Study On Fabrication Of Superconducting MgB₂ Thin Films

As-grown superconducting MgB₂ thin films were prepared by magnetron sputtering and the influence of technological conditions on quality of thin films were preliminary discussed in this paper. The work was made up of two parts: fabrication of targets and deposition of thin films.The first part was fabrication of targets. MgB₂ bulks sintered at different temperatures, with different increasing temperature rates and holding time were analyzed by X-ray diffraction(XRD) and scanning electron microscopy(SEM). The influence of heat treatment on formation of MgB₂ phase and microstructure were investigated. The results were as follows:1. MgB₂ phase could form at the range from 600℃ to 850℃, and 700⁷50℃ was the best range of reaction temperature, because it can improve the connection of grains and reduce the impurity MgB₄. And the density of MgB₂ bulks sintered at 750 ℃ was relatively high.2. The formation of MgO was hardly influenced by different increasing temperature rates, but relatively large temperature increasing rates could contribute to purer MgB₂.3. The formation of MgB₂ phase was very rapid and the influence of holding time on formation of MgB₂ phase was not obvious. But crystallinity of MgB₂ can be improved by prolonging holding time.On base of research on MgB₂ bulks, MgB₂ targets were sintered at 750℃ with heating-up velocity of 60℃/min and kept with 150 minutes in consideration of density and oxidation.And they were analyzed by XRD and transmission electron microscope(TEM).In the second period of experiment, two methods were tried in succession. They were one composite target sputtering and two targets co-sputtering. Analysis such as XRD, atomic force microscopy(AFM), SEM(EDS: energy dispersive spectroscopy) and X-ray photoelectron spectroscopy(XPS) were done to the resultant films. The superconducting critical transition temperatures were also measured using standard four-probe method. The as-grown MgB₂ thin film, whose zero resistance temperature T_c0 was 7.6K, was obtained by single-step growth on the substrate of Si(100) andTiN/Si(100) using one composite target sputtering. The MgB2thin film, whose Tco was 19.6K, was obtained by co-sputtering on the substrate of MgO(lll) using the Mg target and MgB2 target together.The research on thin film deposition using one composite target sputtering and two targets co-sputtering can be concluded as follows:1. It was a key to the performance of thin film whether it is successful or not to assure the appropriate proportion of Mg and B atom deposited on the substrate. And whether they were made into MgB2 and stable or not was also very important.2. It was considerably difficult to get the thin film of stoichiometric MgB2 using one composite target sputtering to control the final thin film composition. By two targets co-sputtering the thin film of stoichiometric MgB2 was easy to get though adjusting the sputtering power rate of Mg target and MgB2 target.3. The MgO in the thin film was mainly caused by targets. So it needs more requirements about the fabrication processing of targets.