Silicon-based full-color stacked structures are emerging as a significant trend in the Micro LED industry. At the ISE 2023 expo earlier this year, Seoul-based Violeds showcased a Micro LED display built with their WICOP Pixel technology, achieving an impressive screen brightness of 4000 nits.
The WICOP Pixel from Violeds is a full-color single-chip display technology that eliminates the need for wire leads, packaging, or lens structures. It stacks the red, green, and blue Micro LED chips vertically.
In recent years, the path to industrializing Micro LEDs has faced several technical challenges, including issues with the yield of mass transfers, substrates, drivers, post-inspection and repair processes, costs, and the low efficiency of red light. These have become critical obstacles to their widespread adoption.
Following Violeds’ lead, organizations like Novus Technologies, Silicon Valley-based Sundiode Inc, South Korea’s Youngwoo DSP, Tsinghua University, Wuhan University, and MIT are all exploring and developing vertically stacked structure Micro LED technology to accelerate its industrialization, even as process technology continues to evolve.
Industry research shows that the primary applications for vertically stacked Micro LEDs are currently in medium and small-sized AR/VR displays, vehicle displays, and smartwatches.
Accelerating the Adoption of New Technology
Under the goal of ultra-high-definition display, Micro LEDs offer low energy consumption, high brightness, excellent contrast, and high reliability. They meet the diverse needs for pixel densities and display sizes, becoming a crucial pillar in advancing display technology and building a new industrial landscape.
The Gaogong LED Industry Research Institute (GGII) forecasts that the global Micro LED market will surpass $3.5 billion by 2025 and potentially exceed $10 billion by 2027.
This substantial market potential has driven the focus of the industry and academia towards speeding up mass production of Micro LEDs. The stacked structure Micro LED technology could be a key pathway for moving Micro LEDs from the lab to large-scale industrial applications.
In December 2022, Novus Technologies announced a significant breakthrough with the successful lighting of a 0.39-inch XGA Micro LED microdisplay, marking progress in their Wafer Level Vertically Stacked Pixels (WLVSP) technology.
Novus’s VSP technology, which stacks multiple material layers vertically, has achieved pixel miniaturization and can enhance the brightness of red light through stacking. This approach effectively addresses a major technical challenge in the Micro LED field.
Overcoming Performance and Cost Barriers
Performance and cost are still key challenges for the mass production of Micro LEDs. On the performance side, due to the size effect, the luminous efficiency of Micro LEDs is lower in smaller sizes, especially in the red light part of full-color displays. Additionally, factors like the size and warping of sapphire substrates affect the yield of Micro LED chips, making it difficult to meet market expectations for cost and capacity.
Novus Technologies has tackled these barriers by adopting a silicon substrate-based approach. They integrated established stacking technology from the Micro LED and integrated circuit fields, unifying the technical scheme at the chip product level.
Earlier, a research team from Korea’s KAIST also integrated red Micro LEDs on a Si-CMOS display driver circuit board using 3D stacking. This method first transfers the Micro LED film layer to the Si-CMOS circuit board through wafer bonding and then generates pixels via photolithography. The team successfully developed a high-resolution display on the Si-CMOS circuit board using a top-down continuous semiconductor process.
In fact, research and development of stacked structure Micro LEDs have been ongoing for several years. In 2021, the Xing research group from Tsinghua University’s Department of Electronic Engineering developed an RGB Micro LED device array design based on a layered structure for full-color lighting and display.
South Korean manufacturer Youngwoo DSP was also selected in 2021 by the South Korean Ministry of Trade, Industry and Energy to lead a national project to develop new Micro LED manufacturing technology based on ultra-small RGB stacked layers.
Silicon Valley-based Sundiode Inc also showcased a full-color Micro LED microdisplay in 2021, composed of a stacked RGB Micro LED pixel array driven by active matrix silicon-based CMOS backplane technology.
Challenges in Developing Stacked Structure Micro LEDs
In February 2021, the US Patent and Trademark Office announced that Apple had obtained a patent related to Micro LED technology, specifically for a “Stacked Hybrid Micro LED Pixel Structure.” This patent brought renewed attention to stacked structure Micro LEDs in the industry.
Recent research by Gaogong LED reveals that several companies have begun exploring silicon-based stacked structure Micro LEDs following the initial technological investigations.
“Stacked structures can significantly improve resolution and enhance device brightness while reducing the requirements for manufacturing precision, thereby improving Micro LED yields,” said the technical head of a packaging company. However, this technology is still in the laboratory stage and far from mass production.
Research also suggests that with a stacked structure, light of three colors will be emitted from different heights, complicating optical design and increasing requirements for the precision of LED spacing and alignment between different layers.
Moreover, color interference from stacked Micro LED RGB chips, the low luminous efficiency of tiny pixels, and compatibility and efficiency issues with red light materials remain significant challenges for future mass production.
Furthermore, while the stacked structure Micro LED is more compatible with silicon substrates, sapphire remains the mainstream substrate. The large-scale use of silicon substrates still faces critical issues such as cost and yield.
Nonetheless, with the R&D advancements by leading manufacturers and university research institutes, new applications for stacked structure Micro LEDs in medium and small-sized fields are expected to emerge.
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