K. D. Gifford

Control of structure and electrical properties of lead‐zirconium‐titanate‐based ferroelectric capacitors produced using a layer‐by‐layer ion beam sputter‐deposition technique

It is demonstrated that the structure and electrical properties of Pb(ZrxTi1−x)O3(PZT)‐based capacitors, involving RuO2 electrodes, can be controlled by depositing intermediate PbTiO3 (PT) layers at the PZT/electrode interfaces. Three different PZT‐based capacitor systems were studied, namely: (a) RuO2/PZT/RuO2/(100)MgO, (b) RuO2/PZT/PT/RuO2/(100)MgO, and (c) RuO2/PT/PZT/PT/RuO2/(100)MgO. Electrical characterization of capacitor (a), without a PT layer, shows about 71% reduction in remanent polarization, while those with one (b) or two (c) PT layers present only 34% and 29% decrease in remanent polarization, after 1010 switching cycles. Electrical conduction measurements (current density versus time) have shown that ion beam sputter‐deposited PZT‐based capacitors with or without PT layers present about an order of magnitude (∼6×10−9 A/cm2, at 100 s) smaller dc leakage current, for comparable physical and electrical parameters, than dc leakage characteristic of PZT sol‐gel‐based capacitors with RuO2 electr...

Electrodes for ferroelectric thin films

Abstract Platinum and ruthenium oxide (RuO2) deposited by ion beam sputter-deposition are evaluated for use as electrodes for PZT thin film capacitors. The effect of deposition temperature, film thickness, and the presence of oxygen on hillock formation in platinum is discussed. It is shown that the hillock density in Pt/Ti/SiO2/Si films can be significantly reduced by properly controlling the processing conditions and film thickness. Stress measurements correlate with the experimental observation that depositing thinner platinum films (<800 A) is an effective means of reducing hillock formation. The use of an intermediate deposition temperature 200–250°C also helps minimize hillock formation. Diffusion of the Ti adhesion layer into and/or through the platinum was significantly reduced by replacing the Ti with a TiOx adhesion layer. RuO2 electrodes are compared to Pt in terms of resistivity, surface morphology, microstructure and film orientation.

Leakage and interface engineering in titanate thin films for non-volatile ferroelectric memory and ulsi drams

Abstract The leakage behavior of Pb(Zr,Ti)O3 PZT and (Ba,Sr)TiO3 (BST) thin films has been studied. The leakage behavior is dependent upon the bottom electrode. PZT films on RuO2 bottom electrodes display a large leakage, predominantly ohmic in behavior, which we have shown to be PZT microstructure-controlled. The leakage of PZT and BST films on Pt display Schottky emission characteristics which is controlled by the film/electrode interface. In the case of PZT films, we have shown that several methods can be utilized to successfully lower the RuO2/PZT/RuO2 system leakage, while retaining the long term performance. These methods include pre-annealing of RuO2 bottom electrode prior to PZT film deposition; addition of buffer layer between RuO2 and PZT film; and PZT film growth via in-situ ion beam sputter-deposition.

Electrical characterization of sol-gel derived PZT thin films

Thin films of lead zirconate titanate (PZT) were deposited on a variety of substrates using the sol-gel process. These included Pt/Ti/SiO/sub 2//Si, RuO/sub 2//SiO/sub 2//Si, RuO/sub 2//MgO, Pt/MgO, and Pt/Ti/MgO. Epitaxial, oriented, and polycrystalline films were obtained after annealing at 700 degrees C for 10 min. Polycrystalline PZT films with RuO/sub 2/ as top and bottom electrodes showed superior hysteresis and fatigue behavior as compared to such films on Pt. Epitaxial films on Pt/MgO and Pt/Ti/MgO displayed excellent fatigue behavior and large remnant polarization as compared to their polycrystalline counterparts on Pt/Ti/SiO/sub 2//Si. The results suggest that different fatigue mechanisms may be simultaneously operating.<<ETX>>

A review of composition-structure-property relationships for PZT-based heterostructure capacitors

Abstract Studies performed by our group on composition-structure-property relationships of Pb(ZrxTi1−x)O3 (PZT)-based heterostructure capacitors are reviewed. The work discussed is related to the synthesis and characterization of ferroelectric PZT and conductive Pt, RuO2, and La0.5Sr0.5CoO3 layers and their integration into heterostructure capacitors suitable for non-volatile memories. The main objective of our research was to determine the influence of deposition parameters and layer processing on the composition, structure, and properties of PZT-based capacitors, with the goal of controlling fatigue, retention, and imprint effects. The work discussed relates mainly to the synthesis of films by ion beam sputter-deposition (IBSD) and pulsed laser ablation deposition (PLAD), where the heterostructures are grown in-situ without exposing the interfaces to uncontrollable atmospheric conditions. Limited comparisons are presented between structural characteristics and properties of PZT capacitors produced by IB...

Control of electrical properties of ion beam sputter-deposited PZT-based heterostructure capacitors

Abstract Previous work on pulsed laser ablation-deposition (PLAD) and sol-gel synthesis has shown that the crystallographic orientation of ferroelectric Pb(ZrxTi1−x)O3 (PZT) films and electrical properties of PZT-based capacitors is controlled by the substrate and/or bottom electrode material type. Similar control is obtainable with the ion beam sputter deposition method. Also, a non-ferroelectric phase that is nucleated during processing on RuO2/MgO substrates can be reduced or eliminated by interposing a layer of PbTiO3 (PT) between the electrode and the PZT film. The use of the PT layer and other proprietary modifications made to the RuO2 bottom electrode result in a substantial reduction in the fatigue of these PZT-based capacitors. The fatigue resistance and low dc leakage current of these capacitors qualify their use in non-volatile ferroelectric random access (FRAM) and dynamic random access (DRAM) memory devices.

Sputter deposition of ferroelectric thin films

Ferroelectric films are typically deposited by a variety of techniques, the two most common being chemical methods (sol-gel, metalorganic decomposition) and sputtering. In this paper we briefly review the sputtering techniques, and then discuss ion beam sputter deposition in greater detail. In particular, ion beam sputter deposition of epitaxial lead zirconate titanate (PZT) films is described. It is shown that the films with compositions close to the morphotropic boundary typically show well-developed ferroelectric hysteresis loops, Pmax around 45 μC/cm2, and Pr around 20 μC/cm2. In comparison with typical polycrystalline sol-gel PZT films, however, coercive fields of thin epitaxial films are large (120-200 kV/cm for 95 nm films). The pulsed fatigue behavior is remarkably similar to a polycrystalline non-oriented sol-gel PZT film investigated for comparison. The similarities suggest that the aging behavior may be dominated by the electrodes, which were Pt in both systems.