialization so far only for television displays. The first microLED TV prototype, Sony’s Crystal-LED, was introduced in 2012, and Samsung’s The Wall came out in 2018. 

Nevertheless, excitement about microLED technology remains high. The technology’s advantages of high-brightness, high-contrast, energy-efficient, and durable display performance hold promise for myriad potential use cases including “augmented/mixed reality (AR/MR), virtual reality (VR), large video displays, TVs and monitors, automotive displays, mobile phones, smart watches and wearables, tablets and laptops.”1 Reflecting this versatility, the microLED market is projected to grow from 2020 revenues of $409 million to more than $18.8 billion by 2026, an impressive CAGR of 89.3%.2

microLED display applications_composite

Applications that can benefit particularly from microLED display performance attributes include AR/MR, VR, large-format display screens, automotive HUD, televisions, smartwatches, mobile phones, laptops, and tablets.

This long maturation timeline is due largely to multiple fabrication and production challenges that manufacturers have had to address to try and achieve commercially viable product yields. “To fabricate a microLED display, many technologies and processes are involved, such as epitaxy, photolithography, chip fabrication, substrate removal, inspection, mass transfer, bonding and interconnection, testing, repair, backplane and drive IC, etc. Several years ago, the major efforts were concentrated in die miniaturization, chip design and mass transfer, etc.”3 More recently, efforts have increased toward integrating “technologies such as inspection, repair, driving, image improvement, light management and high-volume production equipment.”4

Radiant has contributed to addressing these challenges, particularly developing new solutions and methods for “inspection, repair,” “image improvement,” and “light management” of microLED display devices. Because microLEDs are individual emitters, they can exhibit wide variations in luminance and chromaticity from pixel to pixel, which causes a non-uniform display appearance. Innovative new display metrology solutions are enabling manufacturers to measure and correct microLED display uniformity to support high-volume production.

The Challenge of MicroLED Display Uniformity

Variability in luminance and chromaticity can render microLED displays unusable (unsalable) unless corrections are applied to improve appearance. Variability is compounded because each microLED is typically a monochromatic subpixel (red, green, and blue) whose output is combined with other subpixels to produce the overall brightness and color of a single display pixel. This variability at the subpixel and pixel level manifests as a non-uniform appearance across the display, resulting in low yield of acceptable displays, rejection of expensive components, or costly rework.

microLED display uncorrected mura non-uniform

An uncorrected microLED display exhibits variable luminance from pixel to pixel, resulting in a non-uniform, blotchy appearance (Image © Jasper Display).

Measurement of microLED subpixels is necessary to quantify, evaluate, and potentially correct display output. However, microLEDs are challenging to measure accurately due to their variability, size, proximity (small pixel pitch/density), and quantity per display. This makes them equally challenging to correct—especially at the speed needed to support commercial production throughput.

For emissive displays, new measurement methods that can detect and quantify the output of individual pixel and subpixel emissive elements are enabling display uniformity correction. It is now possible to measure and correct the luminance and chromaticity output of each pixel, thereby producing displays with uniform appearance. This process—referred to as pixel uniformity correction, or “demura”—relies on precise subpixel-level luminance and chromaticity measurement to calculate accurate correction coefficients for each microLED. 

Determining the Best Solution for MicroLED Production Quality

Research and testing have shown that a calibrated, high-resolution imaging colorimeter provides both the accuracy and speed needed for microLED production inspection at the pixel and subpixel level. Based on Radiant’s experience working with major LED, OLED, and microLED device manufacturers, the best measurement equipment to address microLED commercial manufacturing demands would include:

  • Imaging colorimeter. One advantage of photometry-based imaging systems such as the ProMetric® I-Series Imaging Colorimeter is efficiency—the ability to detect all meaningful variations across displays in a single image, accomplishing multiple measurements at once: luminance, chromaticity, uniformity, contrast, pixel defects, etc.
  • High resolution. A microLED measurement system must have high-resolution imaging capabilities to distinguish and isolate each pixel and subpixel for measurement, and the efficiency to capture values for every pixel across increasingly high-resolution, pixel-dense displays. For some microLED measurement applications such as wafer-level inspection, an objective Microscope Lens can increase measurement accuracy of subpixels.
  • Low noise. Low-noise imaging capability is also needed. Image noise (which can include read noise, shot noise, or electronic noise), interferes with the clarity of an image. No matter how high the resolution of an imaging system (the number of megapixels (MP) of its sensor), if the system captures significant noise (yielding low signal-to-noise ratio, or SNR), then its effective resolution may be much lower.
     
microled solution_Prometric_TrueTest_microscope lens

An integrated microLED display metrology solution from Radiant includes a ProMetric Imaging Photometer or Colorimeter, an optional Microscope Lens, and TrueTest™ Software for analysis.

Optimizing MicroLED Measurement & Correction

Beyond simply capturing high-resolution, low-noise images, accurate pixel-level measurement of microLEDs relies on a measurement system’s ability to sufficiently isolate each pixel and precisely quantify its output value. Imaging system resolution determines the number of photo-sensing elements (sensor pixels) available to cover each individual display pixel. Applying more sensor pixels per display pixel increases the granularity of data acquired by the imaging system for accurate pixel registration and measurement. As overall display resolution increases, an imaging system’s ability to apply sufficient sensor pixels per display pixel—while continuing to capture measurements for all display pixels in a single image to ensure efficiency—is reduced.

Radiant’s answer to the challenge of accurate microLED measurement and analysis starts with our TrueTest Software, which enables manufacturers to optimize and run tests on a captured image. It includes tools to detect and quantify luminance, chromaticity, uniformity, contrast, pixel and line defects, display mura, and other qualities. Additionally, Radiant has developed two methods that have been proven to significantly improve an imaging system’s ability to isolate and measure subpixels in increasingly high-resolution displays: a “spaced pixel” method and a “fractional pixel” method. Both methods rely on proprietary algorithms and analysis techniques applied via TrueTest.

The spaced pixel measurement method (US Patent 9135851) improves the effective resolution of an imaging system by applying its total image sensor resolution across only a subset of display pixels at one time. This is accomplished by illuminating and measuring subsets of the display pixels for each primary using spaced pixel patterns in each primary color, until all display pixels are measured. This process increases the effective resolution of the measurement at each display pixel, ensures the isolation of each pixel’s output, and thus increases the accuracy of measurement calculations across displays of any arbitrary resolution.

The fractional pixel method (Patent Pending) is a process that first optimizes pixel registration, then enhances pixel measurement, by accurately calculating pixel output captured in fractional quantities across a limited set of image sensor pixels. The method ensures the accuracy of pixel-level measurements for emissive displays of much higher resolution than was previously possible using a single-image capture, thus supporting fast takt times. Using one or both of these methods enables manufacturers to accurately measure and correct microLED display devices at production speeds, thus increasing quality and yield.  

microled demura_before and after

The same microLED display shown previously—before correction (left), and after correction (demura) has been applied (right), resulting in a uniform appearance. (Images © Jasper Display)

To learn more about visual quality and inspection considerations for microLED displays, demura correction solutions, and using the spaced-pixel and fractional pixel methods, read the white paper: Measuring and Correcting MicroLED Display Uniformity. Topics include:

  • How microLED measurement and correction requirements can be satisfied using imaging colorimeters, applying unique equipment specifications, calibrations, and software functions
  • Accurate microLED measurement and registration methods: “spaced pixel” and “fractional pixel”
  • How Enhanced Color Calibration™ reinforces the chromaticity measurement accuracy of CIE-matched tristimulus filter imaging systems
  • The benefits of various measurement and correction methods, demonstrated with test data and real-world application
     
read the white paper_microled uniformity

 

 

 

CITATIONS:

  1. He, X., “MicroLED Displays: Getting Ready for the Future?”, IDTechEx, July 31, 2020.
  2. Micro-LED Market by Application … Display Panel Size, Vertical, and Region - Global Forecast to 2026,” Report from Markets and Markets, May 20, 2020.2. 
  3. Micro-LED Displays 2020-2030: Technology, Commercialization, Opportunity, Market and Players”, IDTechEX Research, overview of report by He, X. (Retrieved 2/11/2021) 
  4. Ibid.
     
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