28 April 2021
Sundiode develops stacked RGB micro-LED pixel devices on a single wafer
Sundiode Inc of Campbell, CA, USA, a Silicon Valley-based company developing micro-LED technologies for display applications including augmented reality (AR) and mixed reality (MR), says that it has achieved fully stacked three-color (RGB) micro-LED pixel devices on a single wafer.
In the stacked-RGB pixel technology patented by Sundiode and developed in collaboration with KOPTI (Korea Photonics Technology Institute), a single pixel features three independently controlled micro-LED subpixels that are stacked vertically to allow full-color emission from essentially the entire area of the pixel. This pixel technology results in a very compact pixel structure and a substantial reduction in the pixel-transfer processing requirement for micro-LED display fabrication, says Sundiode. In addition, the operation of a full-color micro-display consisting of a pixel-array typically sized smaller than a penny is significantly enhanced due to increased utilization of the extremely small pixel area, adds the firm.
Graphic: A diced array of stacked-RGB pixels (left most). R, G and B subpixels of three adjacent pixels of an array with each subpixel lit up separately (round-robin colors, middle three), and the subpixels of the center pixel lit up simultaneously to 5400K white (right most).
The stacked-RGB pixel technology is said to overcome a particularly difficult obstacle to the mass commercialization of micro-LED displays, i.e. a need for laborious pixel-transfer processing. Whereas fabricating a micro-LED display using conventional planar-RGB technologies typically requires transferring discrete R, G and B subpixels using a pick-and-place process, the stacked-RGB pixel technology substantially or even entirely removes such a requirement.
Additionally, flexible in size, Sundiode’s stacked-RGB pixels are suitable for a wide variety of applications ranging from TVs to automobiles to micro-displays. They are especially well suited for applications such as AR and MR requiring high pixel density and precision miniaturization of displays, because an entire array of the devices can be fabricated on a single chip, notes Sundiode. This means that, for micro-displays, an array of the pixels is ready for direct and immediate integration onto a display backplane and, notably, no pixel-transfer processes are required with the stacked-RGB pixel technology.
Graphic: Measured spectrum of a single pixel with all R, G and B subpixels lit up simultaneously to yield nearly the same peak intensity and color space rendered by a single pixel.
To achieve the stacked-RGB pixel device, epitaxial and fabrication technologies were developed on sapphire substrates that allow the stacking of multiple LED junctions where each LED is independently controlled. This multi-junction-LED technology is pivotal in enabling full-color generation, says Sundiode. The well-defined spectral peaks of the three colors lend themselves to excellent color-saturation characteristics and thus a very large color space, the firm adds.
Sundiode plans further development to soon demonstrate a micro-display using the stacked-RGB pixel technology on a silicon CMOS backplane.