Ferroelectric Thin Films on Si-substrate for Tunable Microwave Applications

Doktorsavhandling, 2004

The thesis presents the development of microwave tunable devices based on ferroelectric thin films. The main tasks in this work are the fabrication, optimization, and integration of ferroelectric thin film into Si-MMIC. This work has been motivated by increased interest on advanced tunable microwave components which meet the requirements of low microwave losses, high tunability, easy to integrate to larger systems, and low cost. Dielectric permittivity, tunability, and loss tangent are the most important parameters, characterizing ferroelectrics for application in tunable microwave devices. Most of the efforts in this work have been devoted to the optimization of film fabrication processes in terms of microwave loss reduction. Experimental investigations of these parameters for Na0.5K0.5NbO3 (NKN), (SrTiO3) STO, Ba0.25Sr0.75TiO3 (BSTO) and (Pb, Zr) TiO3 (PZT) films are carried out in a wide frequency, DC bias, and temperature ranges. These measurements give important information about dispersion, temperature, and dc-field dependencies of dielectric permittivity and losses. The latter are useful for understanding the basic physical processes in ferroelectric films and help to optimize film deposition and device fabrication processes in terms of low losses, high tunability, and temperature stability. Coplanar-plate structure is used to integrate the ferroelectric films into the silicon circuits. Planar varactors are fabricated by depositing tunable ferroelectric films on Si/SiO2 substrates using laser ablation, RF magnetron sputtering, and sol-gel technique, followed by metal electrode deposition. Different varactor designs based on as straight gap, interdigital gap, and circular gap have been investigated and further used in the components. Experiments show the capability and potential of tunable devices based on ferroelectric thin films and confirm that they are competitive with other technologies like semiconductors at frequency above 20 GHz. It is shown that NKN films in polar phase are also useful for tunable microwave applications demonstrating low loss and good tunability in a frequency above 20 GHz. BSTO films, which are in paraelectric phase at room temperature, are used to fabricate microwave varactors. These varactors are used in two different phase shifters designs. These tunable devices show more than 60°/dB figure of merit, which is one the best value reported so far.