Development of the micro-vacuum transfer printing for flexible microLED display

Abstract

Inorganic-based light emitting diodes have been spotlighted as light source for next generation displays because of their superior optical properties, fast response time, high stability, and great power efficiency compared to liquid crystal displays and organic light emitting diodes, which have been mainly used as light sources for display applications. In particular, micro-light emitting diodes ($\mu$LED), having a chip size of 100 $\mu$m or less can be utilized as a light source for various flexible and wearable optoelectronic devices such as skin display, biomedical sensors, based on their excellent flexibility, in addition to superior optical and electrical characteristics compared to those of general inorganic-based light emitting diodes. In order to fabricate flexible $\mu$LED, transfer process is essential, which includes picking up the chips from the brittle mother substrate on which the LED epitaxial layer is grown, transferring and electrically connecting the chips to the flexible target substrate. In this reason, various transfer processes have been actively studied and proposed to date, such as transfer by adhesion control of polymeric stamps, electrostatic and electromagnetic forces, lasers assisted transfer. However these transfer techniques still have several disadvantages, including high cost, long process time, low yield, and alignment error and chip damages. In this study, a novel transfer technology using vacuum suction force is developed based on micro-mechanical system (MEMS) process. The flexible vertical red $\mu$LED was fabricated by exfoliating the LED chips of 80 $\mu$m size from the GaAs mother substrate via vacuum suction force, transferring and electrically connecting to flexible target substrate via anisotropic conductive film (ACF). This device exhibited the same electrical and optical properties compared to the device fabricated on the mother substrate, and excellent mechanical stability. Therefore, it was proved that the transfer technology using the suction force of vacuum presented in this study can be applied to the fabrication process of the flexible $\mu$LED display.