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Abstract

The sillenites Bi12MO20 (M = Si, Ge, Ti, Pb, Mn, B1/2P1/2) have been recognised as promising bulk materials for electronic applications because their dielectric properties (5.5 GHz: ε = 37.4 - 41, τf = -19 – (-84) ppm/K, Q x f = 800 - 8100 and Tsin = 680 - 850°C). Among them, Bi12SiO20 (BSO) appears to have the most interesting dielectric properties (5.5 GHz: ε = 37.6, τf = -20 ppm/K, Q x f = 8100 and Tsin = 850°C).
The continuous search for improved properties of electronic devices led us to the preparation of BSO thin films. The following work describes a study of the synthesis chemistry and characterization of BSO thin films prepared by the sol-gel method.
The sols were prepared using Bi(NO3)3∙5H2O and Si(OC2H5)4 as precursors, 2-ethoxyethanol and glacial acetic acid as solvents, ethanol-amine as the chelating agent and formamide as the drying control chemical additive (DCCA). Three syntheses were compared: the synthesis of sol Rh 60, the synthesis of sol Rh 5 and the synthesis of sol Rh 60 with DCCA.
The thin films were deposited on various substrates (Pt/TiO2/SiO2/Si, sapphire, spinel and Si) by the spin-coating method, pre-heated at 250°C and then annealed at 700°C for 1 h.
The chemical interaction that occurred during the sol synthesis and the deposition of thin films was studied by means of Fourier transformed infrared (FT-IR) spectroscopy. The gel point in the sols, which is an important parameter for a sol-gel derived thin film, was determined by use of a Physica MCR rheometer, by following the change in the viscosity of the sols. The TG/DTA/DSC/EGA analyses were performed on sols and thin films, to determine the pre-heating temperature of the thin films, as this is essential for obtaining dense, crack-free BSO thin films. The phase composition of the thin films was characterized by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM), whereas the morphology and the thickness of the thin films were studied with field-emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM).
The quality of BSO thin films critically depends on the molar ratio (Rh) of the sol. We found that sols with the molar ratio Rh 60 are unstable below a concentration of 0.76 M. Furthermore, the rate of the hydrolysis and condensation reaction is too high to obtain dense, crack-free BSO thin films. Therefore, the rate of the hydrolysis and condensation reactions was superseded by reducing the amount of water in the sols, by vacuum drying of the Bi(NO3)3∙5H2O (sols Rh 5).
The sols Rh 5 are stable across the whole concentration range and a reasonably dense microstructure of BSO thin films with a thickness of 200 nm was obtained. The disadvantage of this synthesis process was the poor wetability of the substrates, due to the high viscosity of the sol.
The most promising sol synthesis to achieve dense, crack-free BSO thin films was the synthesis of sol Rh 60 with DCCA and 2-ethoxyethanol. Such thin films resulted in a dense and homogeneous microstructure with a thickness of 250 nm and a 30-nm average roughness (Ra).
The dielectric properties of the BSO thin films were characterized utilizing both low-frequency and microwave-frequency measurement configurations. The low-frequency measurements (100 kHz - 1MHz) were performed with a parallel insulator-capacitor measurement configuration. At 1MHz the following dielectric properties were determined: ε≅40 and tanδ≥6.10-2. The dielectric properties at the microwave frequency were measured via the split-dielectric-resonator method at 10 GHz and in addition also with the sputtering planar capacitor configuration (0.5 - 3 GHz). The relative permittivity and dielectric losses measured at 10 GHz were ε≅40 and tanδ≥17.10-3, which is in good accordance with the values measured at lower frequencies. When measured by the sputtering planar capacitor configuration the dielectric constant ε remains ≅ 40. However, the dielectric losses were significantly higher, tanδ ≥ 2.10-1, indicating that the measuring configuration can strongly affect the measured values of the dielectric properties of the BSO-thin films.