Analysis of Ultra-Thin-Body SOI Subthreshold SRAM Considering Line-Edge Roughness, Work Function Variation, and Temperature Sensitivity

This paper analyzes stability and variability of ultra-thin-body (UTB) SOI subthreshold SRAMs considering line-edge roughness (LER), work function variation (WFV), and temperature sensitivity. The intrinsic advantages of UTB SOI technology versus bulk CMOS technology with regard to the stability and variability of 6T SRAM cells for subthreshold operation are analyzed. Compared with LER, WFV causes comparable threshold voltage variation and much smaller subthreshold swing fluctuation, hence less impact on the UTB SOI subthreshold SRAMs. Even considering LER, the Lg=40 nm UTB SOI 6T subthreshold SRAM cells still provide sufficient margin (μRSNM/σRSNM >; 6 at Vdd=0.3 ~ 0.4 V) while the bulk subthreshold SRAMs with RDF fail to maintain adequate margin. Increasing temperature will increase the Vread, 0 and decrease RSNM because of the degraded subthreshold swing. The RSNM of UTB SOI subthreshold SRAMs show less temperature sensitivity compared with that of bulk subthreshold SRAMs. Due to larger body effect, the back-gating technique is more efficient for the Lg=40 nm and 25 nm UTB SOI subthreshold SRAMs compared with the bulk counterparts. By using lower threshold voltage devices with dual band-gap work functions, the Lg=25 nm UTB SOI subthreshold SRAMs show 31.9% reduction in σRSNM and 55% improvement in μRSNM/σRSNM.