Cu/Cu direct bonding by metal salt generation bonding technique with organic acid and its persistence of reformed layer

In this study, the effect of metal salt generation bonding method on the bonded strength of the solid-state direct bonded interface of copper was investigated. The specimen to be bonded was a block 15 mm × 15 mm × 5 mm cut from 99.9% ingot and a wire (99.9% purity) dimension of φ 1.2 mm. Copper surfaces were modified by boiling in several types of organic acid for predetermined time. Solid-state bonding was performed in a vacuum chamber at bonding temperature of 423-673 K under a pressure of 588 N (bonding time of 0.9 ks). As a result of metal salt generation bonding method, bonded joints were obtained at a bonding temperature 150 K (formic acid) and 100 K (citric acid) lower than that required for non-modified surfaces, and the bond strength was comparable to that of the maximum load. In addition, the durability of the modification effect was investigated by leaving the modified surface to an air atmosphere furnace which kept at 323 K. After the shelf test, solid-state bonding was performed. As a result of peel test, with increase of the shelf time, bond strength was not decreased even for 168 hours when it modified with citric acid, but bond strength decreased in several hours when it modified with formic acid.