We have investigated Pb(Zr,Ti)O/sub 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/sub c/) and a high dielectric constant on the polarization switching (/spl epsiv//sub S/) and a low dielectric constant on the nonswitching (/spl epsiv//sub N/), or essentially a large /spl epsiv//sub S///spl epsiv//sub 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 /spl epsiv//sub S///spl epsiv//sub N/ as a positive effect on the wide operational margin, but increased V/sub 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.
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Inoue et al. (2002) studied this question.
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