Chiharu Isobe

Effect of ozone annealing on the dielectric properties of tantalum oxide thin films grown by chemical vapor deposition

Effects of the heat treatment in flowing oxygen containing ozone of 0.5–1 vol % (O3 annealing) on the dielectric properties of the Ta2O5 thin film (100–200 A) grown on the Si substrate by the chemical vapor deposition method were investigated. The leakage current was drastically reduced from more than 10−3 to 10−9 A/cm2 in an electric field of 3 MV/cm by the O3 annealing at 400 °C. It was also found that the leakage current was decreased and increased reversibly between alternate O3 annealing and O2 annealing (without ozone). These two states of the leakage current can be attributed to the reproducible change of the oxygen vacancies in the Ta2O5 film.

INSULATING PROPERTIES OF TANTALUM PENTOXIDE CAPACITOR FILMS OBTAINED BY ANNEALING IN DRY OZONE

Highly insulating tantalum pentoxide (Ta2O5) capacitor films were obtained by dry O3 annealing at low temperatures ranging from 350 to 500 °C. Typical leakage current density and resistivity of a 10-nm-thick Ta2O5 film measured at 2 MV/cm were 2.5×10−8 A/cm2 and 4.8×1013 Ω cm, respectively, obtained by dry O3 annealing at 450 °C, while leakage current density of the as-deposited film was about 10−1 A/cm2. It was confirmed that the mechanism which improves the insulating properties after dry O3 annealing involves the reduction of the leakage current due to the reduction of the concentration of such impurities as hydrogen and carbon, and the reduction of oxygen vacancies in Ta2O5 films. In addition, amorphous Ta2O5 films annealed in dry O3 show much better insulating properties than polycrystalline films. This can be attributed to the suppression of leakage current at the grain boundaries. Regarding dielectric properties, Ta2O5 films have an average dielectric constant of 28, and the typical SiO2 equivalent...

Fabrication of ferroelectric SrBi2Ta2O9 capacitor films using plasma-assisted metalorganic chemical vapor deposition and their electrical properties

The fabrication of SrBi2Ta2O9 (SBT) films using plasma-assisted metalorganic chemical vapor deposition (P-MOCVD) has been investigated. Optimizing the process conditions under plasma environment, amorphous SBT films were successfully deposited at a substrate temperature below 300 °C, suggesting that the P-MOCVD process effectively utilizes plasma energy to promote the reaction and decomposition of metal organic source molecules. The amorphous SBT films were crystallized to the bilayered perovskite SBT films by a postannealing at 725 °C. Thin SBT capacitors fabricated using P-MOCVD showed a good step coverage and the excellent ferroelectric properties including endurance. Low voltage operation below 1.5 V was successfully achieved using a 75 nm SBT capacitor, in which the signal level derived from the hysteresis curve suggests the feasibility of application to a 64 Mbit ferroelectric random access memories.

Preparation of Ultra thin SrBi2Ta2O9 Films Using Metalorganic Chemical Vapor Deposition Combined with a Modified Annealing Method

We report a capacitor-preparation method using an ultra thin SrBi2Ta2O9 (SBT) film for application to low-voltage-operated ferroelectric random-access memories (FeRAMs). As the film thickness was reduced below 100 nm, the SBT capacitor reached dielectric breakdown at a much lower electric field. Atomic force microscopy (AFM) analysis revealed that the crystallization annealing roughened the interface between the electrode and SBT. We attributed the electrical breakdown to the localized electric field at the roughened interface. By modifying the annealing method for crystallization of SBT, the interface was maintained smooth and flat even after the crystallization annealing, resulting in good performance in terms of the leakage current vs. appled voltage (I–V) characteristics. Employing a 50 nm thick SBT film, by the modified annealing method, a low-voltage operation as well as highly insulating properties was realized; the remanent polarization (2Pr), coercive field (Vc) and leakage current density (J) at 1.5 V were 12.2 µC/cm2, 0.36 V and approximately 5×10-9 A/cm2, respectively.

Effect of interfacial layers on dielectric properties in very thin SrBi2Ta2O9 capacitors

The effect of interfacial layers on the dielectric properties in very thin SrBi2Ta2O9 (SBT) capacitors has been investigated using static measurements. Total permittivity (et) decreased as the film thickness was reduced in both Pt/SBT/Pt and Ir/SBT/Pt capacitors. The contribution of the interfacial capacitance (Cint) and bulk capacitance to the total capacitance indicates that Cint of the Ir/SBT/Pt structure was lower than that of the Pt/SBT/Pt structure, while the bulk permittivity (eb) was essentially the same. The dispersion of all capacitors followed the power law, while the Ir/SBT/Pt capacitor showed a larger dispersion of Cint. These results suggest that the Pt/SBT/Pt capacitor is preferred for obtaining the high performance with less effect of the interfacial layers on the dielectric properties.

Crystallization Mechanism of Ferroelectric SBT Thin Films

We characterize, via selected area electron diffraction and high resolution TEM, the multistage crystallization process by which strontium bismuth tantalate(SBT) thin films are transformed from initial amorphous phase to the fully formed layer-structured perovskite. During the annealing process at 800°C, we found that an intermediate fluorite-like stage is formed after 3min as the SBT precursor crystallizing into the final SBT structure. This new information will be very important for formation and crystallization of ferroelectric SBT films.