Effects of temporal coherence on EUV lensless imaging

One of the potential applications of coherent diffraction imaging (CDI) methods is at-wavelength defect inspection of extreme ultraviolet (EUV) photomasks for future semiconductor technology nodes. Mask defect metrology requires very high resolution as the size of relevant defects is 10 nm or less. In CDI, the resolution is limited by the highest detectable diffraction frequency component. The diffraction signal generated by a small isolated defect drops quickly as the diffraction angle increases and often long integration times are required to achieve a useful signal to noise ratio. With the introduction of states-mixtures into CDI methods, which has been intensively developed in recent years, it is possible to relax the CDI coherence requirements, and thus increase the illumination flux and reduce the exposure time for several types of EUV sources. In this paper, we systematically study the limits of temporal coherence on CDI using EUV synchrotron light and imaging of EUV masks in the reflection mode and we quantify the potential throughput increase for EUV photomask metrology applications.