The challenges that 3D integration present to Failure Analysis require the development of new Fault Isolation techniques that allows for non-destructive, true 3D failure localization.
By injecting a current in the device under test (DUT), the current generates a magnetic field around it and this magnetic field is detected by a sensor above the device. Magnetic field imaging (MFI) is a natural candidate for 3D Fault Isolation of complex 3D interconnected devices. This is because the magnetic field generated by the currents in the DUT passes unaffected through all materials used in device fabrication; the presence of multiple metal layers, dies or other opaque layers do not have any impact on the magnetic field signal. The limitations of the technique are not affected by the number of layers in the stacked devise in samples such as wirebonded stacked memory, Through Silicon Via (TSV) stacked die or even package on package (PoP). The sample is raster scanned and magnetic field is acquired at determined steps providing a magnetic image of the field distribution. This magnetic field data is typically processed using a standard inversion technique to obtain a current density map of the device. The resulting current map can then be compared to a circuit diagram, an optical or infrared image, or a non-failing part to determine the fault location. Today, giant-magnetoresistive (GMR) sensors have been added to the Superconducting Quantum Interference Device (SQUID) sensor to allow higher resolution and Fault Isolation (FI) I at die level.
Magnetic Field Imaging (MFI), using SQUID as the high sensitive magnetic sensor in combination with a high resolution GMR sensor. A solver algorithm capable of successfully reconstructing a 3D current path based on an acquired magnetic field image from both sensors has been developed. The generic 3D inverse problem has no unique solution. Given a particular 3D magnetic field distribution, there are an infinite number of current path distributions that will result in such magnetic field. This ill-posed problem has restricted, so far, the use of magnetic imaging to 2D. A different kind of 3D solver can be constructed, nevertheless capable of obtaining a single solution.
The 3D solver algorithm is not only capable of extracting the 3D current path, but it also provides valuable geometrical information about the device. Accurately being able to position each current segment in a layer allows the FA engineer to follow the current as it vertically moves from one die (or layer) to another. [1,2,3]
Magnetic field imaging and light induced capacitance alteration for failure analysis of Cu-TSV interconnects