Study On Spatial Resolution Of Scanning SQUID Microscope And Its Application On Non-destructive Test Of Creepage Current

Since the discovery of high temperature superconductors in 1986, the applications of Superconducting Quantum Interference Device (SQUID) have drawn increasing attention. This dissertation contains the main results of the research work for the Master's degree of Sciences, including the efforts for improving the spatial resolution of the Scanning SQUID Microscope (SSM ) designed and constructed by the SC6 group of Institute of Physics Chinese Academy of Sciences (IOP, CAS) and the work for applications of the SSM in the Non-destructive Testing (NDT), especially in detecting weak current. To start with, fundamental concepts and main theories of Josephson effect and SQUID were briefly introduced in the dissertaion. Attention was paid to the development of the SSM, which is a new type of magnetic imaging instrument, being able to detect extremely small fields or currents with unparalleled sensitivity. The SSM designed by the SC6 group of IOP, CAS consists a SQUID made by YBCO, the electronics system, the cryogenics maintain system, the vacuum system, the sample mounting and moving system, the data collection system and the control system. The magnetic sensitivity of the microscope is 46pT/Hz1/2 and the spatial resolution is about 400um. The spatial resolution is worse than the best spatial resolution of other type SSMs, of which the reason is that the sample of ours being in room temperature whereas the sample of others lying inside the low temperature dewar, the unavoidable vacuum layer of the dewar leading to the deterioration of spatial resolution. Therefore it is very important to find a technique being able to improve the spatial resolution of SSM for room temperature samples, which is the main task of this dissertation. As a potential candidate of such technology, magnetic concentration device (i.e. concentrator) was systematically studied and a special parameter (i.e., concentration parameter K), for characterizing the feature of the concentrator, was proposed. A simple model was then developed, of which K is dependent on the magnetic permeability of the material and the demagnetization factor of the device. Experimental results of dependences of the concentration parameter on device length and excited field (current) were obtained, respectively. It was established that the magnetic concentrator should be able to improve the spatial sensitivity of the SSM. A real device containing a concentrator for the present SSM in IOP, CAS has been designed and constructed and the experiments of the SSM with this magnetic concentrator device is being carried out. The applications of SSM in NED were also discussed in the dissertation. One of them is the detection of weak currents inside a sample (e.g. short circuit current inside a IC chip). Effort has been concentrated on the inverse process of obtaining two-dimension images of the weak current from the magnetic field measured by the SSM. Considering that what being detected by the SQUID is the vertical component of the magnetic field on a plan where the SQUID lies, whereas the current which produces the magnetic field is actually located in a plan below the SQUID, a TWO PLAN model has been developed. In this model Biot-Savart force laws and Fourier transformation were used to inverse the detected magnetic field into the underneath weak current. It has been shown that the distance between the current and the SQUID, which is important parameter for the spatial resolution, and the noise ratio of the experimental data, which is related to another important parameter of the microscope, i.e., magnetic field sensitivity, have significant effects on the quality of the inverse process.