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EUV lithography requires an exposure system with complex reflective optics and an equally complex EUV dedicated
reflective mask. The required high level of reflectivity is obtained by using multilayers. The multilayer of the system
optics and the mask are tuned to each other. The mask is equipped with an additional patterned absorber layer.
The EUV mask is an optical element with many parameters that contribute to the final image and overlay quality on the
wafer and the productivity of the system. Several of these parameters can be tuned for optimal overlay, imaging and
productivity results. This should be done with care because of possible interaction between parameters.
We will present an overview of the EUV mask contributors to the imaging, overlay and productivity performance for
the 27 nm node and below, such as multilayer and absorber stack composition, reflectivity and reflectivity uniformity.
These parameters will be reviewed in the context of real-life scanner parameters for the ASML NXE:3100 and
NXE:3300 system configurations. The predictions will be compared to actual exposure results on NXE:3100 systems
(NA=0.25) for various masks and extrapolated to the NXE:3300 (NA=0.33).
In particular, we will present extensive multilayer and absorber actinic spectral reflectance measurements of a state-ofthe
art EUV mask over a range of incidence angles corresponding to an NA of 0.33 at multiple positions within the
image field. The ML measurements allow calibrating ML stack for imaging simulations. It allows also the estimation of
mask-induced apodization effects having impact on overlay.
In general, the reflectivity measurements will give detailed variations over the image field of mask parameters such as
ML centroid wavelength and absorber reflectivity which contribute to CD uniformity. Such a relation will be
established by means of rigorous full stack imaging simulations taking into account optical properties of the coming
NXE:3300 system.
Based on this investigation we will propose optimal EUV mask parameters for the 22 nm node EUV lithography and
below, to provide guidance for mask manufacturers to support the introduction of EUV High Volume Manufacturing.
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