High yield electrical connections to micro-LEDs via the chip-first OTFT backplane process

Fabrication of micro-LED displays with high yield and a minimal requirement for re-work is an important pre-condition for successful commercialisation of the technology. Economic production of micro-LED displays also requires the die size to be reduced to dimensions much smaller than has been used for mini-LED displays. This size reduction presents challenges in the attachment of the microscopic dies to active matrix driving systems. Eutectic bonding of micro-LEDs to pads on the backplane commonly requires low melting point metals to be used such as indium or tin, with processing of these materials by difficult to scale processes of thermal evaporation and lift-off. An alternative to eutectic bonding is presented here which reverses the fabrication process, starting with the micro-LED chip first where the anode/cathode pads of the flip-chip device are facing upwards. A planarization layer of organic polymer is formulated and coated on top of the die to provide a level surface and then patterned to form via holes over the pads. Metallization down the vias by sputtering then create ohmic contacts to the anode/cathode pads. This process has been shown to have extremely high die light on yields due to the conventional via process used to make the connection. Active matrix driving of chip-first connected displays requires the use of TFT with low processing temperatures. This is due to the micro-LED sensitivity to elevated temperatures
potentially caused by migration of metal atoms into the quantum well structures. Organic Thin-Film Transistors (OTFT) can be processed at <150°C and has been shown to be a suitable TFT technology for driving micro-LED. This presentation describes the potential for micro-LED device manufacturing using the chip-first approach and the types of display that OTFT may be suitable for in first-generation products.