Simultaneous Transfer and Bonding (SITRAB) Process for Micro-LEDs Using Laser-Assisted Bonding with Compression (LABC) Process and SITRAB Adhesive

The transfer process of moving individual micro-LED chips to a display substrate has been regarded as the biggest technical obstacle for the commercialization of displays based on micro-LEDs. To date, various candidate technologies have not been able to meet the requirements of the transfer process in terms of productivity, yield, capability, known good die compatibility, and cost. In addition, after the transfer process, bonding between the micro-LED and the display substrate is typically accomplished through a process different from the transfer process, which also requires a technical solution. Generally, the bonding process proceeds sequentially after the mass transfer. Therefore, to increase the productivity, yield, capability, and compatibility of both processes, a new method is needed that considers the characteristics of both processes. In this work, we have successfully developed a novel process in which transfer and bonding occur at the same time. This process was named SITRAB, standing for SImultaneous TRAnsfer and Bonding. The SITRAB process was developed by applying our proprietary material called SITRAB adhesive to the LABC (Laser-Assisted Bonding with Compression) process, which we proposed several years ago. The total process time of SITAB was less than 10 s. Its working area could be easily enlarged, according to the homogenized area of the LABC equipment. Because the LABC process uses an IR laser, it is much cheaper than processes that require UV laser equipment. The major features of the STRAB adhesive are that it is fume free, and it is compatible with the LABC and repair process. The adhesive used during the SITRAB process is not cured, so additional transfer and bonding is possible with the applied adhesive on the substrate. A display substrate with a 35×35 micro-LED array was developed using the SITRAB process and the adhesive. The size of the micro LED-chip was about 80 μm× 130 μm, for which the conventional pick-and-place process could not be applied. Two types of solder materials were adopted and proven to be applicable. The repair process was confirmed based on SITRAB technology.