Application of Mueller Matrix Spectroscopic Ellipsometry to determine Line Edge Roughness on Photomasks

We report on Mueller Matrix spectroscopic ellipsometry (MM-SE) to examine undesired asymmetries in structural parameters, i.e. line edge roughness (LER). The investigation was done on a photomask containing line space arrays with intentionally modulated line edges. The Mueller Matrix (MM) elements were measured within the complete azimuth angle range (0 -360 degrees) and a wavelength range from 300 nm to 980 nm. The results are presented in polar coordinates with the azimuth angle and wavelength as the angular and radial coordinate, respectively. It was found that LER significantly impacts the MM elements, which is indicated by the increase of the isotropic character of the array. The experimental data are confirmed by Rigorous Coupled Wave Analysis (RCWA) simulations on perturbed arrays. Based on RCWA the impact of LER amplitudes in the nm range is determined. It was found that both deviation of critical dimension (CD) and LER amplitude impact the MM elements. Based on the intensity ratios of the elements and their spectral distribution both errors create a characteristic finger print, which allows to separate them. Finally, the required measurement precision for LER in the nm range is estimated at 0.001. This precision is challenging but achievable with today's metrology.