(A) study on integrating $(Ba,Sr)TiO_3$ thin film based microwave tunable devices onto Si wafers

Abstract

The prospects of using ferroelectric BST thin films for the fabrication and development of frequency and phase agile microwave devices have increased in the past few years due to improvements in ferroelectric thin film processing techniques as well as innovative circuit designs. In microwave devices, ferroelectric BST thin films are used in as a tuning layer through the nonlinear electric field dependence of the relative dielectric constant. The most critical properties that need to be optimized for tunable microwave devices application are the magnitude of the change in dielectric constant as a function of applied electric field and dielectric loss at microwave frequencies. However, because it is not easy to obtain high tunability and low loss tangent simultaneously, compromises are needed between them in order to optimize the reasonable tunability and loss tangent of the BST tunable devices. To date, considerable efforts have been made to increase the tunability through the control of film stress and orientation and to decrease the dielectric loss of tunable BST thin films by use of dopants such as $Mg^{2+}$, $Mn^{2+}$, $Co^{2+}$, and $Fe^{2+}$ which occupy the B site of the $ABO_3$ perovskite structure. In this study, we report on the effects of Ni acceptor dopants on the microwave dielectric properties for BST films deposited on (100) MgO single crystal substrates by pulsed laser deposition. The behavior of microwave properties in the BST films with Ni doping concentration at high frequency range (10 GHz) was investigated using interdigital capacitor structures. This work demonstrates a potential use for 1 mol % Ni-doped BST thin films in microwave tunable device applications. Currently, BST-based tunable devices have been typically fabricated on small area $(1 x 1 in.^2)$ single crystal substrates such as MgO, $LaAlO_3$, and $Al_2O_3$ which can provide low insertion loss, good lattice mismatch and good mechanical support. However, these substrates a...