Structure and dielectric properties of amorphous tantalum pentoxide thin film capacitors

Amorphous tantalum pentoxide films are currently being studied as a high-k dielectric for high energy-density Metal-Insulator-Metal capacitors. Tantalum pentoxide thin films were prepared through pulsed-dc reactive magnetron sputtering at a high deposition rate (15 Å/s). The films were amorphous as determined by X-ray and electron diffraction through Transmission Electron Microscopy (TEM) at all sputtering conditions of both low and high ion bombardments unlike other oxides such as zirconium oxide. The structure was also confirmed by electron energy loss spectra using anodized Ta₂O ₅ films as a benchmark. After annealing at 750 °C, the films crystallized to the β-Ta₂O₅ phase (x-ray analysis). The dielectric constant and loss of the 2μm-thick films are 21 and 0.3%, respectively, at 1 kHz at room temperature of 25 °C. The amorphous films have a Temperature Coefficient of dielectric constant (TCK) of 2.1 × 10^-3 °C^-1, similar to crystalline forms of Ta ₂O₅ namely, α-Ta₂O₅ and β-Ta₂O₅. Electrical breakdown field of these amorphous tantalum pentoxide films is as high as 400 MV/m with a corresponding energy density of 14 J/cm³. Electrical breakdown is affected by material crystallinity, which is controlled by annealing. The crystallinity is studied both at bulk level through X-Ray diffraction and at the local atomic level through Fluctuation Electron Microscopy (FEM), which is an electron microscopy technique used to study Medium Range Order (MRO) on the length scale of 1-3 nm in apparently diffraction amorphous (TEM and X-ray) materials.