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We present an all-digital Read Stability and Write Margin (WM) characterization scheme for CMOS 6T SRAM array. The scheme measures the cell Read Disturb voltage (Vread) and cell Inverter Trip voltage (Vtrip) in SRAM cell array environment. Measured voltages are converted to frequency with Voltage Controlled Oscillator (VCO) and counter based digital read-out to facilitate data extraction, processing, and statistical analysis. Resistor based voltage divider with 64 voltage levels and 10mV per step is employed to allow sweeping of BL voltage from 640mV to GND for WM characterization. A 512Kb test macro is implemented in UMC 55nm 1P10M Standard Performance (SP) CMOS technology. Monte Carlo simulations validate the accuracy of Vread and Vtrip measurement scheme, and post-layout simulations show the resolution of the digital read-out scheme is 0.167mV/bit.
Date of Conference: 23-25 April 2012