The Study Of Measuring The Parameters Of BST Thin Films Using CPW At The Microwave Frequence

Barium Strontium Titanate thin films (short for BST) have been reseached widely as a key kind of ferroelectric material due to their low leakage current, high dielectric constant at the microwave frequence, and BST's dielectric constant varies with the applied bias DC, which can be used to make some tuned devices such as phase shifters, filters and so on. The BST thin films phase shifter's performance is related to the parameters of BST including dielectric constant, loss angle and tunability, so the measuring of the above parameters of BST are important for designing BST thin films phase shifter.For the Coplanar Waveguide (short for CPW) transmission line is convenient for applying the bias DC to test the tunability of BST thine films, so we use the CPW to measure the above parameters of BST on the microwave frequence of 10 GHz.First of all, We simulated and reseached the CPW's performance of transmitting microwave using software HFSS, including the influence of the widths of CPW's signal line, gap, the sorts and thicknesses of metal, the dielectric constants and thicknesses of BST, and the character of underlays, especially reseached the instance that the width of ground line affected the character impedance and the high modes of CPW.And then, We designed several series nearly one hundred of CPW structures and related calibration kits on the basis of simulation. The widths of the signal line included 10μm, 20μm and 30μm, the width of gap was changed gradually, and the lengths of CPW included three different kind of sizes for the sake of satisfying the testing need of relative wide range of BST's dielectric constant (60-240) and relative wide range of BST's thickness (0.2μm-1.0μm). And we designed their layouts for photo lithography using software L-EDITIn succession, we reseached the relation of bias DC to the S parameters of CPW, emonstrating the S parameter reflect mainly the change of the BST's dielectric constant. Later, we used the HFSS method and the Conformal Mapping method (short for CMM) to compute the parameters respectively and compared with their results. When computing the characteristic impedance of CPW , the dielectric constant and the tunability of BST, the results of the two approached, but when computing the loss angle of BST, the difference was relatibe large reaching to 50%. Finally we build the modle of analyzing the errors of HFSS method, and the key way to minish the final errors was to minish the errors of the testing errors of S parameters and size parameters of CPW. When the testing errors were less than 1%, the final errors can be less than 30%.