Identification of processing defects by focused ion beam (FIB) induced voltage contrast

Processing defects related to multiple stacked metal via and/or contact structures were very difficult to verify to be a high resistance open issue or a bridge/leakage short issue. In this paper, FIB passive voltage contrast was very successfully implemented in 2 Mb SRAM debug to differentiate the two failure mechanisms, high via and/or contact resistance versus substrate defect-induced junction leakage. It was demonstrated in four different cases that passive voltage contrast was used to reveal and locate the specific failure site and followed with pinpoint diagnosis of the abnormality by TEM. It was also known that these defects were invisible to optical microscopy and SEM. The 2Mb SRAM process was a 0.25 μm, three-metal single-poly, 8-inch technology. Shallow trench isolation (STI), self-aligned TiSi₂ silicide (salicide), and triple stacked W-plug processes were integrated in the manufacturing process. The failure mode and failure location were firstly identified by a MOSAID tester. Layer-by-layer deprocessing and top-view SEM examination were used for structural inspection at the failure site prior to FIB imaging. As soon as the fault was isolated by a passive voltage contrast comparison study, precision TEM analyses were utilized as the major tool for structural inspection in both plan-view and cross-section