Impact of noise sources and optical design on defect sensitivity for EUV actinic pattern inspection

Abstract

In this paper, we discuss the impact of various noise sources and optical design on defect sensitivity for bright field EUV actinic pattern inspection. The result shows that an optimum pixel size is needed to maximize the defect signal to noise ratio (SNR) to balance the impact of increasing signal strength and photon 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 throughfocus. 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 to utilize the phase effect of EUV mask absorber to improve the defect SNR by introducing a nominal defocus into the inspection system. A 50% improvement on SNR is achieved by introducing a -50 nm nominal defocus into the bright field inspection system to operate at a higher defect SNR region.