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This paper proposes a new ladder FeRAM ar chitecture with capacitance-coupled-bitline (CCB) cells for high-end embedded applications. The ladder FeRAM architecture short-circuits both electrodes of each ferroelectric capacitor at every standby cycle. This overcomes the fatal disturbance problem inherent to the CCB cell, and halves read/write cycle time by sharing a plateline and its driver with 32 cells in two neighboring ladder blocks. This configuration realizes small 0.35 μm2 cell using a highly reliable ferroelectric capacitor of as large as 0.145 μm2 size, and a highly compatible process with logic-LSI. A slow plateline drive of the CCB cell due to a resistive plateline using an active area is minimized to 2.5 ns by introducing thick M3 shunt-path and distributed M3 platelines. The area penalty of the shunt is 4.7% of an array. A serious bitline-to-bitline coupling noise in edge bitlines up to the noise/signal ratio of 0.38 due to the operation peculiar to FeRAM is eliminated by introducing activated dummy bitlines and their sense amplifiers. The design of 16 cells in a ladder block is optimal for effective cell size, cell signal, and active power dissipation. A new early plateline pull-down read scheme omits "0"-data rewrite operation without read disturbance. A 64 Kb ladder FeRAM with the CCB cells and the early plateline pull-down read scheme achieves a fast random read/write of 10 ns cycle and 8 ns access at 150°C.
Date of Publication: March 2011