High-precision Cu pad height measurement using quad-detector SEM signals for hybrid bonding

Semiconductor integration is advancing toward increasingly three-dimensional structures, which elevates the importance of wafer hybrid bonding. In this process, Cu pads must be recessed by several nanometers to ensure proper bonding, requiring sub-nanometer measurement precision. We propose a method to efficiently evaluate Cu pad height using quad-detector backscattered-electron (BSE) signals integrated in the scanning electron microscopes (SEMs). We extracted surface-height information by analyzing signal differences from multiple directions. Using 350-nm-sized Cu pads, we confirmed sensitivity to height variation through comparison with atomic force microscopy. A repeatability test demonstrated a precision of 0.2 nm (3σ). The proposed method enables high-precision, high-throughput in-line metrology for hybrid bonding processes.