Modern integrated circuits are made up of literally millions of active and/or passive devices such as transistors and capacitors. These devices are initially isolated from each other, but are later interconnected together to form functional circuits. Typical interconnect structures include lateral interconnections, such as metal lines (wirings), and vertical interconnections, such as vias and contacts. Interconnections are increasingly determining the limits of performance and the density of modern integrated circuits. On top of the interconnect structures, bond pads are formed and exposed on the surface of the respective chip. Electrical connections are made through bond pads to connect the chip to a package substrate or another die. Bond pads can be used for wire bonding or flip-chip bonding. In a typical bumping process, interconnect structures are formed on metallization layers, followed by the formation of under-bump metallization (UBM) and solder balls. Flip-chip packaging utilizes bumps to establish electrical contact between a chip's I/O pads and the substrate or lead frame of the package. Structurally, a bump actually contains the bump itself and the UBM located between the bump and an I/O pad. An UBM generally contains an adhesion layer, a barrier layer and a wetting layer, arranged in that order, on the I/O pad. The bumps themselves, based on the material used, are classified as solder bumps, gold bumps, copper pillar bumps and bumps with mixed metals. Usually, a material used for the solder bump is so-called Sn—Pb eutectic solder. Recently the semiconductor industry has been moving to “lead (Pb) free” packaging and lead-free device connector technology. This trend increasingly results in the use of lead free solder bumps and lead free solder balls to form connections with integrated circuits and packages. The use of lead free solder is safer for the environment, safer for workers in the industry and safer for consumers than lead based solder bumps or solder balls. Failure analysis revealed that Electromigration (EM) is a reliability concern in flip chip solder joints, which is initiated by the formation of intermetallic compounds (IMC) at the interface between UBM and solder alloys. Under EM, the continued growth of IMC accelerates the dissolution of UBM materials into solder alloys. This rapid dissolution and migration will cause an open circuit failure and interfacial cracks, which will degrade the joint strength and device lifetime.
USPTO Backgrounds
USPTO Backgrounds