Superb sinterability of the Cu paste consisting of bimodal size distribution Cu nanoparticles for low-temperature and pressureless sintering of large-area die attachment and the sintering mechanism

Pressureless low temperature Cu-Cu bonding by sintering Cu nanoparticle (NP) paste is a promising method to realize die attachment in power electronics and third-generation semiconductor devices. However, the low reliability of sintered Cu paste joints due to poor sintering microstructure and processing defects is a challenging and urgent issue to be solved. The present work develops a novel Cu paste consisting of bimodal-size Cu NPs with special wrapping structure by means of a one-step method and use of reducing hybrid solvents. The sintered Cu paste matrix shows relatively dense sintered microstructure, despite using mild process condition of pressureless low temperature sintering, and sintered Cu-Cu joints in large-area dummy die attachment exhibit high shear strength up to 29.5 MPa after pressureless sintering at 280 degrees C for 10 min in N-2 atmosphere. High strength of joints is ascribed to the strengthening effect of bulky Cu phase formed in sintered microstructures. After thermal aging tests, the strength of joints is increased to over 40MPa, indicating exceptional long-term reliability The bimodal size Cu NP paste is capable of sintering by adopting relatively mild process yet endows sintered Cu-Cu joints with robust reliability, thus exhibits a broad application prospect in the packaging field of high power electronics.