Cu pad surface height evaluation technique by in-line SEM for wafer hybrid bonding

Wafer-to-wafer hybrid bonding is a key technology for achieving high-density three-dimensional interconnections in semiconductor devices. This technology directly bonds Cu pads formed on the surface of two wafers, where the surface height of the Cu pad compared to the SiCN surrounding the Cu pad have to be within a few nm. We have developed a method to measure the Cu pad surface height with sub-nm precision by using a top-view scanning electron microscope image. The proposed method is based on the physical principle that the difference in the backscattered electron (BSE) signals of the opposing detectors is dependent on the slope. It estimates the slope of the target with the BSE signal and then calculates the height of the target on the basis of this slope. We compared the Cu pad height measurement results by this method with those by atomic force microscopy and found that ours provided measurement precision on the sub-nm order and demonstrated the capability for evaluation of layout dependency and intra-wafer distribution. Because of its speed and alignment capability, our proposed method is promising for Cu height control in wafer-to-wafer hybrid bonding.