Perovskite ferroelectric thin films for tunable microwave applications

Simple and cost effective sol-gel technique was utilized to deposit highly oriented thin film perovskite ferroelectrics for their possible applications in tunable microwave devices. We optimized the process methodology to synthesize highly (100) oriented Ba0.5Sr0.5TiO3 (BST50) and Ba0.6Sr0.4TiO3 (BST60) thin films on (100) strontium titanate and (100) lanthanum aluminate substrates by sol-gel technique and to achieve best possible dielectric properties for tunable microwave devices. The annealing temperature and time were found to have effect on the epitaxial quality, phase transition behavior, and dielectric properties of the BST films. With increase in annealing temperature the grain boundaries disappear and the grains coalescence occurs with increase in annealing time and the epitaxy is improved. Eight element coupled microwave phase shifters were designed using the standard lithographic method and phase shift & loss characteristics were measured between 12--18 GHz range (Ku band). The degree of phase shift was increased from 221 to 308° (measured at 14.5 GHz in an electric field ranging from zero to 30V/mum) with the improvement in the epitaxial quality of the films. An insertion loss of 8.435 dB, phase shift in the order of 320° (20--340V) and kappa value (phase shift per dB loss) of about 38°/dB was achieved in Ba0.6Sr0.4 TiO3 films deposited on LAO (100) substrate.; We have also studied the effect of Mn doping on the degree of texturing, surface morphology, dielectric properties, and phase transition behavior of BST50 thin films. With 3 at % Mn doping the degree of (100) texturing and grain size of BST50 thin films were markedly improved, which led to an increased tunability from 29% (undoped) to 39% (3 at % Mn doped); measured at 1 MHz and 2.34V/mm bias field. The phase shift measurements at 16 GHz showed that the degree of phase shift increases from 239° to 337° with 0 to 3 at % Mn doping. The insertion loss also increased from 5.4 dB (undoped) to 9.9 dB (with 3 at % Mn doped), so that there is no effective improvement in the k factor, which remains in the range of 33--44°/dB.; It would be desirable for the ferroelectric thin films to have moderate dielectric constant while having low losses in order to obtain high figure of merit, hence hetero-structured thin films of BST with low loss materials like MgO and MgTiO3 were synthesized. We have therefore studied the effect of thickness of the BST50 and MgO (and MgTiO3) layers on the structural, microstructural, & dielectric properties, and the investigations on the leakage current characteristics of these heterostructured thin films. (Abstract shortened by UMI.)...