Low-Temperature Bonding of Porous Cu Bumps With a 1.5- μ m Fine Pitch

This article reports a method for bonding Cu bumps with a 1.5-$\mu $m fine pitch by using porous Cu. The porous Cu is fabricated as a capping layer by selectively removing Sn elements from Cu–Sn intermetallic compounds (IMCs) of Cu–Sn bumps. Under a bonding force, the porous Cu can deform so that it can adapt to bump height variations, avoiding the use of chemical mechanical polishing (CMP). This allows bonding of standalone Cu bumps with fine pitches by avoiding the friction in CMP that may cause collapse of small and standalone bumps without lateral support of dielectrics. The high surface energies and (111) crystal orientation of porous Cu facilitate low-temperature direct bonding. These features relax the pretreatment and bonding conditions and allow bonding of porous Cu bumps with a pitch of$1.5~\mu $m at$200~^{\circ }$C and 10MPa. Test results show a void-free bonding interface with high strength, low resistance, and good reliability.