Review on our recent research activities on sol-gel derived highly textured barium strontium titanate thin films for microwave dielectric applications

摘要

Barium strontium titanate (BST) thin films are attractive candidate for dynamic random access memories (DRAM) and frequency agile microwave electronics (FAME). For DRAM applications synthesis of polycrystalline thin films is sufficient, whereas for microwave dielectric applications highly textured or epitaxially grown BST thin films are required as the presence of high angle grain boundaries in polycrystalline films increases the dielectric loss and consequently limits the device performance. We have optimized the process methodology to synthesize highly textured BST thin films on lattice matched strontium titanate (STO) and lanthanum aluminate (LAO) substrates by using sol-gel technique. The high dielectric tunability and low dielectric loss of these films were attractive to fabricate eight element coupled micro-strip phase shifters with figure of merit 44o/db. We have adopted several novel approaches to improve the dielectric characteristics of textured BST thin films. As for example, uniform manganese (Mn) doping in BST lattice was found to improve the degree of phase shift with the simultaneous increase of insertion loss and as a result there is no effective improvement in the figure of merit, which remains in the range of 33-44o/db. Graded Mn doping, on the other hand, has been demonstrated as a promising approach to reduce the temperature coefficient of capacitance (TCC), loss tangent and leakage current of BST thin films. Alternate layer deposition of hetero-structured magnesium oxide (MgO): BST thin films is demonstrated to be another approach to synthesize dielectric films with a higher figure of merit for microwave dielectric applications. Although the dielectric constant and tunability are modified by MgO insertion, the dramatic reduction in the dielectric loss tangent effectively increased the figure of merit of the hetero-structured thin films as compared to pure BST films.