November_EDFA_Digital

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 26

after the bare substrate termination, corresponding to the top of the C4 bump. Physical failure analysis (PFA) of the device later verified the EOTPR results, revealing that the open fault arose froma passivation layer on the C4 pad, as shown in the inset of Fig. 4, which is located 105 µm from the bare substrate termination. MICROELECTROMECHANICAL DEVICES The short trace length of MEMS devices can prove challenging for EOTPR investigations, due to their lack of reference features for localizing the fault. A common workaround is to introduce artificial faults in the reference devices, which can be used to extract the fault location in the failed device. Figure 5 shows a schematic and cross section of a MEMS device used in one such case. The device consists of aMEMS unitmounted onto a substrate, with the electrical connection between the MEMS device and the solder ball made by a through-silicon via (TSV) and a redistribution layer (RDL). The EOTPR waveform obtained for a KGD (green trace), reference device (blue trace), and the failed unit (red trace) are shown in Fig. 6. The reference sample was preparedby cutting the surface signal trace in the RDL of a separate KGD. The approximate location of the cut is shown in Fig. 5. The open peak for the failed sample was calculated at 320 µm after the cut in the RDL. Later PFA, shown in the inset of Fig. 6, reveals that the fault arose from a crack in the TSV interface, which was separated from the artificial defect by 325 µm.

waveform, highlighted in Fig. 4, clearly demonstrates the open failure in the device, which is calculated at 100 µm

Fig. 3 EOTPR results from a 2.5D case study. The EOTPR waveformfromthe failedunit (red line) clearly shows an open fault indicated by the positive peak in the waveform. This occurs between the substrate (black curve) and interposer (blue curve) terminations.

Fig. 4 Zoomed-in EOTPR waveform of the fault, and inset, SEM image of a cross section through the TSV of the failed unit showing a thin layer of passivation, highlighted by the red box between the TSV and RDL Cu.

Fig. 5 Top: Cross section through a MEMS device showing the location of the open in the RDL of the reference device. Bottom: Schematic of the MEMS device.

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