Influence of rework ashing temperature on the phase segregation and CuAl2 precipitation in Al(Cu) metal line stacks

Metal bridging was identified as the root cause for yield degradation for the wafers that were processed through metal litho rework operations. The metal film contains TiN/Al(0.5%Cu)/TiN/SiON/SiO₂ stack with designed thickness at each associated layer. The objective of this study was to investigate and understand the formation of metal bridging and define new operation conditions to improve yield for reworked wafers. DOE was carried out on short loop (SL) wafers to identify key modulators in the rework loop. Plausible mechanism was proposed as CuAl₂ precipitation along the grain boundary (GB) and as a result local masking to leave etch residues at interconnect interface. Scanning electron microscope (SEM) and focused ion beam (FIB) were used to locate and review etch residues after plasma etch. In-line Etest data indicated that ashing temperature was the key modulator for metal bridging. Top-view images and cross-section micrographs showed that the bridging/etch residues appeared at the bottom between metal space lines. Yield was recovered to match non-rework wafers under new rework operation conditions.