Sintering of oxide-free copper pastes for the attachment of SiC power devices

The introduction of wide-band-gap semiconductors such as silicon carbide (SiC) in power electronic devices has allowed operation temperatures beyond 300°C. However, these high operational temperatures are not suitable for traditional die-attach materials such as solder. Therefore, a bonding material that is stable beyond 300°C, with high electrical and thermal conductivities and low cost is essential. In this regard, copper (Cu) nanoparticle-based pastes are promising candidates due to their stability at high temperatures and elevated thermal and electrical conductivities. Furthermore, the recent introduction of oxide-free Cu pastes based on amine-passivated Cu nanoparticles enables these materials to be processed in inert atmosphere.We report here on attaching SiC dies to direct-bonded copper ceramic substrates by sintering oxide-free Cu pastes. Applying bonding pressure during sintering improves the die-attach quality by reducing the percentage of residual voids and increasing shear strength. We succeeded in further reducing the percentage of voids by using a paste containing a solvent with a low boiling point and high vapor pressure. Furthermore, we developed a novel dual-layer process to deposit Cu paste that yields residual percentages of voids in the die attach as low as 5% and a shear strength of 29 MPa. Finally, the reliability of the die attach by oxide-free Cu paste was investigated with thermal shock cycling.