Design and Experimental Validation of Radiation Hardened by Design SRAM Cells

The design and electrical characterization of a total ionizing dose hardened by a design static random access memory (SRAM) cell using annular layout and guard rings are presented. Since foundry SRAM cells can be validated during process development and manufacturing ramp but radiation hardening by design cells cannot, we use a specialized test structure to validate the cell design here. Stability, manufacturability, and hardness are experimentally investigated using a 4 kbit SRAM structure, fabricated on one version of the foundry 90 nm process. The structure, combined with a novel test and simulation based extraction procedure, allows direct measurement of the as-fabricated cell electrical characteristics. Variation of the SRAM switching points due to irradiation as well as the individual transistor threshold voltage variability is measured in the SRAM array test structure. Irradiation tests show negligible impact on switching voltage and increase in the standby current less than 1.5% after 2 Mrad(Si). The effects on the cell margins are also analyzed. The specific SRAM cell layout, which uses a very low aspect ratio, is intended to minimize multibit upset of horizontally adjacent cells. This impact is also discussed with measured heavy ion results.