Improved low temperature gold-gold bonding using nanoporous powder bump using vacuum ultraviolet irradiation pre-treatment

This paper describes a novel low temperature Au-Au bonding method using nanoporous Au-Ag powder and vacuum ultraviolet irradiation in the presence of oxygen gas (VUV/O₃) pre-treatment. The nanoporous powder, which was fabricated by dealloying Ag-Au alloy sheet, was used to form the bump structure on the Au substrate by simple filing process, while an Au-coated Si substrate was used as the chip. The VUV/O₃ treated bumps and chip was bonded under a bonding pressure of 20 MPa at 200 °C for 20 min in a vacuum atmosphere of 1 kPa. A ligament size of the nanoporous structure on powder surface was found to be grown dramatically during bonding process. The tensile strength reached 10.1 MPa which is 2.3 times higher than that without VUV/O₃ treatment. This suggests that organic contaminants on the each ligament surface were effectively removed by VUV/O₃ treatment, and consequently, the diffusion of gold atoms in the nanoporous powder was significantly promoted to change into bulk structure. The proposed method will be highly a promising method for 3D-LSI and MEMS packaging.