Preparation And Properties Of Niobium Thin Films In Superconducting Gravity Instruments

The superconducting coil is an important part of the core detector in superconducting gravity instruments,which forms a magnetic spring oscillator with the test mass.Its main function is to provide levitation force for the test mass,and detect the gravity signal in combination with SQUID.The quality of the superconducting coil is particularly important.The superconducting coil prepared by the traditional craft has some disadvantages,such as non-uniform thickness,low preparation efficiency and poor correlation.In addition,it can not make curved coils as well.All of these will affect the comprehensive performance of the superconducting gravity instruments.In this paper,the high quality superconducting film was prepared by magnetron sputtering,and the investigation on the preparation of superconducting coil based on the method was carried out.In this study,the superconducting niobium films were prepared by DC magnetron sputtering.Superconducting niobium with purity of 99.995%was selected as the sputtering target,high-purity argon was used as the working gas,and intrinsic silicon wafer was used as the substrate material in the experiment.Different sputtering parameters were used to deposit niobium films on silicon wafer,the reasons for the differences included sputtering rate,resistivity,micro morphology,crystal structure and superconducting properties were analyzed by stylus profiler,scanning electron microscope(SEM),X-ray diffraction(XRD)and physical property measurement system(PPMS).The effects of different background vacuum,sputtering current,sputtering pressure and substrate temperature on the basic properties of niobium films were analyzed by variable-controlling approach.The sputtering current and sputtering pressure are the main factors that affect the deposition rate of niobium films,and also the important factors that lead to difference of the surface morphology and conductivity of niobium films.It is good for niobium atom diffusion and reducing the resistivity of niobium film by adjusting the substrate temperature reasonably,but it has little effect on the sputtering rate of niobium films.PPMS was used to measure the superconductivity of niobium films.The highest critical temperature of Nb films is 6.26K when the background vacuum is 8.8×10^-4Pa,and the sputtering parameters are 350W,1.2Pa and 350?.The composition of niobium films was analyzed by energy dispersive spectrometer.It is found that the content of oxygen atom is the fundamental factor which leads to the difference of superconductivity.Finally,the annealing experiment was carried out and the critical current density was measured.The critical current density of niobium films is higher than 1.6×10²A/cm².