The semiconductor manufacturing industry has experienced continued growth by doubling in the number of components per integrated circuit every two years, following Moore’s Law . The introduction of Extreme Ultraviolet (EUV) lithography at λ = 13.5 nm is a necessary step to continue with this trend . However, at this short wavelength, one has to operate in reflective mode with 6 degrees of illumination angle, which implies numerous changes in the current lithography infrastructure, including the production of defect-free masks and metrology tools allowing defect inspection for current and future lithography nodes. The standard approach to address this problem involves expensive and complex EUV optics. At the XIL-II beamline (SLS, Switzerland), we are developing an actinic pattern inspection platform based on coherent diffraction imaging (CDI) . Due to the absence of optical elements in the experimental setup, our tool – called RESCAN (reflective-mode EUV mask scanning lensless imaging microscope) – offers unique insights into EUV metrology: both amplitude and phase of the lithography masks can be imaged , and inspection can be performed through-pellicle, which is a requirement for any mask inspection tool that needs to be integrated in the lithography process . Furthermore, the method can be applied to investigate the imaging properties of different absorber materials that are currently under consideration as alternative routes to mitigate the known mask 3D effects .
In this talk, I will describe RESCAN and its working principle, and I will illustrate its capabilities through recent examples. I will show how programmed defects down to 35 nm on mask can be detected using die-to-die and die-to-database approaches, and how this can be done on different absorber materials. These studies will bring me to discuss about the sensitivity and resolution limits of CDI as a potential technique in EUV mask technology.
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