For its latest generation of high performance logic applications, Motorola employs a 0.13 µm CMOS technology with shallow trench isolation (STI). The contact dimension and spacing requirements for the dense areas of the circuitry, such as the cache, are quite aggressive. We recently encountered single bit and massive array failures, which were traced to an electrical short between tungsten contacts. We report here the failure analysis, which involved electrical and physical testing techniques.

Several hundred units were subjected to autoclave stress as part of the qualification of a new fast static RAM. Many units failed after autoclave stress, and these parts recovered after conventional depotting using nitric acid and a hot plate. Based on the recovery of the units, the failures were determined to be fuse-related because the nitric acid cleared the fuse cavities during depotting. Chemical analysis after thermally extracting the die from the package revealed an antimony-rich material in failing fuse cavities. Source of the antimony was linked to antimony trioxide added to the plastic package as a fire retardant. However, it was unclear whether the antimony-rich material caused the failure or if it was an artifact of thermal depotting. A new approach that did not thermally or chemically alter the fuse cavities was employed to identify the failing fuses. This approach used a combination of back-side grinding, dimpling, and back-side microprobing. The antimony-rich material found in the fuse cavity was confirmed using SEM and TEM-based EDS analysis, and it is believed to be a major contributing factor to fuse failures. However, it is unclear whether the short was caused by the antimony-rich material or by a reaction between that material and residual aluminum (oxide) left in the fuse cavity after the laser blows.

Temperature sensitive single bit failures at wafer level testing on 0.4µm Fast Static Random Access Memory (FSRAM) devices are analyzed. Top down deprocessing and planar Transmission Electron Microscopy (TEM) analyses show a unique dislocation in the substrate to be the cause of these failures. The dislocation always occurs at the exact same location within the bitcell layout with respect to the single bit failing data state. The dislocation is believed to be associated with buried contact processing used in this type of bitcell layout.