Temperature stable BaSrTiO3 thin films suitable for microwave applications

Abstract

In this paper, the properties of an optimized BaSrTiO3 thin film, deposited on an alumina substrate using a sol-gel process, are presented. The real and imaginary parts of the permittivity and the tunability have been measured over 7 decades of frequency and in a temperature interval of 320°C, which provides a good knowledge of the material properties for microwave applications. The dielectric properties of the films show a good stability in frequency and in temperature. From –80°C to 20°C, the permittivity changes less than 2% and from –75°C to 100°C, the tunability stays higher than 90% of its maximum value. The frequency dependence of the relative permittivity of the thin film is rather small since it only varies from 375 at 1kHz to 350 at 5GHz. As a main consequence, the tunability which attains almost 60% under a bias field of 400kV/cm, is very stable in frequency up to 5GHz. The dielectric losses tan δ, measured up to 1GHz, stay below 0.02 for the complete frequency range. Although the material is in the ferroelectric phase, the hysteresis effect is quite negligible, which results in a well-determined permittivity value for a given electric bias field. The characterized thin film has been integrated into a reflectarray cell allowing a dynamic control of the reflected phase. The measured phase-shift value is close to the simulated one, showing the performance of the material.