Measurement of asymmetric side wall angles by coherent scanning Fourier scatterometry

The accurate reconstruction of geometrical grating parameters with small scattering volumes, such as side wall angles (SWAs), is nowadays still a challenging task for optical scatterometry as state-of-the-art inspection tool in semiconductor manufacturing. We propose a measurement technique which enables the precise determination of asymmetric SWAs with values that deviate by less than 1 degrees from the ideal 90 degrees. We verify the validity of our method through rigorous simulations of silicon line gratings with grating periods on the order of a few micrometers. Each grating line consists of a fine sub-grating with 40 nm pitch and 20 nm critical dimension. Our approach is based on coherent high-NA Fourier scatterometry, extended by a lateral scan over the sample. An additional 180 degrees-shearing element enables self-referencing by coherently superposing the positive diffraction orders with their negative counterparts (and vice versa). The information about the asymmetric SWAs is uniquely encoded in the phase of the resulting sinusoidal intensity signal and can be retrieved by tracking the position of the extremum of the curve over different pupil coordinates. The setup may be extended by a reference arm and a structured aperture stop, thus optimizing the sensitivity towards SWA. Likewise, the flexibility and the range of measurable samples increase.