A sunken phase in aluminum-copper interconnects as a new kind of stress void

A new kind of stress void in Al-1.5% Cu interconnects, a sunken phase (or depleted region), was noticed as an intermediate phase which evolved to a complete void by further diffusion during later processes. It has a unique hexagonal shape with a flat bottom plane and was formed not only at grain boundaries but inside grains. The formation of sunken phases is dominant on large metal pads and with tensile interlayer dielectrics (ILDs) such as ozone-PE-TEOS glass and spin-on-glass (SOG). The evolution sequence from a circular phase, a precursor of a sunken phase, to a complete void is described based upon microstructure observations by optical microscope and scanning electron microscopy (SEM)