Silver Adhesive Layer for Enhanced Pressure-Free Bonding Using Copper Nanoparticles

Pressure-free Cu nanoparticle bonding between two Cu plates with an Ag adhesive layer was examined. Insertion of the Ag adhesive layer considerably enhanced the bonding strength at firing temperatures between 523 K and 673 K. The bonding strength generally increased with the firing temperature. The strength enhancement of the Ag adhesive layer was observed even for a very thin (3 nm) Ag layer, and there was no obvious dependence of the thickness of the Ag layer on the bonding strength for Ag layers of thickness up to 200 nm. Ag atoms from the adhesive layer diffused away to the bonding layer with an increase in the firing temperature. The elemental mapping images showed that the Ag had two morphologies: thin Ag layers existing between particulate Cu grains, and fine Ag particles dispersed in coarse Cu crystals. The microstructure near the interface between the Cu nanoparticle bonding layer and Cu plate used as the substrate suggests that the enhancement effect of the Ag layer originates in the active migration of the Ag layer itself.