Effect of imprint on operation and reliability of ferroelectric random access memory (FeRAM)

The effect of the imprint on the operation of the ferroelectric random access memory (FeRAM) was estimated by a quasitheoretical model, including the hysteresis shift along the voltage axis and the depolarization induced by the hysteresis shift. Based on the experimental data of the imprint up to 90 days, the hysteresis shift and the depolarization are expressed by a stretched exponential equation as the aging time, and a hyperbolic tangent as the shifted voltage, respectively. Using this model, nonlinear phenomena of bit-line voltage change with the aging time is successfully predicted, and the aging reliability for two transistor and two capacitor (2T/2C) cell architecture with the imprinted capacitors is estimated for the first time. This technique is applied to the evaluation of aging reliability for the Pb(Zr,Ti)O₃ and (Pb,La)(Zr,Ti)O₃ capacitors, revealing that the latter has higher reliability than the former