Precise optical constants: determination and impact on metrology, simulation, and development of EUV masks

Abstract

The interaction of matter and light can be described based on optical constants, shortly called δ&β. These constants provide the fundamental basis for the design of any optical system. In the Extreme Ultraviolet (EUV) spectral range, however, the existing data for many materials or compounds is very sparse, non-existent or exhibit considerable discrepancies between different sources. This is further complicated since the scaling effects stipulate the optical response of a thin film to differ from bulk. Oxidation, impurities or interdiffusion significantly affect the optical response of a system to EUV radiation. For this reason, the Physikalisch-Technische Bundesanstalt (PTB) is establishing a new database in cooperation with other European partners. This database, designated as the Optical Constants Database (OCDB) can be accessed online freely (OCDB.ptb.de). This data collection shall support further development of various fields from new metrological techniques, like EUV scatterometry to computational lithography in the EUV. This is demonstrated exemplarily here by the interplay between δ&β and the dimensional parameters with respect to a structured TaTeN EUV photomask. It is equally important either direction, to derive structure parameters from the measured EUV scattering as vice versa to predict the EUV response from the geometrical structure. In addition, the impact of varying δ and β on the expected imaging performance will be investigated by simulating typical lithographic image metrics like Critical Dimension (CD), best focus position, image contrast (NILS) and non-telecentricity for the imaging of through pitch L/S and 16 nm vertical Lines with 32 nm pitch in a NA=0.55 scanner for TaTeN mask absorber as typical representatives of high-k absorber materialsand as an example of the effect on imaging simulation.

Pushing the boundaries of EUV scatterometry: reconstruction of complex nanostructures for next-generation transistor technology

“…Future studies can improve on the method by incorporating measured optical constants from samples, produced under circumstances identical to the actual structured surfaces. 18 Solving these points will improve the accuracy of the fits.…”

“…For the FEM model, the energy-dependent optical constants of all materials must be given, because the resulting field distribution sensitively depends on their values. 17,18 In the used energy range between 115 eV and 145 eV, we use tabulated data from the CXRO data base. 19 The corresponding densities are additional model parameters.…”