Simulation of transient radiation upset in a 0.18-μm CMOS SRAM by accurately modeling a 6T memory cell

Abstract

Static random-access memory (SRAM) circuits exposed to pulsed X- or γ-rays will suffer transient radiation upsets, which poses a significant challenge to their reliability. The transient radiation upsets are caused by global photocurrents, which are governed by the interconnect resistance of the power supply lines and the local photocurrents generated in each memory cell. Thus, accurate modeling of local photocurrents in each memory cell is essential for predicting the global photocurrents and transient radiation upset thresholds of SRAM circuits. In this study, the local photocurrents generated in a 0.18-μm complementary metal-oxide-semiconductor (CMOS) SRAM cell are extracted using a physics-based model of the actual device structure, rather than an analytical or experimental model. The memory cell containing six transistors and parasitic structures is accurately modeled in three-dimensions. Its responses to pulsed γ-rays are simulated with an in-house parallel semiconductor device simulation program, which enables the extraction of accurate local photocurrents. These local photocurrents, together with the interconnect resistance of the power supply lines in SRAM circuits, are modeled and simulated with an in-house parallel SPICE circuit solver. Using this approach, the global photocurrents in the 0.18-μm CMOS SRAM circuit irradiated with γ-rays are obtained and the known rail-span-collapse effect is observed. Finally, the transient radiation upset threshold of the SRAM circuit is predicted numerically and verified experimentally.