Pitch scaling of interconnects is required for 3D system integration with the industry shifting to bumpless bonding technology. However, hybrid metal/dielectric bonding requires tight process control of planarity after chemical mechanical polishing (CMP) to avoid bonding voids. Due to its sub-angstrom resolution, atomic force microscopy (AFM) is typically used to assess the nano-topography but conventional systems suffer from increased noise floor at high scanning speeds making it unsuitable for high-volume manufacturing (HVM). Here, we validate a novel in-line high-throughput AFM system (QUADRA) by reporting the topographical parameters of 250 nm and 1 μm size copper nano-pads at high scanning speeds that reach tens of wafers per hour throughput.
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.
Wafer bonding is a key technology for many advanced chip technologies. For 3D integration, advanced stacking schemes and high-density packaging put a stringent requirement on the bonding reliability. Bonding quality can be characterized by the absence of voids at the bonding interface, as the voids delimit the complexity of the subsequent processing and integration steps. Therefore, in-line and non-destructive inspection techniques for void detection are crucial for early-stage detection and full process integration. In this work, we perform a comprehensive study on bonding void detection for 3D integration. We fabricate bonded Si wafers with programmed bonding voids with size from 10 nm to 20 μm. We combine different inspection and review tools, including acoustic, optical, electron beam etc., for bonding void detection at different process steps of the fabrication with different top Si thicknesses.
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