## 3 Defect Localization  
### Magnetic Field Imaging  
To locate the defect, both fail and reference samples were scanned with the SQUID sensor for an overview scan first and then the GMR sensor for a zoomed-in scan of the die area by Neocera Magma EFI HiRes, as shown in Figures 2 and 3. AC and RF scans were acquired for the reference sample. The AC and RF scans of the reference sample helped us understand the way the current is supposed to propagate in the sample when there is no failure. Only RF scans were acquired for the failing sample. We used a two-probe RF technique for the failing sample to try to isolate the failure channel from the whole grid. For the AC scan in channel 6, the current input from the pad of upper surface J12 travels through the TOP_M2 layer to the TOP_M1 layer, then passes the TSV down to the lower surface, travels through the BTM_M1 layer, then passes the TSV back to the upper surface, it repeated many times, and finally output from the pad of the upper surface J3. The current image is shown in Figure 4.  
Figure 4: Overlays of the current images with the optical images: (**a**) SQUID AC scan; (**b**) GMR AC scan. Figure 3: Failing sample under the GMR sensor. The main circuit path, as shown in the AC scan, shows a weaker signal. The relative intensity of the signal in the fail is lower compared to the reference. The fact that we see a weaker signal in the fail sample points to a possible failure in that path. The RF signal followed the whole circuit rather than the single line connecting J12 to J3 in the AC case. For some reason, the RF signal propagated more strongly in the top part of the circuit for both the reference and the fail. This complicated the analysis. The RF scan image from pin J12 on reference and fail sample is shown in Figure 5. The same behavior can be observed from pin J3. For this reason, a two probe RF technique on pins J3 and J12 was performed. This can help the RF signal to propagate through the path of interest. Two RF probes were connected to pins J3 and J12 (Channel 6) at the same time. Some unusual behavior in the frequency sweep for the failing sample was noticed in comparison with the frequency sweep for the reference sample, as shown in Figure 6. The reference sample sweep has only one voltage dip, where the RF signal drops significantly. The failed sample sweep shows two dips at two different frequencies. The typical behavior is to only have a single dip. Scans are performed at different frequencies. The reference sample current density data were taken at 43.0 MHz, with one probe on the left side and one probe on the right side. As shown in Figure 7, the signal shows no significant decay along with the green cursor at all three modes. Figure 5: RF scan from pin J12: (**a**) Reference sample; (**b**) Failed sample. Figure 6: RF frequency sweep curve: (**a**) Reference sample; (**b**) Failed sample. Failed sample current density data were taken with both probes at the same time with two different frequencies, as shown in Figure 8. The signal decays in the same general area for both datasets along the green cursor. The red arrows point to that area of signal decay. The yellow cross shows the position of the possible open location, as shown in Figures 9 and 10. The linear decay shows us more precisely where the current signal disappears. The linear decay works best with the signals that propagate in one plane. This particular signal seems to changes planes at the same time as it is decaying. We may experience some overshooting from the actual open location. The possible location of the opening was on the right side of the sample in the proximity of the fifth via (see blue arrow in Figure 10). Given the proximity of the location to the via, the fifth via was viewed as the likely opening location. Figure 8: RF scan with two probes on failed sample: (a) F = 42.5 MHz; (b) F = 48.0 MHz. Figure 10: Optical images overlay with Current images. Figure 4:0 RF scan on reference sample at 43.0 MHz: (a) Left probe; (b) Right probe.