A Scalable Shield-Bitline-Overdrive Technique for Sub-1.5 V Chain FeRAMs

Abstract

A ferroelectric capacitor overdrive technique with shield-bitline drive has been demonstrated and verified by a 130 nm 576 Kb test chip with a 0.7191 μm2 cell. First, cell signal degradation and bitline-to-bitline coupling noise worsened by wide cell signal distribution in low voltage scaled FeRAMs are discussed. Next, the shield-bitline-overdrive technique is presented. This technique applies a 0.24 V higher bias to the ferroelectric capacitor through bitline-to-bitline coupling during the read operation without increasing device stress, and eliminates bitline-to-bitline coupling noise. The measured tail-to-tail cell signal is improved by 100 mV and effectively doubles for 1.3 V array operation. The area penalty of the proposed scheme is 0.9% of the 576 Kb cell array, and the access time penalty is 5 ns. The effect of this technique will be enhanced by cell shrink as the bitline-to-bitline coupling ratio increases. A tail-to-tail cell signal window of more than 200 mV is expected in 1.3 V 256 Mb and 1.2 V 512 Mb chain FeRAMs, whereas the tail-to-tail cell signal window without overdrive would degrade to 95 mV for 256 Mb and 60 mV for 512 Mb.