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

Abstract

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 °C. A well-crystallized orthorhombic phase with strong a-axis orientation was obtained at an annealing temperature of 650 °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 °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 °C, while crystalline thin films, annealed in the temperature range 650–750 °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 1012–1015 Ω cm at an applied electric field of 1 MV/cm in the annealing temperature range of 500–750 °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/°C for films annealed in the temperature range 500–750 °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 °C. For a 0.15-μm-thick film, a unit area capacitance of 3.0 fF/μm2 and a charge storage density of 22.3 fC/μm2 were obtained at an applied electric field of 0.5 MV/cm.