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™ is used to heterogeneously integrate GaN blue μLEDs of dimension <; 100 μm in a matrix array configuration along with Si dielets seamlessly on biocompatible Poly Di-Methyl Siloxane (PDMS) substrates. The GaN μLEDs, 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 μLEDs. The assembled μLED matrix array on PDMS has a pixel resolution of > 200 PPI. Electrical characterization of the μLEDs at different process stages indicates no significant degradation in operation. The developed μLED mass transfer and heterogeneous assembly process on PDMS could pave the way for manufacturing soft, conformable displays for wearable and implantable applications.