For barium strontium titanate (Ba0.6Ti0.4TiO3, BST) films used in tunable microwave devices, they must have excellent structural characteristics and outstanding combination of dielectric properties i.e., a low loss tangent over the range of operating direct current (DC) bias voltages, a moderate dielectric constant for impedance matching purpose, a large variation in the dielectric constant with applied dc bias (high tunability, in particular high tunability at low applied dc bias), etc. To achieve such a high objective, the following two great improvements are carried out. A normal sol-gel method is modified to prepare multilayer BST films layer by layer. Each multilayer BST film is composed of six layers, where each layer is preheated at 600 ℃, thus the layers from the first layer to the sixth layer are successively preheated once to six times. Thus each BST film is smooth and dense, and contains almost no organic residues. On the other hand, as a new doped mode, Ce/Mn alternate doping is performed. For every six layer-BST films, when the odd number layers are doped with Ce, then the even number layers are doped with Mg, vice versa. The above two improvements result from the fact that Ce doping, Mg doping and Y and Mn alternate doping could make BST thin films significantly improve the dielectric tunability, reduce the dielectric loss, and improve the combination of dielectric properties, respectively. According to the above two improvements, 1 mol% Ce and 3 mol% Mg alternately doped BST thin films are prepared on Pt/Ti/SiO2/Si wafers (substrates). The prepared BST films are denoted by the doped element as follows: Ce/Mg/Ce/Mg/Ce/Mg with Ce doped BST layer is referred to as the first layer (for short Ce/Mg) and Mg/Ce/Mg/Ce/Mg/Ce with Mg doped BST layer as the first layer (Mg/Ce), and the structure and dielectric properties of the films are studied. X-ray diffraction results show that two films present cubic perovskite structures, mainly grow along (110) crystal face, and show strong crystallization. SEM results indicate that the surface morphologies of two films are greatly improved, and Ce or Mg doped BST layer as the first layer can be well matched with the substrate. The surface of the Ce/Mg film is more uniform and denser with slightly smaller grains and weaker crystallization. XPS results demonstrate that the non-perovskite structures on the surfaces of two films are significantly reduced. Each of the two films has high tunability at low applied dc bias and high figure of merit (FOM). The Mg/Ce film shows more stable combination of dielectric properties in a high frequency range. The Ce/Mg film shows more excellent combination of dielectric properties and higher dielectric strength in a low frequency range, where when the testing frequency is 100 kHz, 10 V, 20 V and 40 V applied dc bias voltages correspond to tunabilities of 47.4%, 63.6% and 71.8%, and FOMs of 71.8%, and 27.1, 77.5 and 86.5, respectively. Such good dielectric properties can fully satisfy the requirements for tunable microwave device applications. The relevant mechanisms are also analyzed.
