Reexamination of SRAM Cell Write Margin Definitions in View of Predicting the Distribution

Four definitions of static random access memory (SRAM) cell write margins (WMs) were reexamined by analyzing the dependence of the WM on the SRAM cell transistor threshold voltages (Vth´s) in order to find a preferable definition. The WM is expected to obey the normal distribution if the differential coefficients of the WM to Vth´s are constant over a wide range of Vth variations. This means that the write yield can be easily predicted by a small number of measured samples. Using SPICE in 45-nm technology, we examined which definition had Vth linearity, as well as giving an accurate write limit. The distribution predicted from the linearity was verified by the Monte Carlo simulation. As a result, the definition proposed by Gierczynski was found to be the most suitable definition for predicting the distribution and the write yield.