Reliability and processing of ferroelectric thin film capacitors with emphasis on fatigue and etching

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Virginia Tech


Ferroelectric materials are characterized by a reversible spontaneous polarization in the absence of an electric field. The characteristic polarization response of a ferroelectric material to an applied electric field enables a binary state device in the form of a thin film ferroelectric capacitor that can be used to store digital information. This provides opportunities for the development of high speed, low cost and low power, nonvolatile memory devices. The development of commercial ferroelectric memory devices has however been hampered by (a) several reliability issues including fatigue, leakage current, aging, time dependent dielectric breakdown, retention and imprint and (b) processing problems including the development of a low temperature thin film deposition process and the development of a patterning technology.

Lead zirconate titanate (PZT) is now widely considered as the most promising material for ferroelectric memory applications as a result of its excellent ferroelectric properties and wide operating temperature range. However, it is commonly found that metal electroded-PZT capacitors (e.g., Pt/PZT/Pt) show a loss of switchable polarization with cumulative switching cycles. This phenomenon is known as fatigue and is the one of the critical problems affecting the lifetime of ferroelectric memories.

This research is primarily focused on the problem of fatigue. On the basis of a quantitative model, various guidelines to minimize the degradation problem have been derived. The model attributes fatigue to domain pinning by space charge that is caused by defect (e.g. oxygen vacancy) entrapment at various interface sites such as electrode-ferroelectric interface, domain boundaries and grain boundaries. Two different approaches to minimize the problem have been outlined : (a) control of the defect density and (b) control of the interface state. The control of interface state was achieved by replacing the metal electrodes with conducting oxide electrodes such as RuO₂. The oxide electrode/PZT capacitors were characterized for their diffusion barrier properties, perovskite phase formation, interface nature and ferroelectric properties. The results indicate that these oxide electroded PZT films are good candidates for nonvolatile memory applications. However, the leakage current levels at the operating voltages are far higher than the metal counterparts. Simultaneous minimization of fatigue and leakage current in PZT films was achieved by using multilayer metal/conducting oxide electrodes (e.g., Pt/RuO₂).

The control of defect density was attained by (a) donor doping to compensate for the oxygen vacancies (e.g, Nb doping in PZT) and (b) utilizing ferroelectric materials that have a low intrinsic defect concentration. As a result of the latter approach, novel ferroelectric materials belonging to the layer-structure family of oxides have been identified as excellent candidates for fatigue free nonvolatile memory applications. Laser ablated SrBi₂(TaxNb1 - x)₂O₉ (0<x<1) films showed very good hysteresis characteristics (remnant polarization value of 11 µC/cm², coercive field of 60 kV/cm), no fatigue was observed up to 10⁹ switching cycles and very low leakage current densities. Furthermore, the formation and properties of these films were characterized. It was found grain size and orientation played a major role in determining the properties of these films. C-axis oriented films were found to exhibit almost no polarization.

An additional objective of this research was to identify an etching technology (process integration issue) for patterning of the ferroelectric capacitors. The etching process should provide high etch rates, good etch anisotropy, high etch selectivity and minimal post etch residues. It has been shown that a reactive ion etch process with CCl₂F₂/O₂ as the etch gas mixture can meet these requirements. A detailed process study has been conducted to determine the mechanism of etching.



ferroelectrics, thin films, reactive ion etching, lead zirconate titanate, strontium bismuth tantalum oxide, laser ablation