High resolution 3600ppi full color Silicon Display for AR glasses and HMD

HMDs and AR glasses are expected to be the next generation communication devices to replace smartphones. There are many prototypes and early products using several display devices. Various display devices have been proposed, including LCD, micro OLED, Laser Beam Scan (LBS), LED LCOS, and Laser LCOS. LCD is one of the major display device for VR HMDs, however, it is very heavy and has a limitation to pixel density. Furthermore, although the laser-based display devices are compact and can achieve high brightness, the image quality of the display is not excellent because of speckle noise, one of the specific issue of laser-based display. Micro OLED and LED LCOS are at a high level of technological maturity and widely used to HMDs, but their brightness is not enough to the outdoor AR. Micro LED has been attracting a lot of attention as a display device to solve disadvantages in other display devices.
We have developed full color micro LED, "Silicon-Display", and demonstrated the first prototype with 1,053 ppi. Figure 1 shows the process-flow of Silicon Display, RGB full-color micro LEDs using color conversion. Blue micro LEDs are formed on a sapphire substrate and one LED array contains 352 x 198 micro LED dies of 24 um x 8 um in size. The cathode (N-type electrode) and anode (P-type electrode) are fabricated for each micro LED die to apply driving voltage independently to each die. LSI with the circuit driving the LEDs is fabricated on Si wafer. The Au bump electrodes are fabricated in accordance with the pitch of the LED dies. LED and LSI chips are divided into chips, and then LED chip is bonded to LSI chip with Au electrodes. After that, the sapphire substrate of the LED is removed by laser lift-off, resulting in a blue monochromatic micro LED display. QDs (Quantum Dots) are patterned on LED dies to generate red and green emission.
Figure 2 shows a schematic diagram of a pixel structure including RGB sub-pixels. To convert blue emission from LEDs to green and red, QDs are fabricated. Color filters are fabricated on the top surface of the QDs to improve color reproducibility. Light shielding walls (LSWs) are also fabricated to prevent optical cross talk.
We developed the 1,053 ppi prototype, however, there is a strong demand for higher resolution for the practical AR application. Therefore, we have been working on a 3,600 ppi prototype. Figure 3 shows the difference between 1,053 ppi and 3,600 ppi. The small size of micro LED dies makes brightness low due to small active emission area. To solve this problem, we designed and applied the common cathode structure. The area of micro LEDs contributing to light emission in one pixel was improved from 23% to 38%. As a result, brightness of 11 knits was achieved.
The current brightness is not sufficient for outdoor AR applications. We plan to improve the brightness furthermore by improving the QD performance and the LSW structure.