Compositional design of Pb(Zr, Ti)O3 for highly reliableferroelectric memories

We have investigated Pb(Zr,Ti)O₃ (PZT) film composition suitable for highly reliable ferroelectric RAM (FeRAM) application. To obtain a wide operational margin for 2T/2C (two transistors and two capacitors) FeRAMs, the PZT film capacitor is needed to have a low coercive voltage (V_c) and a high dielectric constant on the polarization switching (ϵ_S) and a low dielectric constant on the nonswitching (ϵ_N), or essentially a large ϵ_S/ϵ_N ratio. Concerning the B-site composition in the perovskite structure, it is found that lowering the Zr/Ti ratio from 47/53 to Ti-richer ones increases the ratio of ϵ_S/ϵ_N as a positive effect on the wide operational margin, but increased V_c as a negative effect. Taking the balance of these factors into consideration, it is concluded that an optimum composition, such as Zr/Ti=30/70, provides the maximum operational margin. The A-site composition, on the other hand, affects the long-term reliability of a PZT capacitor. The endurance to the fatigue and imprint are enhanced by reduction of the Pb-excess and dope of La in the A-site. A La-doped PZT (Zr/Ti =30/70) capacitor is successfully integrated to the 8 kbit FeRAM macro with double-layer Al wiring to confirm the feasibility of this capacitor