edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 28 NO.4 6 Based on inspection needs, users can balance factors such as time-to-data (urgency), required data precision, and the level of structural detail, and accordingly select the appropriate imaging condition, ranging from fast micro-CT for quick screening to high-resolution nano-CT for detailed analysis. Electrical fault isolation (e.g., lock-in thermography, emission microscopy).[9-11] Lock-in thermography identifies electrically active defects under realistic operating conditions, including electromigration hotspots, resistive shorts, leakage paths, and high-resistance inter- connects that directly impact performance and power efficiency. The technique operates by applying a periodic electrical stimulus to a device under test, generating localized heating at defect sites due to Joule heating. An infrared camera synchronously detects the resulting temperature modulation, and lock-in signal processing extracts phase and amplitude information to enhance weak thermal signals while suppressing noise. This enables high-sensitivity localization of electrically active defects in both lateral and axial (depth) directions within complex advanced packaging structures. Enhanced LIT analysis of a defect in a 4-layer HBM stack along lateral direction is shown in Fig. 4. The right side shows the raw amplitude response at 21 Hz, where heat diffusion causes significant spreading, limiting precise localization. The left image of Fig. 4 demonstrates improved lateral localization using phase-based heat propagation inversion, reducing the apparent hotspot size by over an order of magnitude. Engineers can first judge the true lateral defect position from the image plane. Furthermore, Fig. 5 shows enhanced LIT analysis of a defect in a four-layer HBM stack along axial direction. The right image of Fig. 5 presents a reconstructed thermal cross-section, enabling quantitative depth estimation of the defect at approximately 670 µm below the surface. The data in this graph are synthesized through recursive extrapolation for inverting the heat propagation under consideration of the sample’s internal structure, the corresponding thermal diffusion coefficients, and the phase distribution measured across the sample’s surface. From this graph, the depth of the hot spot can be approximated to be 670 µm underneath the sample’s surface. The left Fig. 3 A reconstructed vertical slice and 3D render of the region of interest scan of HBM device obtained by high-resolution x-ray nano-CT.[6] Fig. 4 Comparison of before (right) and after (left) results of lateral-resolution-enhanced LIT 3D localization of a defect in a 4-layer HBM device.[10]
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