Influence of postdeposition annealing on the enhanced structural and electrical properties of amorphous and crystalline Ta2O5 thin films for dynamic random access memory applications

Tantalum oxide (Ta2O5) thin films were fabricated on Pt-coated Si, n(+)-Si, and poly-Si substrates by metalorganic solution deposition technique. The effects of postdeposition annealing on the structural, electrical, and optical properties were analyzed. The Ta2O5 films were amorphous up to 600 degrees C. A well-crystallized orthorhombic phase with strong a-axis orientation was obtained at an annealing temperature of 650 degrees C. The refractive index was found to increase with annealing temperature and a value of 2.08 (at 630 nm) was obtained for films annealed at 750 degrees C. The electrical measurements were conducted on metal-insulator-metal (MIM) and metal-insulator-semiconductor capacitors. The dielectric constant of amorphous Ta2O5 thin films was in the range 29.2-29.5 up to 600 degrees C, while crystalline thin films, annealed in the temperature range 650-750 degrees C, exhibited enhanced dielectric constant in the range 45.6-51.7. The high dielectric constant in crystalline thin films was attributed to orientation dependence of the dielectric permittivity. The dielectric loss factor did not show any appreciable dependence on the annealing temperature and was in the range 0.006-0.009. The frequency dispersion of the dielectric properties was also analyzed. The films exhibited high resistivities of the order of 10(12)-10(15) Omega cm at an applied electric field of 1 MV/cm in the annealing temperature range of 500-750 degrees C. The measurement of current-voltage (I-V) characteristics in MIM capacitors indicated the conduction process to be bulk limited. The I-V characteristics were ohmic at low fields, and Poole-Frenkel effect dominated at high fields. The temperature coefficient of capacitance was in the range 52-114 ppm/degrees C for films annealed in the temperature range 500-750 degrees C. The bias stability of capacitance, measured at an applied electric field of 1 MV/cm, was better than 1.41% for Ta2O5 films annealed up to 750 degrees C. For a 0.15-mu m-thick film, a unit area capacitance of 3.0 fF/mu m2 and a charge storage density of 22.3 fC/mu m(2) were obtained at an applied electric field of 0.5 MV/cm. (C) 1999 American Institute of Physics. [S0021-8979(99)07914-1].