On the Use of Assist Circuits for Improved Coupling Fault Detection in SRAMs

As technology scales down, the density of SRAM devices increases drastically, and their storage capacity grows at the same time. Moreover, SRAMs become more prone to physical defects in each technology node, which therefore increases the need of effective tests with high fault coverage. It has been shown that resistive-bridging defects induce coupling faults that may increase defective parts per million levels if not well covered during manufacturing test. In this work, we study the reuse of read and write assist techniques, commonly applied to improve the functional margins of SRAM core-cells, to improve the coverage of coupling faults. This analysis is based on extensive injection of resistive bridging defects in core-cells of a commercial low-power SRAM. We show that assist circuits can be leveraged to increase the sensitization of defects causing coupling faults by 10-12%, however, they need to be used carefully.