Localization of Electrical Defects in Hybrid Bonding Interconnect Structures by Scanning Photocapacitance Microscopy

We report a scanning photocapacitance microscopy technique for the localization of open electrical defects in hybrid bonding wafer-to-wafer (W2W) interconnect structures for 3-D system integration. Whereas the well-known optical beam induced resistance change (OBIRCH) method is effective for the localization of resistive opens and shorts, it cannot be applied on full electrical opens. Our approach uses a focused laser beam to stimulate the photocapacitance of the W2W interconnect, and by measuring its response, we can verify the electrical continuity of the structure. We show how illuminating the interconnects with the light of a suitable wavelength leads to an increase of the depletion capacitance due to carrier generation in the silicon substrate. Our measurement instrument uses a commercially available sigma-delta ( \Sigma -\Delta ) capacitance-to-digital converter (CDC) chip with a capacitance sensing resolution down to 4 aF. We demonstrate this methodology on large open-failed W2W interconnect chains with sub-micrometer hybrid-bonded interconnect pad dimensions and confirm our results by optical and transmission electron microscopy (TEM).