Electrical characterization of ferroelectric capacitors for non-volatile memory applications

Ferroelectric materials show a spontaneous electrical polarization that can be reversed in sense by an applied external electric field. It should, therefore, be feasible to build a ferroelectric memory device that can store information in digital form.

Ascertaining the suitability of a ferroelectric material for use in memory devices requires an understanding of electrical properties of the thin-film capacitor. There are a number of electrical characterization methods which can be used to investigate these electrical properties. The polarization mechanism can be studied by the most fundamental characterization technique for ferroelectrics, the hysteresis loop, which is derived by plotting polarization against applied field. Fatigue, retention and imprint, which are specific ferroelectric lifetime characterization methods, are employed to determine the rate of capacitor degradation as well as the mechanisms responsible for it. The DC conductivity characterization techniques, including leakage current, resistivity degradation and time dependent dielectric breakdown (TDDB) are used to study the electrical current properties and charge transport mechanism in memory applications. Finally, the AC conductivity (complex impedance) characterization method, introduced here for the first time for ferroelectric capacitors, permits further understanding of the charge transport mechanism of ferroelectric materials. However, this characterization method is not directly used to evaluate the application of ferroelectrics in memory devices, but it can provide a further physical understanding of ferroelectric capacitors, such as the understanding of fatigue.

PbZr_xTi_1-xO₃ (PZT), a ferroelectric material with a pseudo-cubic perovskite-type structure, has been the material of choice in all major ferroelectric random access memory (FRAM) development programs to date. However, degradation problems such as fatigue and imprint that affect the lifetime of ferroelectric capacitors have moderated the progress of using PZT in commercial ferroelectric memories. Recently, SrBi₂Ta₂O₉ (SBT), a ferroelectric material that crystallizes in a layered structure, has been identified as a promising candidate for FRAM applications as a result of its fatigue-free behavior. In this research, the performance of PZT and SBT as ferroelectric memory materials is compared by using the aforementioned electrical characterization methods.