Superior strength and strengthening mechanism of die attachment joints by using bimodal-sized Cu nanoparticle paste capable of low-temperature pressureless sintering

Recently, there has seen an increasing demand for reliable and cost-effective die attachment materials to be used in power electronics and third-generation semiconductors. The present work develops a facile yet efficient approach to fabrication of bimodal-sized Cu nanoparticles (NPs) by a one-step method and the Cu NP paste capable of low-temperature pressureless sintering for die attachment. The synthesized Cu NPs exhibit a special wrapping conformation of large Cu NPs (similar to 160 nm) surrounded by a large number of small ones (similar to 9 nm). The special wrapped structure of Cu NPs is heritable to Cu NP paste, which greatly increases initial density of Cu NPs and enables the paste to be sintered completely at low temperature without pressure. Sintered dummy die joints show very high shear strength of 48 and 54 MPa, respectively, after pressureless sintering at 260 and 280 degrees C for 20 min in nitrogen, which is attributed to the strengthening effect of sintered microstructures featured by a large amount of bulky Cu phase formed as a result of the heritable effect of large Cu particles in bimodal-sized Cu NPs. This pressureless sinterable Cu NP paste shows a great potential for use in die attachment of power electronics.