Impact of noise sources and optical design on defect detection sensitivity in extreme ultraviolet actinic pattern inspection tool

Abstract

We discuss the impact of various noise sources and the optical design in bright field extreme ultraviolet (EUV) actinic inspection of mask features for defects in the patterned absorber. It is shown that an optimum pixel size is needed to maximize the defect signal-to-noise ratio (SNR) to balance the trade-off in increasing signal strength with shot noise from defect signal and the background pattern intensity (mask layout image) and speckle noise from the mask blank roughness. Moreover, we consider defocus showing that the EUV mask phase effect has an asymmetric impact on pattern defect SNR’s through-focus behavior. The impact of defocus limits inspection performance based on defect SNR. Using critical defect sizes in a case study, we show the defect SNR performance of the limiting case and discuss the possibility of utilizing a nominal defocus in the inspection system to leverage the phase effect of EUV mask absorber to improve the defect SNR. A 50% improvement in defect SNR is shown to be possible by introducing a −50 nm nominal defocus into the bright field inspection system.