A Heterogeneously Integrated, High Resolution and Flexible Inorganic μLED Display using Fan-Out Wafer-Level Packaging

In this work, a die first Fan-Out Wafer-Level Packaging (FOWLP) process called FlexTrate (TM) is used to heterogeneously integrate GaN blue mu LEDs of dimension < 100 mu m in a matrix array configuration along with Si dielets seamlessly on biocompatible Poly Di-Methyl Siloxane (PDMS) substrates. The GaN mu LEDs, prefabricated on c-plane sapphire substrates, are released using Laser Lift-Off (LLO) with a Diode Pumped Solid State (DPSS) laser source and assembled on the flexible substrate using a novel high yield (> 99%) mass transfer technique based on thermoplastic adhesive bonding. A nearly 100% LLO yield was achieved by the use of an electroplated Ni stress buffer on top of the mu LEDs. The assembled mu LED matrix array on PDMS has a pixel resolution of > 200 PPI. Electrical characterization of the mu LEDs at different process stages indicates no significant degradation in operation. The developed mu LED mass transfer and heterogeneous assembly process on PDMS could pave the way for manufacturing soft, conformable displays for wearable and implantable applications.