As an important branch of micro LED display applications, the development of micro micro LED display devices is on the fast track. If successful, this technology will significantly change the future competition path for in-vehicle HUDs, projection entertainment screens, and AR/VR headsets.
Choose micro LED micro display solutions with different color architectures
Micro LED realizes the application of micro display video, and there are a variety of color schemes. For example, monochrome micro LED panel chip + color conversion layer technology. Among them, the color transformation layer is mainly made of QD quantum dot materials. Another example is the three-chip micro LED display system, that is, three panel chips process three primary colors separately. In addition, there is a monolithic color solution, which integrates RGB trichromatic LED crystals onto a single substrate.
In addition to the above three colorization schemes, some AR applications do not need to be fully colored. Some devices adopt a monochrome display scheme to achieve the function of mixed reality information fusion, which can fully meet the needs of basic geometric graphics, data information fusion, and even night vision and night battle information fusion. However, from the mainstream direction, overcoming high-efficiency colorization is still the main direction of micro LED micro display.
For example, Jade Bird Display (JBD)'s "Hummingbird" X-cube light engine has been mass-produced in May 2023 and will reach an ultra-high lumen efficiency of 12.5lm/cc per unit volume by the end of the year, and a high-quality optical waveguide can achieve an eye-catching brightness of 1800 nits. With a volume of only 0.4cc and a weight of 1.0g, it is the world's lightest AR projection light engine. The product solution adopts three-piece colorization technology.
Recently, the media reported that the mainland Taiwanese company Chitron has successfully mass-produced 0.49-inch full high-definition (FHD) resolution Micro LED microdisplays, which are mainly used in AR glasses. With a brightness of over 250,000 nits and a color gamut of over 100%, this microdisplay is currently the brightest full-color microdisplay in the AR application market. Compared with JBD's three-chip solution, each micro LED panel core is 0.13 inches, and the panel chip size of the colorized single panel solution is 0.49 inches.
It can be seen that the choice of different color schemes will have a significant impact on the minimum size of the panel chip. At the same time, the integration of a single color LED on a single panel chip also has the advantage of manufacturing process. However, who will win the final result of three-chip color and single-chip color technology also depends on the final level of volume control of the product terminal. - Whether it's AR or VR, the lighter the better.
Brightness levels are the first threshold for product usability
In addition to the different color schemes, the brightness level is the focus of competition between different micro LED micro display solutions. ——From AR applications to VR, HUD, and then to projectors, the demand for brightness of micro LED micro display panel chips continues to increase. And every time you cross a product, it's an order of magnitude improvement.
At present, JBD has achieved the record brightness of 1 million nits of blue light for micro LED micro display, 5 million nits for green light, and 1 million nits for red light. These metrics look high, but the nits are measured in square meters, and they fall on a 0.13-inch micropanel chip, which is equivalent to the 5 lumen output of a red, green, and blue light engine. The eye-catching effect on AR is equivalent to 1,800 nits (i.e., measured per square meter of frame).
In June 2023, Kopin, an American microdisplay manufacturer, announced that it has developed a monochrome Micro LED microdisplay with ultra-high brightness and a resolution of 2K x 2K. Micro LED microdisplay with a brightness of up to 3 million nits. However, the chip size is 1 inch, which severely limits the effectiveness of the brightness in the eye. It is reported that the brightness of the eye is 1150nits.
It can be seen that AR applications, as well as VR applications, require not only high brightness, but also smaller size—small size not only means that the headset is lighter, but also means that the optical waveguide is designed to effectively utilize brightness.
However, in HUD and projection applications, due to the reduced volume sensitivity of the device, the micro-panel chip at the inch level will not affect the design efficiency of the optical system - it is generally believed in the industry that when applied to HUD and projection, the micro LED micro-display can appropriately enlarge the product panel area. For example, a three-piece 0.5-inch panel. The larger substrate area is more friendly to drive design, high current bearing, thermal power consumption, and larger scale LED crystals. It is conducive to the realization of the market application of high-brightness products.
JBD MicroLED 0.31 micro display pixel size 5 μm, used in AR glasses, HUD head-up display, embedded projection and light field display and other fields
At present, AR, as the first main target market for micro LED microdisplays, generally requires a single color of 0.5 inches or less, and three colors of 0.2 inches or less. This is an important reason for limiting the final luminance flux lumens (non-nit unit standard) of the product. For example, in 2023, Taiwan's Hon Hai Technology Group and Ennostar's Jingcheng Semiconductor jointly developed a new 0.12-inch microdisplay with a pixel density of more than 6,500 PPI and a blue light brightness of 200,000 nits. - Small size, high pixel density, suitable for AR applications, but also affects brightness levels.
The resolution is increased, and the needs of different applications are different
On the one hand, it is because there is no other microdisplay application on AR that can be as miniaturized as MICRO LED. For example, LCOS and DLP both require additional light sources, which increase the volume and lower electro-optical efficiency, and micro OLED is successful in VR, but for AR the pixel density is not enough and the resolution is limited.
The biggest advantage of Micro LED in AR is its size. The 6500ppi sample developed by Hon Hai and Ennostar mentioned above is an example. It is difficult for other microdisplays to achieve this pixel density and may require additional light sources.
However, even with 6500PPI and 4 micron pixels, if you experience Full HD video on AR, it is still "stretched". The good news is that AR, as an auxiliary hybrid display tool, focuses more on superimposing digital information when viewing reality, and has relatively low resolution requirements - AR can tolerate lower resolution, combined with micro LED high light efficiency, high brightness, and high resolution, forming a miniaturized volume advantage.
For VR or HUD and projector applications, micro LED displays face more challenges from competing technology products, and even the challenges of highly mature traditional technologies. However, the high resolution required by the latter three is still not the main bottleneck of micro LEDs. - On VR, HUD, and projectors, the bottleneck of micro LED micro display is brightness and cost. Micro LEDs currently have 4 micron pixels, which are similar to the 5.4 microns of DLP and the 3.X microns of LCOS. Micro LED pixel density has reached the current level of 3000-4000PPI typical for micro OLED (for example, the Apple Vision Pro features two 1.42-inch Micro OLED screens provided by Sony with more than 23 million pixels and a pixel density of 3386ppi).
However, in VR, micro LEDs can rely not only on high pixel density, but also on high brightness, which can be combined to achieve a smaller chip size. This is important for reducing product weight and chip costs in the future. IN THIS REGARD, MICRO OLED SEEMS TO THEORETICALLY HAVE A HIGHER PIXEL DENSITY, SUCH AS 10000PPI, BUT THE BRIGHTNESS LAGS BEHIND MICRO LEDS. This is also the reason why the industry is still optimistic that micro LED will eventually win AR and enter the VR market. On HUDs and projectors, micro LEDs may have a final chip cost equivalent to DLP or LCOS (because all three are complex functional integrations on silicon-based circuitry and are currently at the same level of pixel density), but the overall design does not require an additional light source – and thus the overall cost is lower.
Electro-optical efficiency and energy saving, the breakthrough direction of Micro LED
If the HUD or projector uses a micro LED micro display panel chip, then there is no need for a light source component. Even if the light source for projection and other light sources is high-efficiency such as LEDs and lasers, the energy consumption of micro LED microdisplays will be reduced by at least 60% at least 60% at the same imaging brightness because they do not require spectroscopic, time-sharing, or polarization processing during imaging.
On the other hand, how high the brightness of micro LED micro display can be has always been the main direction of industry research and development. For example, the efficient heat dissipation design is conducive to improving the carrying power of the system and achieving high luminous brightness. For example, micro LEDs overcome more technical bottlenecks and convert more electrical energy into light at the target frequency instead of heat, which more directly increases brightness and reduces thermal power levels. The latter is also the micro LED micro display, in addition to the massive integration process, another core exploration direction.
At present, the photoluminescence efficiency of quantum dots such as cadmium selenide and perovskite has exceeded 85%, which is higher than that of traditional rare earth phosphor materials, but the luminous efficiency of encapsulated LED devices is generally 100-200Lm/W. In this regard, there is room for improvement in material formulation, quantum efficiency, light extraction rate, light angle, etc. In particular, solving the problem of quantum performance degradation of red micro LED below 5 nanometers is a key bottleneck in the industry.
In 2023, there were media reports that the Korea Photonic Technology Research Institute (KOPTI) announced that it had successfully developed a high-efficiency Micro LED product with an internal quantum efficiency that can be maintained within the range of 90% and is not affected by the size of the chip and different injection current densities. - In this regard, the industry has made significant progress after 2020, and the brightness level of micro micro LEDs has increased by about 4 times. This means that the practical application of micro LED micro display has laid the foundation.
Of course, from the perspective of potential scenarios and projection displays that require the highest brightness of micro LEDs, micro LED micro displays still need to continue to improve. The mainstream consumer products of projectors have a brightness of 1000-3000 lumens. This is equivalent to a 0.5-inch three-chip color micro LED micro display chip panel, with tens of millions of nits of brightness. This is an order of magnitude gap from the current industry accumulation. In addition to the mass transfer manufacturing process, it is still important to improve brightness and electro-optical efficiency.
To sum up, the technological progress of micro LED microdisplay is very fast, but it is far from meeting the large-scale mature application of various styles of terminal products, and it also has many tests and bottlenecks such as process yield, brightness, core performance, and cost. As the "last technology choice" among the various microdisplay panel chips, it will take more time for micro LED to achieve positive results.