Interface Structure
A thin-film metal layer stack sits between the microchip bond pad and the solder flip-chip interconnect to provide electrical connection. This under-bump metallization structure ensures a low-resistance contact while protecting the delicate aluminum or copper wiring underneath from solder contamination.
Metallurgical Barrier
A barrier metal within the stack blocks the diffusion of tin from the solder bump into the underlying pad metallization during reflow and operation. This under-bump metallization layer prevents the formation of brittle intermetallic compounds that cause structural weakness and failure. Nickel or titanium-tungsten is commonly selected to perform this diffusion-blocking function.
Mechanical Support
Adhesion layers at the base of the metal stack bind strongly to both the silicon dioxide passivation and the bond pad metal. Without this robust under-bump metallization layer, the solder bump would easily peel off the wafer under mechanical or thermal stresses. The metal stack distributes the physical loads and prevents the propagation of cracks into the active silicon.
Processing Method
Physical vapor deposition or electroplating applies the sequential layers of metals across the entire wafer surface before photolithographic patterning defines the bump sites. Wet etching or lift-off processes then remove the excess metal between the bond pads. This sub-micron fabrication process requires precise thickness control to ensure uniform bump height and reliable electrical contact across the thousands of interconnects on a single high-density semiconductor die.
