Novel methods for SPC defect monitoring: Normalizable diversity sampling: Defect inspection

The key to robust SPC control is the inline signals provided by metrology tools (e.g. CD-SEM, Overlay, film thickness measurement) and defect inspection tools (e.g. Surfscan, Broadband Plasma (BBP), Laser Scanning). Wafer defect inspection tools like Broadband Plasma find anomalies and provide defect coordinates to report their locations. Defects reported by the inspection tools are then sampled, or in other words, a sub-set of those defects are chosen for Scanning Electron Beam (SEM) Review. Classification of the defect type is provided using SEM. Due to SEM review throughput limitations, not every defect reported by inspection can be sampled for review. Therefore, the theoretical ideal sampling technique would generate an accurate representation of the true defect population on a wafer solely based on a limited review sample. The paper discusses the methodology for selecting such a review sample, termed diversity sampling. This scheme samples defects based on properties (location on wafer / die, design location, optical characteristics) instead of sampling solely based on defect location. Compared to random sampling, this technique demonstrates reduced error between the normalized defect density reported and the true defect density actually present on the wafer.