Use of EUV scatterometry for the characterization of line profiles and line roughness on photomasks

Abstract:
Scatterometry is a versatile metrology for characterizing periodic structures, regarding critical dimension (CD) and other profile properties. With respect to small feature sizes on future lithography photomasks, the short wavelength of extreme ultraviolet (EUV) radiation is advantageous since it minimizes diffraction phenomena and increases the sensitivity to roughness. The advantage of the short wavelength can also be used for investigations of DUV photomasks. For EUV masks, only EUV radiation provides direct information on at-wavelength mask performance. The intensity of the measured diffraction orders carries information about the absorber line profile like top CD, sidewall angle, height, and top corner radius. It is shown that it is feasible to derive information on the absorber line profile in periodic areas of lines and spaces by means of rigorous numerical modeling with the finite element method (FEM). We demonstrate the determination of line profile parameters for chrome on glass (CoG) and EUV masks. EUV Reflectometry on CoG masks is successfully used to determine absorber line heights. A good correlation (0.2 nm rms) between CD values determined using EUV scatterometry and CD-SEM is demonstrated for an EUV mask. A clear correlation between diffuse scatter intensity and CD uniformity, respectively line roughness, is also obtained.