edfas.org 15 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 28 NO. 4 FAULT LOCALIZATION THROUGH SCHEMATICS AND LAYOUT REVIEW AND MICROPROBING ANALYSIS The schematic and layout were reviewed to map the suspected component with respect to its circuit block, functionality, and electrical response. Review revealed that the unpowered circuit blocks during LEM analysis were identified as the oscillator driver circuitry and the reference circuitry. Meanwhile, the circuit block exhibit- ing a resistance change during OBIRCH analysis was identified as the Ibias circuitry. Further review indicated that the anomaly is highly likely on a resistor R1 in the start-up circuitry within the Ibias circuitry block of the transformer driver die of the unit. The anomaly on the resistor R1 may be causing the start-up current to decrease when the unit is subjected to cold temperature prior to power up—making the start-up current insufficient to establish the bias circuitry. The unestablished bias circuitry would cause the other circuit blocks to be unpowered causing no oscillation on the transformer drivers. Microprobe analysis—a technique that uses fine‑pitch electrical probes to directly contact specific nodes on the die to measure voltages, currents, or resistances during active biasing—was performed on the Ibias circuitry of the transformer‑driver die to validate the suspected anomaly at resistor R1. The results revealed that the suspected resistor R1 exhibited a significantly higher resistance than its expected design value, indicating an open‑resistance condition. See Fig. 5. VIA SIDEWALL NANOGAPS UNVEILED Focused ion beam (FIB) uses a finely focused ion beam to mill and expose precise cross-sections of the die. FIB was performed on multiple contacts and vias connected to the suspected resistor R1, and the resulting sections were inspected through SEM, which uses a focused beam of high energy electrons to generate high resolution surface and cross-sectional images. Results did not reveal any significant anomaly initially. Further FIB-SEM inspection was requested to retake the images with a higher magnification on the area of interest (AOI), focusing on the interface of the vias and metals to check for any anomalies. Additionally, it was requested that the images be taken using a lower acceleration voltage at SEM, as the initial FIB cross-section images were observed to have been taken at a higher acceleration voltage. Using lower acceleration voltage at FIB-SEM, results showed nanogaps on the sidewall of the vias connecting the suspected resistor R1 and VDD (Figs. 6 and 7). Fig. 6 Images showing the area of interest (AOI) sites at FIB. (continued on page 18)
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