Reflectance and ellipsometry, measured as functions of wavelength or angle, are becoming increasingly popular for assessing the dimensions and profiles of periodic structures for process control in microlithography applications. In this study, we investigate the effects that variations in profile have on the optical signature. We model variations in a periodic structure using rigorous coupled wave analysis, by calculating the reflectance of a superstructure in which the profiles are randomly modulated from their nominal profile. We investigate, as an example, a nominal grating consisting of 100 nm silicon lines having a vertical sidewall angle, a pitch of 200 nm, and a height of 100 nm with a wavelength of 532 nm. We vary the edge positions, the edge profiles, the line heights, and the trench depths and find that the Stokes reflectance can be modified from its nominal value by a relatively large amount, especially in the case of line-width variations. We find that the reflected field can be approximated by the mean field reflected by a distribution of periodic gratings and that the field does not represent the field from the average profile. When fitting results to more than one modeled parameter, the changes that are observed can be enough to shift the deduced parameter in some cases by more than the rms variation of that parameter. The diffuse reflectance (the non-specular diffraction efficiency) is found to increase with the variance of the fluctuations.
Citation: Journal of the Optical Society of America A-Optics Image Science and Vision
Issue: No 10
Pub Type: Journals
critical dimension metrology, diffraction, ellipsometry, gratings, reflectance, scatterometry