Wafer bonding layer characterization and inspection by Electrochemical Impedance Spectroscopy (EIS)

To address the limitations of current wafer bond quality assessment methods for 3D integration, this study introduces a novel, non-destructive Electrochemical Impedance Spectroscopy (EIS) technique employing a specialized ball-to-rod magnetic probe. This novel approach overcomes the time-consuming and often destructive nature of conventional methods like direct mechanical testing, scanning acoustic microscopy (SAM) and transmission electron microscopy (TEM). The key innovation lies in the magnetic probe's ability to perform highly localized impedance measurements, which directly correlate physical bonding defects to their unique impedance responses. The technique's accuracy was validated through experimental measurements in well-bonded regions (∼622 kΩ) which closely matched theoretical correlations on a 12-in. SiCN bonded wafer. Subsequent 7 × 7 grid mapping measurement system revealed the method's high sensitivity to spatial degradation, identifying poorly bonded regions with resistances up to 2 MΩ and pinpointing localized defects (voids, particles) later verified by Near-Infrared (NIR) imaging. Furthermore, this study quantified process improvements in bonding quality with post-annealing treatments, mirrored by a significant reduction and homogenization of interfacial resistance across the bonded wafer, with values dropping from a non-uniform 2–3 MΩ to a more uniform ∼1 MΩ. A rigorous 37-point polar map of the annealed wafer was adopted to characterize the remaining non-uniformity, revealing a distinct radial degradation with average impedance increasing from a low of ∼0.3 MΩ at the center to a less uniform ∼1.2 MΩ at the edge. This work, therefore, validates EIS with a novel probe setup as a reliable, non-destructive, high-resolution and highly sensitive technique for characterizing localized defects in bonded wafers, offering significant potential for real-time process monitoring for wafer-level 3D integration.