Micro LED technology utilizes micron-sized LEDs as pixel-emitting units, offering multiple advantages such as high brightness, high contrast, high pixel density, and modularity. It is currently regarded as the most suitable display technology for AR.
Reportedly, Micro OLED has gradually become the mainstream display solution in newly launched AR headsets in recent years, thanks to its advantages in lightweight design, high pixel density, contrast ratio, response time, and power consumption. However, due to its shortcomings in brightness, Micro OLED is primarily paired with optical solutions like Birdbath and freeform optics, which have high light efficiency but are limited in terms of lightweight design, making them more suitable for indoor applications. On the other hand, Micro LED, through silicon-based technology combined with micron-scale LEDs, can meet the performance requirements of AR headsets, such as high pixel density, high brightness, and low power consumption. It can also achieve lightweight design by integrating with waveguide optical systems.
Thus, from an application perspective, AR headsets will be the main driving force behind innovations in Micro LED technology. This need has become even more critical and urgent, especially after Apple canceled its Micro LED smartwatch project.
Micro LED stands out with its remarkable parameters and smaller size. Micro LED is a microdisplay technology based on LEDs, where each pixel emits its own light. There are two types of Micro LEDs: high-density and low-density. The type used in AR and VR is the high-density Micro LED, with a PPI exceeding 2000 and LED particle sizes ranging from 1-10μm. Micro LED’s core advantages include: 1) high brightness (ranging from tens of thousands to even millions of nits, with diffractive waveguides enabling in-eye brightness exceeding 1000 nits); 2) fast response time (in the nanosecond range); 3) smaller size, making it suitable for consumer-grade AR (JBD’s full-color Micro LED optical engine “Hummingbird” is 0.4cc, compared to LBS/Micro OLED’s 0.5-1cc and new LCOS/DLP’s 1-2cc). Additionally, Micro LED boasts high contrast, wide color gamut, low power consumption, and long lifespan.
AR Micro LED does not involve mass transfer technology but faces challenges like sidewall effects, low red-light efficiency, and complex full-color implementation.
1. AR Micro LED does not involve mass transfer technology: Mass transfer (moving Micro LEDs from the silicon wafer substrate to the driving backplane) is a core challenge in the mass production of Micro LEDs, requiring extremely high transfer yield and precision. However, since AR displays are small, with pixel sizes and spacing of only a few micrometers, mass transfer technology is not involved.
2. Micro LED sidewall effects: As Micro LED pixel sizes decrease and the perimeter-to-area ratio of the chips increases, more surface recombination occurs at the sidewalls, leading to an increase in non-radiative recombination rates and a decrease in optoelectronic efficiency. Additionally, the ICP etching process during device fabrication exacerbates sidewall defects. Sidewall effects negatively impact the power efficiency of Micro LEDs.
3. Insufficient red light efficiency in Micro LED: Blue and green LEDs are grown on substrates like sapphire, silicon carbide, or silicon using ternary materials such as InGaN. However, red LEDs are primarily grown on GaAs substrates using quaternary AlGaInP materials. Compared to blue and green light, reducing the size of AlGaInP-based red Micro LEDs leads to a more pronounced efficiency drop. Material innovations, such as InGaN-based red Micro LEDs, and technical optimizations are the main approaches to addressing this issue. In October 2023, JBD announced that the brightness of its 0.13-inch Micro LED red chip had surpassed 1 million nits, thanks to a new generation of AlGaInP epitaxial technology, which significantly mitigates the effects of non-radiative recombination on the Micro LED surface and slows the steep drop in luminous efficiency for red Micro LEDs smaller than 5μm. Combined with chip passivation technology, this breakthrough addresses the size-effect bottleneck of red light.
4. The complexity of full-color Micro LED technology: Producing full-color Micro LEDs at scale is difficult and costly, with few products available. According to LEDinside, there were 9 new Micro LED AR glasses released or launched in 2023 (compared to just 3 in 2022), of which 6 featured monochromatic Micro LEDs.
Currently, the main technologies for achieving full-color Micro LEDs can be divided into three categories: color-mixing technology, quantum dot technology, and monolithic stacking technology.
1. Color-mixing technology: The only full-color Micro LED technology that has been mass-produced in the AR field is color-mixing technology. 1) X-Cube color-mixing (prism color-mixing): Red, green, and blue monochromatic panels are mounted on three faces of an X-Cube (prism), and the three colors are combined and emitted through a fourth face. A set of microlenses then collimates and projects the light. X-Cube module volume is smaller than 1.4cc. 2) Waveguide color-mixing: Independent red, green, and blue light engines are used to achieve color-mixing, typically paired with multi-layer waveguides or multiple waveguide coupling ports.
2. Quantum dot technology: This technology converts UV/blue LED light into different colors by exciting quantum dots or phosphor materials. Due to the larger size of phosphor particles, quantum dots are typically used. Quantum dots, once excited, can be tuned to emit RGB light, achieving full-color displays through color ratio control. Quantum dots have a narrow half-peak width and a broad absorption spectrum, as well as high luminous efficiency, resulting in high color purity and saturation. However, challenges remain in real-world applications, including poor material stability, short lifespan, and uneven color distribution.
3. Monolithic stacking technology: Monolithic full-color Micro LEDs offer broader application potential, with wider fields of view, smaller optical engines, and simplified system-level integration for AR glasses, reducing optical losses and achieving higher waveguide collimation efficiency. 1) In February 2023, an MIT team developed a full-color vertically stacked Micro LED using 2D material layer transfer, achieving an array density of 5100 PPI with sizes of only 4μm and a stacking height of 9μm. 2) In August 2023, JBD released the world’s first 0.22-inch, 2K resolution, monolithic full-color vertically stacked Micro LED prototype called Phoenix. The total thickness of Phoenix’s stacked layers is less than 5μm, minimizing cavity absorption losses. The use of native epitaxial materials allows for high flux density light emission, with brightness reaching up to 1 million nits. Additionally, the native color solution enables narrow full-width half-maximum (FWHM) spectra, resulting in higher color quality and purity. The prototype is expected to enter mass production in 2025.
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