A 0.9V 64Mb 6T SRAM cell with Read and Write assist schemes in 65nm LSTP technology

A low Vmin, 6T SRAM is designed in 65nm LSTP (low standby power) technology using read and write assist schemes. In this work, we reduced the Vmin of SRAM cell from 1.2V to 0.9V by using assist schemes. Static noise margin (SNM) is improved by using Compensated Wordline Underdrive (WLUD) scheme as read assist. Using this read assist, maximum WLUD of 261mV is achieved for SNM critical PVT i.e., FS/0.9V/125C. SNM achieved is 465mV ensuring that it qualifies required cell sigma robustness. The underdrive at other PVT corners is lesser therefore it limits the impact on speed and write margin (WM) at their respective critical PVT. Negative BL approach using capacitive coupling is used as write assist in which a small capacitor of 10fF is placed in the write driver to achieve required bitline undershoot. BL undershoot obtained is 166mV which improves the WM at critical PVT i.e., SF/0.9V/-40C and thereby write-ability. WM at this PVT is 286mV. By enabling lower voltage of operation upto 45% of dynamic power reduction is achieved in the design.