In-situ characterization and extraction of SRAM variability

Abstract

Measurement and extraction of as fabricated SRAM cell variability is essential to process improvement and robust design. This is challenging in practice, due to the complexity in the test procedure and requisite numerical analysis. This work proposes a new single-ended test procedure for SRAM cell write margin measurement. Moreover, an efficient decomposition method is developed to extract transistor threshold voltage (VTH) variations from the measurements, allowing accurate determination of SRAM cell stability. The entire approach is demonstrated in a 90 nm test chip with 32 K cells. The advantages of the proposed method include: (1) a single-ended SRAM test structure with no disturbance to SRAM operations; (2) a convenient test procedure that only requires quasi-static control of external voltages; and (3) a non-iterative method that extracts the VTH variation of each transistor from eight measurements. The new procedure enables accurate predictions of SRAM performance variability. As validated with 90 nm data of write margin and data retention voltage, the prediction error from extracted VTH variations is <; 4% at all corners.