MicroLED displays require encapsulation to provide essential protection from environmental factors, offer mechanical support, and ensure long-term reliability and performance. The typical encapsulation method is to laminate a transparent cover plate (such as glass plate) to a microLED display using optically clear adhesive (OCA). However, this encapsulation causes some optical issues, such as a significant degradation of the display brightness and edge light leakage of the tiles. Light leakage at the tile edges results in undesirable tiled microLED display seam visibility. Two major reasons for these issues are: 1) the encapsulation changes the light emission properties of the microLEDs due to the alteration of the surrounding optical condition of the light emission surfaces; 2) light confinement and guiding by the waveguide formed by the cover plate and OCA layer.

Corning’s Scattering Layer

One way to increase brightness is to introduce a scattering layer on the backside of the cover glass. While this works, it also increases crosstalk and haze, depending on the characteristics of the scattering medium.

In paper 89-2, Corning experimentally investigated this tradeoff between luminance, scattering film properties, and crosstalk. In their work, Corning patterned the scattering layer and conducted a modeling study on the method. The modeling results indicate that this method is an effective way to conquer the tradeoff issue between luminance enhancement and crosstalk suppression.

Tianma Border Processing

Tianma described improvements they have done to process the edges of their glass-based microLED panels for tiling applications in both automotive and video wall applications as well as new planarization techniques to improve the application of the black matrix to reduce light emission at the tiles edges.

For automotive displays with a fairly high spatial resolution, a very thin border area is needed to allow for a consistent pixel pitch from tile to tile. Paper 97-2 described key advancements on their 7.05-inch transparent microLED panel in this area and more:

• Circular instead of square apertures to reduce diffraction effects
• Vertical shift registers were moved inside the active area to provide a clean border area
• Implementation of a narrow horizontal edge compression zone to maintain green microLED pitch from tile to tile
• Optimization of the encapsulation layer to minimize tile warpage
• Modification on the edge chamfer design to reduce pixel-to-pixel distance without compromising display integrity
• Using a new tile alignment method to aid in applying adhesive to achieve a seam of less than 30 microns

Transparent borderless microLED displays used as tileable screens still face several challenges. These include higher reflection and lower transmittance in pixel compression zones due to high metal density, backlight leakage, visual tiling seams, and reliability risks caused by ultra-narrow borders. Further research should be carried out to improve the performance of transparent seamless displays through innovative designs and optimized processes, laying a solid foundation for the future integration of microLED technology in automotive applications.

AUO Efforts to Reduce the Seam

In paper 97.4, AUO noted they have two three-sided transparent microLED-based panels that can be tiled to create larger display. The 16.1-inch tile can be assembled into a 2×8 array to make a 60-inch display. They also have a 42-inch tile that can be assembled into a 1×2 configuration to reach 64 inches, as shown on the exhibit floor. Any tiled display must consider two conditions: the bright and dark states. In the paper, they focused on the dark state as this is the tougher problem to solve.

The goal of the paper was to develop some objective assessment of the seam to enable AI-based inspection. Using a camera-based system to image the tiled modules illuminated by a lightbox, they developed a structured similarity measurement based in the frequency domain to better capture any shifts or rotations of the two panels.

This methodology allowed the company to assess the optimal physical gap, the best optical gap filling material, edge geometry and surface roughness effects. The results suggest filling the gap with index-matched bubble-free epoxy and laser cutting the edge to get a mirror-like surface with no chamfer and a roughness of around 0.3 microns.