Temperature stable BaSrTiO3 thin films suitable for microwave applications

• BST thin film were deposited by sol-gel method on alumina substrates
• Dielectric properties examined over a range of 320 °C and over 7 decades in frequency
• The tunability and permittivity stay stable up to 5 GHz
• The figure of merit stays higher than 90% of its maximum over a band of 120 °C

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 1 kHz to 350 at 5 GHz. As a main consequence, the tunability which attains almost 60% under a bias field of 400 kV/cm, is very stable in frequency up to 5 GHz. The dielectric losses tan δ, measured up to 1 GHz, 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.