Fabrication and Reflow of Indium Bumps for Active-Matrix Micro-LED Display of 3175 PPI

Abstract

Indium is acknowledged as a preferred material for micro-light-emitting diodes (micro-LEDs) flip-chip bonding within the industry, due to its favorable economic characteristics and low melting point. However, indium bumps fabricated via photolithography and thermal evaporation often exhibit irregular shapes and varying heights, and they readily oxidize in air to form a tightly adhered oxide layer (In₂O₃), leading to flip-chip bonding failures and blind pixels. The reflow process can not only remove the oxide layer but also enhance bump uniformity. Nevertheless, literature on indium reflow predominantly focuses on planar substrates, with limited studies on high-resolution micro-LED chips for flip-chip bonding. This paper details the preparation of micro-LED chips with a pixel density (pixel per inch, PPI) of 3175. An indium bump array with a diameter of approximately 5 μm was prepared on the micro-LED chips using thermal evaporation technology. The influence of reflow time and temperature on indium bumps was thoroughly investigated by the formic acid reflow process, revealing that under the conditions of 270 °C and 180 s, the indium bumps with a narrower size distribution could be reflowed into spherical shapes on the micro-LED structure. Furthermore, an inversely proportional relationship was discovered between mesa/metal layer height and indium bump growth, which influenced the reflow effect. Ultimately, micro-LED chips were integrated with si complementary metal–oxide–semiconductor (CMOS) driver chips through flip-chip bonding technology, resulting in the successful functioning of the devices.