Flexible, Heterogeneously Integrated microLED Displays in Elastomeric Substrates using Fan-Out Wafer-Level Packaging
In recent years there has been a growing demand to develop next-generation flexible displays that can achieve higher brightness, dynamic range, and contrast ratios compared to LCDs or OLEDs. This demand is driven by new application spaces such as wearable Augmented Reality (AR), conformable vehicular Head UP Displays (HUDs), and emerging medical fields such as Optogenetics, where implanted high brightness displays are used to stimulate biological cells such as neurons. The only display technology that can meet these requirements is flexible microLED displays. microLEDs are light-emitting devices made from compound semiconductor materials like GaN or InP that have demonstrated unparalleled brightness (>106 cd/m2), color quality, response times (in ns range), and lifetime (>100,000 hours). The task of fabricating flexible microLED displays has, however, proven to be complicated. Inorganic microLEDs cannot be fabricated directly on flexible organic substrates due to the high processing temperatures and lattice matching considerations. Instead, they are fabricated on a growth substrate, released from it, and then assembled onto a target flexible substrate using a massively parallel transfer process, simply called mass transfer. However, flexible microLED display technology has not picked up due to (1) complex and expensive mass transfer processes that suffer from yield issues & (2) primitive flexible electronic integration approaches that use coarse interconnects and are not well suited to the heterogeneous integration of micron-sized LEDs. To overcome these challenges, we have developed a novel microLED mass transfer process based on thermoplastic adhesive (HD3007) bonding that is much simpler to implement, low cost, and can potentially attain many high yields (> 99%) and panel-level scalability. We also use a novel Fan-Out Wafer-Level Packaging (FOWLP) technology called FlexTrateTM to heterogeneously integrate 50 X 100�m2 blue InGaN/GaN microLEDs with Si CMOS display driver ICs at < 40�m interconnect pitch to demonstrate a high density, functional, high resolution (> 150PPI) flexible microLED display. Detailed analytic and experimental studies of the various process steps, especially the Laser Lift-Off (LLO) process that is used to release GaN microLEDs from the growth substrate, is conducted in this thesis.