Preparation And Characteristic Study Of PZT Films For FeRAM Application

In recent years, the new devices combining ferroelectric materials with semiconductor devices have been widely used. Compared with the traditional EEPROM and FLASH memory, the new non-volatile Ferroelectric Random Access Memories (FeRAM) have many advantages, such as low operating voltage, low power consumption, long-time retention, quick writing-operation and outstanding anti-radiation characteristics, which make it very suitable for embedded applications. Therefore, this paper mainly focused on the fundamentally experimental research of the ferroelectric film materials, which are the key components of the embedded FeRAM.Taking the intrinsic material properties and preparation conditions, as well as the commercial future into consideration, we chose PZT(52/48) to be the ideal ferroelectric film material for application to FeRAM. Ferroelectric ceramic PZT targets with 20mol% excess PbO were prepared by the conventional solid-state route. XRD patterns on the polished ceramic targets indicated pure perovskite phase. A series of ferroelectric thin films were prepared by rf magnetron sputtering. Fabrication parameters, such as work pressure, substrate temperature, and post annealing were examined in terms of their influences on the ferroelectric characteristics of the ferroelectric films. Experimental analysis showed that the optimal deposition conditions for PZT film are as follows, work pressure: 3Pa, substrate temperature: 300°C, sputtering atmosphere: pure argon, annealing temperature: 650°C, annealing time: 20 minutes. Considering it is hard for a single PZT film to satisfy the needs put forward by FeRAM, we inserted a thin PLT seed layer between PZT film and the electrode so as to improve the ferroelectric properties of PZT films. Experiment results indicated that PLT seed layer would increase the remnant polarization and enhance the fatigure resistance of the PZT films. Therefore, PZT films with satisfying electrical properties could be acquired through the prior deposition of a thin PLT seed layer. Pt/PLT/PZT/PLT/Pt/TiO₂/SiO₂/Si is an optimized structure, and suitable for FeRAM use. By analyzing the effects of various PLT seed layer thickness on the ferroelectric properties of PZT film, we found that when the thickness increased, the ferroelectric properties got better. In order to get the optimal fabrication conditions of the PLT seed layer, we examined the effects of PLT fabrication conditions on ferroelctric characteristics of PLT/PZT/PLT structure, and summarized the following optimal fabrication parameters of depositing PLT film, work pressure: 3Pa, substrate temperature: 600°C, sputtering atmosphere: Ar/O₂ = 6:1. In the end, optimized PLT/PZT/PLT ferroelectric structure with excellent characteristics was successfully prepared on Pt substrate. Measured results indicated that the films possessed excellent ferroelectric characteristics. When an electric field of approximate 500kV/cm was applied, the film had a larger remanant polarization (2Pr=52.7μC/cm2), lower coercive field (2Ec=130kV/cm). Moreover, PLT/PZT/PLT structure showed almost no polarization fatigue even after 1010 switching cycles.