Fabrication and bonding of bumps on 3175 PPI AlGaInP-based red Micro-LED and its CMOS driver

Journal article, 2024

In recent years, people have increasingly demanded both the quantity and quality of information conveyed by display technology. Therefore, micro light emitting diode (Micro-LED) display technology is crucial in facilitating immersive interaction between individuals and information, such as in augmented reality (AR) and virtual reality (VR), due to its advantages of high contrast, high brightness, long lifespan, and low power consumption. However, to excel as a widely used display technology, an unavoidable topic is achieving its full-color technology. As a popular research object for red LED luminescent materials, AlGaInP has certain research value in realizing the full-color technology of Micro-LED. Due to its lattice matching with GaAs, it is frequently grown on GaAs substrates. Nevertheless, the GaAs substrate’s light absorption for AlGaInP will result in a diminished light extraction efficiency for red light. Hence, it is worthwhile to investigate the resolution to the issue of enhancing the light extraction efficiency of AlGaInP red Micro-LED. Simultaneously, the high-speed response and small size of the complementary metal oxide semiconductor (CMOS) chip enable the Micro-LED driven by it to achieve higher performance and higher pixel density. However, in reports on the production process of high pixel density Micro-LED display screens, the emphasis is typically placed on the production of Micro-LED chips, while the fabrication of bumps and the bonding process on CMOS chips receives less attention. Whether the process on the CMOS chip can be done well is also the key to the industrialization of Micro-LED displays in the future. In order to further promote the development of Micro-LED display technology, this paper proposed and optimized a scheme to transfer the red Micro-LED chip to the CMOS driver and bond with it. Initially, the red Micro-LED chip with equipped cathode and anode electrodes was transferred from the GaAs substrate to the sapphire substrate using benzocyclobutene (BCB) glue, which avoided the absorption of red light by the GaAs substrate and enhanced the hardness of the substrate. Subsequently, indium bumps were fabricated on the red Micro-LED chip following the transfer of the substrate. Furthermore, various processing techniques were employed for the etching of the surface passivation layer of the CMOS chip, depending on the thickness of the passivation layers. When etching a thicker passivation layer, an additional inductively coupled plasma (ICP) etching step was introduced to replace the oxygen plasma cleaning, which resolved the issue of residual photoresist in lithography and the problem of increased aperture size caused by oxygen plasma cleaning. Ultimately, we fabricated metal bumps on a CMOS chip measuring 2.23 μm in height. The chip itself has a size of 0.7 inches, a pixel pitch of 8 μm, a resolution of 1920×1080 and a pixel density of 3175 ppi. Afterwards, we bonded the CMOS chip to the red Micro-LED chip with indium bumps. Finally, it can successfully drive the Micro-LED chip to exhibit the specified characters through the CMOS chip. This work is of great significance for the fabrication of bumps and the bonding process of CMOS driver with high pixel density and high resolution, as well as the production of red Micro-LED displays.