Proposed Specification of EUVL Mask Substrate Roughness

A revised specification of mask substrate roughness was proposed at the 1st International EUVL Symposium in Dallas in 2002 [1]. This document describes the reasoning behind the proposed revision in more detail. The specification of mask substrate roughness should be based on its effect on lithographic performance. The effects of mask roughness can be considered according to the spatial frequency. At high frequencies (f > M x NA/λ) corresponding to spatial periods too small to be resolved, light is scattered outside the angular acceptance of the camera effectively reducing the reflectivity of the mask. At lower frequencies, f > M x NA/λ, light is scattered within the acceptance angle of the camera and can degrade the aerial image quality. The loss in reflectivity due to high-spatial frequency roughness (HSFR) is given by R/R0 = exp(-(4πσ/λ)2), (1) where R0 is the peak reflectivity of the coating on a smooth substrate, σ is the HSFR after multilayer coating. The relationship between top surface roughness and substrate roughness depends on the multilayer deposition process and significant smoothing of substrate roughness has been demonstrated [2]. Ultimately the specification of HSFR may be best decided based on the multilayer deposition process. For the present we may adopt a worst-case scenario of no smoothing in which case the top surface roughness is the same as that of the substrate. At very high spatial frequencies, light scattered from the individual interfaces of the multilayer coating no longer adds in phase and the effect of the roughness is diminished. For a typical Mo/Si multilayer coating this occurs at a scattering angle of about 15 degrees from specular and corresponding spatial-frequency of 0.02/nm (50 nm spatial period). If a 2% (relative) loss in reflectivity is allowed due to the HSFR of the mask substrate then one arrives at the following specification, HSFR < 0.15 nm (0.004/nm < f < 0.02/nm). (2) The low frequency limit is M x NA/λ rounded down to 0.004/nm (250 nm spatial period), where the magnification M=0.25, NA=0.25 and λ=13.5 nm