Detection of printable EUV mask absorber defects and defect adders by full chip optical inspection of EUV patterned wafers

Abstract

The ability to rapidly detect both printable EUV mask adder defects as well as mask absorber defects across the entire mask image field is a key enabler for EUV lithography. Current optical wafer-based inspection techniques are only capable of detecting repeater defects on a Die-to-Die basis for chiplets within the image field. Larger server-type chips that encompass the entire mask image field cannot rely on such a scheme, since the presence of the defect in every die prevents their detection. In this study, a prototype optical wafer defect inspection methodology designed to detect repeater defects over the entire image field, termed Die-to-Baseline Reference Die (D2BRD), is investigated. The sensitivity of this inspection technique is demonstrated and compared to eBeam inspection over a range of defect sizes for both opaque and clear type mask absorber programmed defects. Moreover, the D2BRD methodology is used to monitor printing defect adders present in a lithographic defect test mask, as well as 7 nm metal mask layer. Using defect repeater analysis, SEM review and patch image classification of full chip wafer inspections over several mask cycles, the D2BRD scheme is shown to allow the unambiguous identification of both mask adder and absorber "native" mask defects, while suppressing random process defects. Thus, this methodology has the potential to help define the risk assessment of mask adder defects in the absence of an EUV pellicle, and can play an integral part of the wafer print protection strategy.