Enhanced dielectric properties of modified Ta2O5 thin films

 Tantalum oxide (Ta₂O₅) is a promising high dielectric constant material for the DRAM applications because of its ease of integration compared to other complex oxide dielectrics. The dielectric constant and thermal stability characteristics of bulk Ta₂O₅ samples were reported to enhance significantly through small substitutions of Al₂O₃. However, this improvement in the dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ is not clearly understood. The present research attempts to explain the higher dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ by fabricating thin films with enhanced dielectric properties. A higher dielectric constant of 42.8 was obtained for 0.9Ta₂O₅–0.1Al₂O₃ thin films compared to that reported for pure Ta₂O₅ (25–30). This increase was shown to be closely related to a-axis orientation. Pure Ta₂O₅ thin films with similar a-axis orientation also exhibited a high dielectric constant of 51.7, thus confirming the orientation effect. The leakage current properties and the reliability characteristics were also found to be improved with Al₂O₃ addition. Received: 24 November 1998 / Reviewed and accepted: 7 December 1998     

In situ STM and EQCM studies of tantalum electrodeposition from TaF5 in the air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide

The electroreduction of 0.5 M TaF₅ on Au(1 1 1) and on polycrystalline gold substrates was investigated at room temperature in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, [Py_1,4]TFSA, by cyclic voltammetry, in situ scanning tunneling microscopy (STM) and electrochemical quartz crystal microbalance (EQCM). The electrochemical reduction of TaF₅ in the employed ionic liquid occurs in several steps. The first redox process is attributed to the reduction of TaF₅ to TaF₃, which likely occurs in the solution, as EQCM indicates no mass change. The electrodeposition of tantalum occurs only in a very narrow potential window and is preceded by the formation of various non-stoichiometric tantalum subhalides. Attempts to deposit micrometer thick tantalum layers at room temperature fail, presumably because of kinetic reasons.     

Synthesis, densification, and mechanical properties of TaB2

Tantalum diboride (TaB₂) was synthesized by reducing Ta₂O₅ using B₄C and graphite at 1600 °C under flowing Ar. The powder had an average particle size of 0.4 μm with both needle-like and rounded particles. The TaB₂ powder was hot pressed to relative densities of 97% at 2000 °C (3.6 μm grain size) and 98% at 2100 °C (5.3 μm grain size). Mechanical properties were measured for TaB₂ hot pressed at 2100 °C and were comparable to those of the commonly studied diborides, ZrB₂ and HfB₂. The Young's modulus was 551 GPa, Vickers' hardness was 25.6 GPa, flexure strength was 555 MPa, and fracture toughness was 4.5 MPa-m^1/2.     

Decay of capacitance on ceasing formation of anodic oxide films on tantalum

The decrease with time in the capacitance of anodic oxide films on tantalum after ceasing film growth by reducing the field in the oxide was investigated. No dependence was observed of the amount and rate of change in relative dielectric permittivity on the initial current density. The kinetics of this effect are therefore different from those for the decay of the latent ionic conductivity.     

Distribution Behavior of Nb,Ta, and Pa as Homologues of Db in Cation Exchange and Solid-Liquid and Liquid-Liquid Extractions by Aliquat 336 from Hydrochloric Acid

Detailed chemical studies on transactinide elements will clarify their characteristic properties caused by strong relativistic effect. In this work, to realize chemical study on transactinide element ₁₀₅Db, cation exchange, and solid-liquid and liquid-liquid extractions of micro amounts of Nb, Ta (homologues of Db), and Pa (pseudo homologue) by Aliquat 336 were performed employing hydrochloric acid. Clearly different chloride complex formations and distribution reaction kinetics were observed among these elements, and the present results are useful for studying the chemical properties of Db. Based on these results, we propose suitable experimental conditions for Db.     

Grain subdivision and recrystallization in oligocrystalline tantalum during cold swaging and subsequent annealing

A coarse-grained ingot of high-purity tantalum was deformed by swaging at room temperature to a strain of 1.28. During annealing at 900°C for 30 min two neighboring grains were observed to behave quite differently. Electron backscattering diffraction (EBSD) results show noticeable differences in terms of the misorientations developed in both grains. The grain developing larger misorientations recrystallized much more readily than the other. The result is interpreted in terms of the differences in grain subdivision into strongly misoriented regions.     

Texture characterization of Cu interconnects with different Ta-based sidewall diffusion barriers

X-ray diffraction is used to assess the texture of narrow lines and study the impact of different sidewall diffusion barrier materials. All the Ta-based barriers developed a strong 〈1 1 1〉 texture in the scaled geometry, with little effect from sidewall growth. Comparisons were made with blanket wafers, demonstrating the pined grain structure in the narrow lines and contrasting change in texture due to re-crystallization in the unconstrained film. Furthermore, patterned lines showed significant anti-symmetric plane distribution influenced by high strains and twinning along the lines.     

Improvement of Ta-based thin film barriers on copper by ion implantation of nitrogen and oxygen

Magnetron sputtered polycrystalline Ta and Ta(Si) barriers for copper metallization schemes were modified by nitrogen as well as oxygen high dose ion implantation to improve their thermo-mechanical stability. Ion bombardment changed the initial polycrystalline microstructure to amorphous-like. In contrast to pure Ta, Ta(Si) layers were already amorphous or nanocrystalline after deposition. In this case, the annealing temperature at which formation of a well crystallized structure occurs increased by approximately 100 K as a result of the implantation. In order to demonstrate the improvement in the barrier properties of the implanted Ta films, the intermixing of Ta and Cu at the interface of corresponding layer structures was measured as a function of the annealing temperature by depth profiling using Auger electron spectroscopy (AES). The thermal stability of Ta and Ta(Si) barriers increased from 600 °C/1 h for the non-implanted layers up to 750 °C/1 h after implantation of nitrogen or oxygen.