High-throughput in-line SEM Metrology for Cu Pad Nanotopography for Hybrid Bonding Applications

Abstract

Wafer-to-wafer hybrid bonding relies on precise surface nanotopography control with the chemical mechanical polishing (CMP). To ensure good Cu-to-Cu and dielectric-to-dielectric bonding, atomic force microscopy (AFM) is used as a reference tool where one of the key metrics, Cu pad height, has to be closely monitored to ensure it is within a narrow recess range (0 to -5 nm). Throughput of such a metrology step is critical and has to keep a balanced compromise between collecting quality large data/statistics versus reducing wafer queue times for faster processing and preservation of the bonding interface. In this work, we present the results of a new and fast metrology approach based on in-line SEM for leveraging the 2D resolution of SEM for 3D metrology i.e., nanotopography. Our technique is based on the use of directional, shallow angle back-scattered electron (BSE) imaging, followed by image analysis for topography induced shadows. Output of the analysis is "height_index" which correlates well with AFM Cu pad height results on 500 nm and 350 nm sized Cu pads, where we achieve root mean square errors of 0.42 nm and 0.47 nm respectively. In addition, BSE images are screened for Cu pad defectivity based on gray level outlier presence, to help detect Cu pads with excess recess or improper fill for potential electrical yield risk areas. These approaches are scalable following the interconnect pitch scaling path and can offer valuable inputs for CMP process optimizations.