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Scanning SQUID microscope
A Scanning SQUID Microscope is a sensitive near-field imaging system for the measurement of weak magnetic fields by moving a Superconducting Quantum Interference Device (SQUID) across an area. The microscope can map out buried current-carrying wires by measuring the magnetic fields produced by the currents, or can be used to image fields produced by magnetic materials. By mapping out the current in an integrated circuit or a package, short circuits can be localized and chip designs can be verified to see that current is flowing where expected.
==High temperature scanning SQUID microscope==
A high temperature Scanning SQUID Microscope using a YBCO SQUID is capable of measuring magnetic fields as small as 20 pT (about 2 million times weaker than the earth’s magnetic field). The SQUID sensor is sensitive enough that it can detect a wire even if it is carrying only 10 nA of current at a distance of 100 µm from the SQUID sensor with 1 second averaging. The microscope uses a patented design to allow the sample under investigation to be at room temperature and in air while the SQUID sensor is under vacuum and cooled to less than 80 K using a cryo cooler. No Liquid Nitrogen is used. During non-contact, non-destructive imaging of room temperature samples in air, the system achieves a raw, unprocessed spatial resolution equal to the distance separating the sensor from the current or the effective size of the sensor, whichever is larger. To best locate a wire short in a buried layer, however, a Fast Fourier Transform (FFT) back-evolution technique can be used to transform the magnetic field image into an equivalent map of the current in an integrated circuit or printed wiring board.〔J. P. Wikswo, Jr. “The Magnetic Inverse Problem for NDE”, in H. Weinstock (ed.), SQUID Sensors: Fundamentals, Fabrication, and Applications, Kluwer Academic Publishers, pp. 629-695, (1996)〕〔E.F. Fleet et al., “HTS Scanning SQUID Microscopy of Active Circuits”, Appl. Superconductivity Conference (1998)〕 The resulting current map can then be compared to a circuit diagram to determine the fault location. With this post-processing of a magnetic image and the low noise present in SQUID images, it is possible to enhance the spatial resolution by factors of 5 or more over the near-field limited magnetic image. The system’s output is displayed as a false-color image of magnetic field strength or current magnitude (after processing) versus position on the sample. After processing to obtain current magnitude, this microscope has been successful at locating shorts in conductors to within ±16 µm at a sensor-current distance of 150 µm.〔L. A. Knauss, B. M. Frazier, H. M. Christen, S. D. Silliman and K. S. Harshavardhan, Neocera LLC, 10000 Virginia Manor Rd. Beltsville, MD 20705, E. F. Fleet and F. C. Wellstood, Center for Superconductivity Research, University of Maryland at College Park College Park, MD 20742, M. Mahanpour and A. Ghaemmaghami, Advanced Micro Devices, One AMD Place Sunnyvale, CA 94088〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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