Fabrication and Characterization of High Sensitivity SQUID Magnetometers for Biomagnetism

• Main Authors: Chiu-Hsien Wu, 吳秋賢
• Other Authors: 楊鴻昌
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/64241442827228733336

博士 === 國立臺灣大學 === 物理研究所 === 93 === The fabrication and characterization of high-Tc sensitivity SQUID magnetometers for biomagnetism at 77 K were investigated. To pursue the high reproducibility and quality step edge for high-Tc SQUIDs, we have developed two-step procedures of fabricating very good step-edge substrates. We have characterized high-Tc step-edge SQUIDs connected in series. The flux-to-voltage transfer function increase with the number of SQUIDs. The flux noise, SΦ of the serial SQUID arrays scale as 1/N1/2. Second, we fabricated a serial low-noise direct-current superconducting quantum interference device (SQUID) magnetometer on a 10 mm × 10 mm SrTiO3 (100) bicrystal substrate. This field sensitivity around 30–80 fT/Hz1/2 has been obtained at 77 K in the white noise region. The latter noise has been avoided by optimizing the geometry of the pickup coil as well as by using the so-called flux dam. As a result, the 1/f noise of SQUIDs has been much improved. Finally, using atomic Force microscope (AFM) and scanning electric microscopy (SEM), we observe an inhomogeneous distribution and microstructural defects on YBCO grain-boundary Josephson junctions. SEM images show that these defects cause the grain boundary of YBCO thin film grown on the bicrystal to a few submicron depth of the groove in film. The existence of these defects is expected to affect the superconducting current and the motion of the magnetic flux in the films and hence generate noise in the devices.