Novel copper particle paste with self-reduction and self-protection characteristics for die attachment of power semiconductor under a nitrogen atmosphere

A novel die-attach material, Cu particle paste with self-reduction and self-protection characteristics, was designed by simply adding ascorbic acid (AA) into Cu paste. The self-reduction characteristic is due to the addition of AA into the paste, as it reduces the Cu oxide layer on the metal Cu, even at room temperature. The self-protection characteristic is due to the decomposition of AA preventing further oxidation during sintering process. These characteristics are beneficial for the sintering of Cu particles and for enhancing the bondability. The increase to bonding strength is the direct result of the concentration of AA in the Cu paste. The bonding strength of Cu joints sintered from Cu paste with 1.3 wt% AA addition was as high as 24.8 MPa, thus making it attractive as a die-attach material. On the other hand, the bonding strength of those sintered from Cu paste without AA addition was only 9.7 MPa. These results strongly suggest that the self-reduction and self-protection characteristics resulting from AA addition are necessary and crucial for sintered joints fabricated from readily oxidized Cu particle paste. This novel Cu paste is an efficient, safe, and environmentally-friendly die attachment material, especially well-suited for applications in high power semiconductor devices.