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

We have investigated Pb(ZrTi)O-3 (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,) and a high dielectric constant on the polarization switching (epsilon(S)) and a low dielectric constant on the nonswitching (epsilon(N)), or essentially a large epsilon(S)/epsilon(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 epsilon(S)/epsilon(N) as a positive effect on the wide operational margin, but increased V. 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.