Electrodeposited Copper Recrystallization Mechanics in High Density Circuit Stackups
Thermal cycles in high density stackups drive copper recrystallization, causing grain growth and vacancy coalescence that trigger microvia interface failures.
Bamboo Microstructure
Morphological State
Crystalline arrangement where the individual grains of a metal interconnect span the entire width or thickness of the conductive path. A bamboo microstructure typically forms in narrow copper traces or vias when the grain size becomes comparable to the feature dimensions. This geometry eliminates grain boundaries that run parallel to the direction of current flow.
It leaves only transverse boundaries that resemble the joints in a bamboo stalk.
Conductive Reliability
The formation of this specific grain layout inhibits the movement of atoms along grain boundaries during high current density operation. In conventional fine grained copper, the network of boundaries provides a high diffusivity path for mass transport under electrical stress. Because a bamboo microstructure lacks these continuous longitudinal paths, the rate of electromigration decreases.
This leads to a longer mean time to failure for narrow interconnects in power delivery circuits. The transverse boundaries still allow for some diffusion but the overall flux of metal atoms is much lower than in polycrystalline structures.
Thermal Annealing
Control of the grain growth process during fabrication determines the final grain size. If the annealing temperature or duration is sufficient, grains will grow until they impinge on the sidewalls of the trace.