Electroplating of Cu/Sn bumps with ultrafine pitch and high uniformity for micro-LED interconnection

Electroplating of metal bumps is a promising method for the integration of micro-light-emitting diodes (Micro-LED) due to its advantages of fast deposition speed, low cost, and suitability for the fabrication of ultra-fine-pitch bumps. In this study, we report the fabrication of ultra-fine pitch and large-scale Cu/Sn bumps by electroplating method. The effects of cathode-anode distance, plating solution agitation rate and current density on the morphology, microstructure and uniformity of electroplated Sn bumps were systematically investigated. The results show that the distance between the cathode and anode would affect the current distribution during the plating process. Appropriate stirring of the plating solution during electroplating enhances convective mass transfer efficiency. This aids in the timely replenishment of solute consumption at the cathode interface, thereby improving the quality of the bumps. However, excessive stirring velocity may cause the surface of the Sn bumps to become rough. The uniformity and top surface morphology of Sn and Cu/Sn bumps could be controlled by adjusting the current density which appropriately can enhance the polarization of the cathode and also improve the plating efficiency. Finally, Cu/Sn bumps with ultra-fine pitch of 8 mu m, pixel array of 1920 x 1080 and with smooth morphology and good uniformity were achieved. This fabrication strategy of Cu/Sn bumps is beneficial for the interconnection of Micro-LED display.