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Y. Li and D. Goyal
development, LIT could still be a very promising method for the non-destructive
fault isolation of resistive failures in 3D packages [13].
18.3.1.4 Scanning Superconducting Quantum Interference Device
(SQUID) Microscopy (SSM)
Scanning Superconducting Quantum Interference Device (SQUID) Microscopy
(SSM) is a generic name for FI techniques using a SQUID sensor to map the magnetic
field produced by a current injected into the failing structure of the device under test
(DUT) [14]. It is sometimes called Magnetic Field Imaging (MFI), or Magnetic
Current Imaging (MCI) [15, 16]. For short and high resistance open failures, SQUID
sensor typically operates at kHz frequency range. By increasing the bandwidth of
SQUID electronics to RF range, SSM is capable for isolating open failures. The high
frequency application of SSM is also called Space Domain Reflectometry (SDR)
[15]. SSM has been applied as a non-destructive fault isolation technique for electrical failures in conventional packages [14–16]. Magnetic field generated by the
input current inside the failed unit is detected and processed using a Fourier Transform inversion technique to obtain current density map of the sample. The x and y
locations of the failure can be determined by comparing the current density map from
the failed unit to a circuit diagram or a golden unit. The z dimension of the failure
location can be estimated based on the fact that the distance between the minimum
and the maximum magnetic field around a straight current path in the unit is twice
the total distance from the SQUID sensor to the current path [16]. The distance from
the sensor to the sample surface is fixed for each SSM scan, therefore the distance
from the sample surface to the current path could be estimated. Figure 18.10 illustrates how the x, y, and z locations of a short failure in a 3D package with stacked
die configuration can be defined by SSM. As shown in Fig. 18.10a, x and y location
of the short can be determined by the current analysis, while the z location of the
defect is estimated to be 560 μm away from the Si surface, by subtracting the sensor
to sample distance (393 μm) from the sensor to current path distance (953 μm),
which is half of 1906 μm, the distance between the minimum and the maximum
magnetic field around the current path (Fig. 18.10b). By comparing to the design
file, the failure is isolated to be in the micro bump area between the two dice, shown
in Fig. 18.10c [16].
An alternative method to acquire z information of the failure by SSM is to perform
the analysis of the magnetic field signal by a different algorithm so that a 3D current
path can be calculated instead of a 2D current path by the conventional method [17].
It has been demonstrated that the method is promising to map 3D current path in 3D
packages with stacked die configuration [17].
SSM is a complementary technique to LIT, as it provides current mapping instead
of locating the dissipated heat from short defects. For example, Defects causing short
failures with extremely small resistance may not always get captured by LIT. High
resistance routing in the failing structure, like TSV, could also shadow the dissipated
heat coming from short defects. The phase shifts obtained by LIT from multiple
Y. Li and D. Goyal
development, LIT could still be a very promising method for the non-destructive
fault isolation of resistive failures in 3D packages [13].
18.3.1.4 Scanning Superconducting Quantum Interference Device
(SQUID) Microscopy (SSM)
Scanning Superconducting Quantum Interference Device (SQUID) Microscopy
(SSM) is a generic name for FI techniques using a SQUID sensor to map the magnetic
field produced by a current injected into the failing structure of the device under test
(DUT) [14]. It is sometimes called Magnetic Field Imaging (MFI), or Magnetic
Current Imaging (MCI) [15, 16]. For short and high resistance open failures, SQUID
sensor typically operates at kHz frequency range. By increasing the bandwidth of
SQUID electronics to RF range, SSM is capable for isolating open failures. The high
frequency application of SSM is also called Space Domain Reflectometry (SDR)
[15]. SSM has been applied as a non-destructive fault isolation technique for electrical failures in conventional packages [14–16]. Magnetic field generated by the
input current inside the failed unit is detected and processed using a Fourier Transform inversion technique to obtain current density map of the sample. The x and y
locations of the failure can be determined by comparing the current density map from
the failed unit to a circuit diagram or a golden unit. The z dimension of the failure
location can be estimated based on the fact that the distance between the minimum
and the maximum magnetic field around a straight current path in the unit is twice
the total distance from the SQUID sensor to the current path [16]. The distance from
the sensor to the sample surface is fixed for each SSM scan, therefore the distance
from the sample surface to the current path could be estimated. Figure 18.10 illustrates how the x, y, and z locations of a short failure in a 3D package with stacked
die configuration can be defined by SSM. As shown in Fig. 18.10a, x and y location
of the short can be determined by the current analysis, while the z location of the
defect is estimated to be 560 μm away from the Si surface, by subtracting the sensor
to sample distance (393 μm) from the sensor to current path distance (953 μm),
which is half of 1906 μm, the distance between the minimum and the maximum
magnetic field around the current path (Fig. 18.10b). By comparing to the design
file, the failure is isolated to be in the micro bump area between the two dice, shown
in Fig. 18.10c [16].
An alternative method to acquire z information of the failure by SSM is to perform
the analysis of the magnetic field signal by a different algorithm so that a 3D current
path can be calculated instead of a 2D current path by the conventional method [17].
It has been demonstrated that the method is promising to map 3D current path in 3D
packages with stacked die configuration [17].
SSM is a complementary technique to LIT, as it provides current mapping instead
of locating the dissipated heat from short defects. For example, Defects causing short
failures with extremely small resistance may not always get captured by LIT. High
resistance routing in the failing structure, like TSV, could also shadow the dissipated
heat coming from short defects. The phase shifts obtained by LIT from multiple