Single Bit SRAM Failure Case Study

Abstract Static random-access memory (SRAM) is a type of device that requires the highest reliability demands for integration density and process variations. In this study, we focus on single bit cell SRAM failures. These failures can be categorized as Hard bit cell failure, where bit cells fail the read or write operation under both higher and lower supply voltages, and Soft Bit cell failure, where failures occur at either higher or lower voltage. The analysis on SRAM Soft failure is further divided as VBOX High and VBOX Low failure, which depends on the failure mode supply voltage. With transistor dimensions continuously shrinking, the analysis of SRAM errors imposes tremendous challenges due to their small footprint. In this paper, a thorough failure analysis procedure is described for solving an SRAM yield loss issue. Different analysis techniques were applied and compared to narrow down the failure to the final root cause, including nanoprobing, Focus Ion Beam (FIB) cross-section, Scanning Spreading Resistance Microscopy (SSRM), Transmission Electron Microscopy (TEM), Electron Energy Loss Spectroscopy (EELS), Scanning Capacitance Microscopy (SCM), and stain etch.