Process optimization of preparing CMOS backplane for ultrahigh pixel density red Micro-LED display

Abstract

In this paper, an AlGaInP-based red Micro-LED display measured 17.78 mm (0.7 in), with a resolution of 1920 x 1080, a light-emitting mesa size of 6 μm, a pixel pitch of 8 μm and a pixel density of 3175 PPI was designed and fabricated with a CMOS driver backplane. The metal bump preparation technology of the CMOS driver backplane was optimized to enhance the bonding yield and create an optimal display effect. Improper sizing of the etched window in the SiO2 insulation and passivation layer can have a detrimental impact on the metal bump preparation and subsequent bonding process. By optimizing the settings of lithography and dry etching, the appropriate size of the etched aperture in the passivation layer was achieved. The high density, small size, and large aspect ratio of the photoresist openings for the bump fabrication made it challenging to remove the photoresist following the metal evaporation. To successfully remove the photoresist, it is important to carefully choose suitable experimental conditions for the removal. Afterwards, an 8 μm AlGaInP-based red Micro-LED display was effectively integrated with CMOS using flip-chip bonding technology. This work may be of reference value to those who work on ultrahigh density red Micro-LEDs that is challenging but crucial for future full color micro displays.