A single pixel features three independently controlled micro-LED subpixels that are stacked vertically

Sundiode, a Silicon Valley based company developing micro-LED technologies for display applications including AR and MR, has announced the achievement of fully stacked 3-colour (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-colour 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. In addition, the operation of a full-colour micro-display consisting of a pixel-array typically sized smaller than a penny is significantly enhanced due to increased utilisation of the extremely small pixel area.

The picture above shows 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 colours, middle three), and the subpixels of the center pixel lit up simultaneously to 5400K white (right most).

The stacked-RGB pixel technology overcomes a particularly difficult obstacle to mass commercialisation 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 attractive for a wide variety of applications ranging from TV to automobile to micro-display, and 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. This means, for micro-displays, that 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.

To achieve the stacked-RGB pixel device, epitaxial and fabrication technologies were developed on sapphire substrates that allow stacking multiple LED junctions where each LED is independently controlled. This breakthrough multi-junction-LED technology is pivotal in enabling the full-colour generation. The well-defined spectral peaks of the three colours lend themselves to excellent colour-saturation characteristics and thus a very large colour space.

A measured spectrum of a single pixel with all R, G, and B subpixels lit up simultaneously to yield nearly the same peak intensity and colour space rendered by a single pixel.

Sundiode plans further development to shortly demonstrate a micro-display using the stacked-RGB pixel technology on a Si CMOS backplane.