Electrodeposited Nanowires for High Density Interconnects of GaN-Based MicroLEDs

Abstract

For next-generation applications, fully controllable gallium nitride (GaN)-based micro light emitting diode (microLED) arrays will play a crucial role. These feature high pixel densities, high intensities, and high efficiencies. Well-established top-down etching processing schemes of pristine LED wafers allow for an easily accessible and scalable fabrication approach for microLED arrays down to the single digit µm dimensions with dies featuring several 100,000 on a square centimeter. At this scale, classical bonding approaches like wire bonding fail to harvest the true potential of such an array due to inherit technological restrictions. To allow for the individual controllability of every pixel a highly scalable integration approach to a CMOS backplane is necessary. Established processes for the realization of such an integration often feature major drawbacks. Indium bumps deposited via thermal evaporation generally fulfill the scale requirements but fail to adapt to the unique challenges of such hybrid integration. High non-planarity of the substrate, high wafer bow, and mismatching thermal expansion coefficients can lead to diminishing bonding yields and hence functioning microLEDs during the high-temperature bonding step necessary for indium bumps. Additionally, Indium is an inferior electrical and thermal conductor to other metals and limits the active performance of the device while also restricting its usage area due to its low melting point. In the currently ongoing EU Project SMILE, we are developing GaN-based microLED arrays up to 512 x 512 pixels and thoroughly examine established bonding technologies as well as an in-house developed metal nanowire approach. The necessary perquisites for electrochemical deposition, namely the conductive seed and the removal thereof can easily be integrated into the device’s processing scheme. Nanowires are electrochemically deposited out of a liquid solution at room temperature onto the microLED array and the CMOS backplane. The direction, length, and diameter of the nanowires can easily be controlled to accustom different proportions and bonding geometries. Inline monitoring via the recorded amperogram allows for the differentiation of the individual stages of nanowire growth during the electrochemical deposition. Mechanically stable and electrically sufficient interconnects can then be realized by pressing the nanowires of both substrates into each other. The high aspect ratio of the wires results in a high surface area and allows for nanowires of both substrates to intertwine forming stable connections with large contact surfaces at low pressures without any additional heating steps necessary. Due to the variability of the wire length and ongoing intertwining during the bonding, non-planarities and wafer bow can be accommodated. This flexible bonding behavior is a unique advantage of bonds based on metal nanowires. The junctions approach the conductivity of bulk material and are crucial for the thermal management of the microLED array. The final LED device generates a substantial amount of heat, which cannot be cooled meaningfully via convection to the surrounding air or by its radiation. Consequently, a capable thermal conduction towards the silicon chip is necessary for heat management. Here the metal nanowires play a crucial role in providing this thermal connection via the bonding interface, which potentially allows for improved device performance over other bonding technologies.