Abstract
An accelerated evaluation method for the SRAM cell write margin is proposed using the conventional Write Noise Margin (WNM) definition based on the “butterfly curve”. The WNM is measured under a lower word line voltage than the power supply voltage VDD. A lower word line voltage is chosen in order to make the access transistor operate in the saturation mode over a wide range of threshold voltage variation. The final WNM at the VDD word line voltage, the Accelerated Write Noise Margin (AWNM), is obtained by shifting the measured WNM at the lower word line voltage. The WNM shift amount is determined from the measured WNM dependence on the word line voltage. As a result, the cumulative frequency of the AWNM displays a normal distribution. Together with the maximum likelihood method, a normal distribution of the AWNM drastically improves development efficiency because the write failure probability can be estimated from a small number of samples. The effectiveness of the proposed method is verified using the Monte Carlo simulation.
Highlights
The recent progress of process technology has caused various fluctuation problems in device characteristics due to transistor area reduction
An accelerated evaluation method for the Static Random Access Memory (SRAM) cell write margin is proposed using the conventional Write Noise Margin (WNM) definition based on the “butterfly curve”
Together with the maximum likelihood method, a normal distribution of the Accelerated Write Noise Margin (AWNM) drastically improves development efficiency because the write failure probability can be estimated from a small number of samples
Summary
The recent progress of process technology has caused various fluctuation problems in device characteristics due to transistor area reduction. Since the write operation is strongly affected by the Vth of the SRAM cell access transistors, the conventional WNM is expected to be sensitive to Vth variation of the SRAM cell access transistors. Takeda et al described these problems and maintained the importance of a normal distribution of the WNM [5] They proposed a new write margin definition which is sensitive to Vth variation of the access transistors and follows a normal distribution. We propose an accelerated evaluation method for the SRAM cell write margin using the conventional butterfly curve based WNM definition. The write margin obeys the normal distribution even under the conventional WNM definition.
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