High-Throughput Characterization of Nanoscale Topography for Hybrid Bonding by Optical Interferometry

Hybrid bonding requires nanoscale topography control of the surfaces to be bonded. Such topography is conventionally characterized by atomic force microscopy (AFM), which provides high accuracy but very limited throughput. In this study, we present a protocol to analyze the nanoscale metal recess and its wafer-level uniformity by phase shift interferometry (PSI) with a throughput three orders of magnitude greater than typical AFM analysis. Using an automated protocol, we analyzed ~ 10 million Cu pads and constructed a recess uniformity map for a full wafer. It confirmed that our CMP process could control the wafer-level recess variation within a few nm. While PSI has clear advantage in the data throughput, the limitations of PSI should also be considered. PSI has a limited lateral resolution of ~ 0.5 μm compared to its nanoscale z-resolution. The apparent Cu recess measured by PSI from Cu/SiO2 samples is greater than the physical recess by an offset of tens of nm, based on the different phase change on reflection from different areas. This offset value varies with Cu surface oxidation or different thickness of transparent SiO2. If SiO2 is thicker than ~ 2 μm, the structures below the layer do not make significant impact. When the sample configuration and surface conditions are the same over the sample(s) to be analyzed, PSI can efficiently characterize the topography variation of numerous Cu pads on a die, a wafer, or multiple sets of wafers. This technique can be useful in controlling the process conditions to reduce the variation in critical topography parameters and improve hybrid bond yield.