Scanning SQUID Microscopy

SQUID (superconducting quantum interference device), is the world's most sensitive magnetic flux sensor. In Wellstood's group we develop scanning SQUID microscopes to measure extremely weak magnetic fields with high spatial resolution. We have used the SQUID microscopes in wide variety of applications such as

    - paring symmetry of the order parameter in YBCO superconductor,
    - eddy current technique for nondestructive evaluation,
    - low noise and high performance SQUID for gravity wave detector,
    - NDE of superconducting wires,
    - non-destructive fault isolation (FI) of integrated circuits,
    - study of magnetic properties of magnetic materials such as CMR materials, Fe₃O₄, etc.

Both LTS and HTS dc SQUIDs are fabricated at the CSR.

A trilayer Nb-SQUID fabricated on Si substrate.

A HTS YBCO-SQUID on STO bicrystal substrate. The square washer SQUID has the outer dimension of 60 mm and the inner hole of 20 mm. The bridge is ~4 mm wide.

Scanning SQUID Microscopes

The microscope is an instrument which can measure magnetic field patterns and convert them into images. This usually involves scanning a magnetic sensor over the surface of a magnetic sample and converting the recorded magnetic field strengths into a false-color or grayscale images. Because magnetic fields are not visible to the eye, a magnetic microscope allows the study of properties of matter which are otherwise not observable. Since it turns out that many things produce magnetic fields, there is a wide variety of samples suited to this type of microscopy.

The cold stage of a liquid-Helium cooled scanning SQUID microscope.

Liquid-nitrogen cooled microscope on a support stand. The white part at the bottom is the cold end and holds the SQUID and sample. The rest of the probe consists of the motion controls and SQUID electronics.

The two microscopes below were designed to image objects at room-temperature. The sensors used in these microscopes are YBCO dc SQUIDs. We use a 25 mm - thick sapphire window to separate air from vacuum and maintain the SQUID at 77K. The sample is placed on the xyz scanning stage in air. With care, the sample can be placed within 50 mm of the SQUID. These two microscopes perform a variety of measurements, including imaging static fields from magnetic materials, non-destructive eddy current probes, low-frequency FI on IC's, and high-frequency imaging of rf and microwave sources.

A scanning SQUID microscope cooled by liquid nitrogen.

A scanning SQUID microscope cooled by Joule-Thompson cryo-refrigerator. The vacuum enclosure is less than 1' tall. A commercial version is also available at Neocera, Inc.

Applications:

Physical Failure Analysis of Integrated Circuits

Magnetic field genreated by current flow in a tape package. The Si substrate of the IC is in the middle of the picture. The magnetic image is overlayed on the picture of the sample. In our microscope, we detect the normal component of the magnetic field. In this magnetic image, red and blue are the opposite signs of the magnetic field (+/- 25 nT). White trace is the current path.

Picture shows part of a multi-chip module (MCM). The current is applied to the two wires shown on the right of the package carrier. The short-circuit is known to be inside a blue circle (~12 cm in diameter). The exact location is unknown.

Magnetic Bz(x,y) image over the electrical short-circuit in the MCM generated by 86 mA current. Maximum amplitude of magnetic field in this image is about 50 nT. The white path is roughly a zero field representing current path. SQUID-sample separation isabout 340 mm. This is because the current carrying layer is buried under many layers of metals and insulators.

Current density image, J2(x,y), obtained from converting magnetic image from left. Surprisingly, the inverse technique clearly show a meander pattern in the middle, in which it is not obvious in the magnetic image. The line-width of the current path is found to be about 75 mm. Short-circuit was found by comparing this image to circuit desing drawing.

Eddy Current for NDE

Picture shows a tri-layer of aluminum lap-joint simulating a part of an airplane's wing structure (scale 1:4). A very fine crack was created by an EDM machine in the second layer between the top surface and the support strut. The location of the defect is between the second and the third rivets as shown at the top of the sample. The thickness of the aluminum sheet is about 0.2 mm.

A picture shows a quadrature-phase (resistive regime) eddy current image taken with a driven current at 45 kHz. The SQUID-sample separation is about 0.5 mm. The anomaly created by the defect is clearly shown between the second and the third rivets.

A picture shows an in-phase (inductive regime) eddy current image taken simutaneously with the image on the left. The contrast is not as good as the quadrature-phase image, i.e. the sample is in the resistive regime. The contrast also depends on the geometry of the excitation coil.

SQUID for Gravity Waye Detector

A two-stage SQUID system for gravity wave detector.

Selected Publications

1.    A. Mathai, D.Song, Y.Gim, and F.C. Wellstood, Appl. Phys. Lett. 61, 598 (1992).
2.    R.C. Black, A.Mathai, F.C. Wellstood, E. Dantsker, A.H. Miklich, D.T. Nemeth, J.J. Kingston, and J. Clarke, Appl. Phys. Lett.62, 2128 (1993).
3.    A. Mathai, D.Song, Y.Gim, and F.C. Wellstood, IEEE Trans. Appl. Supercond. 3, 2609 (1993).
4.    R.C. Black, F.C. Wellstood, E. Dantsker, A.H. Miklich, J.J. Kingston, D.T. Nemeth, and J. Clarke, Appl. Phys. Lett. 64, 100 (1994).
5.    R.C. Black, F.C. Wellstood, E. Dantsker, A.H. Miklich, D. Koelle, F. Ludwig, and J.Clarke, Appl. Phys. Lett. 66, 100 (1995).
6.    A. Mathai, Y.Gim, R. C. Black, A. Amar, and F.C. Wellstood, Phys. Rev. Lett. 77, (1995).
7.    I. Jin, F. C. Wellstood, IEEE Trans. Appl. Supercond. 9, 2931 (1996).
8.    F. C. Wellstood, Y.Gim, A. Amar, R.C. Black, and A. Mathai, IEEE Trans. Appl. Supercond. 7, 3134 (1997).
9.    I. Jin, A. Amar, T. R. Stevenson, F. C. Wellstood, A. Morse, W. W. Johnson, IEEE Trans. Appl. Supercond. 7, 2472 (1997).
10. S. Chatraphorn, E. F. Fleet, R. C. Black, and F. C. Wellstood, Appl. Phys. Lett. 73, 984, (1998).
11. E. F. Fleet, S. Chatraphorn, F. C. Wellstood, S. M. Green, L. A. Knauss, IEEE Trans. Appl. Supercond. 9, 3704 (1999).
12. E. F. Fleet, S. Chatraphorn, F. C. Wellstood, L. A. Knauss, IEEE Trans. Appl. Supercond. 9, 4103 (1999).
13. S. Chatraphorn, E. F. Fleet, F. C. Wellstood, L. A. Knauss, T. M. Eiles, Appl. Phys. Lett. 76, 2304 (2000).
14. E. F. Fleet, S. Chatraphorn, F. C. Wellstood, S. M. Green, L. A. Knauss, Submitted to Rev. Sci. Instr.

Center for Superconductivity Research, University of Maryland, College Park, MD 20742-4111
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