Decorrelation of fitting parameters by Mueller polarimetry in conical diffraction

Abstract

We used full Mueller polarimetry in conical diffraction geometries to characterize 1D holographic optical gratings etched in bulk silica with a patterned photoresist layer. We studied four different samples corresponding to different stages of etching, with a Mueller polarimeter based on ferroelectric liquid crystals, operated in the visible. Two samples were also characterized by standard spectroscopic ellipsometry (SE) in the UV-VIS range (300-800 nm). The measured spectra were fitted with a Rigorous Coupled Wave Analysis code with different models of grating profiles. With the Mueller spectra the model adequacy could be assessed from the stability of the optimal values of the fitting parameters when the azimuthal angle was varied. The conclusions were found to be in agreement with AFM images of the sample, while the fits of the SE data were too poor to provide any information in this respect. A key issue for process control is resist-silica interface localization, a difficult task due to the low index contrast for these two materials. In fact, strong correlation occurs between resist and silica thicknesses when SE spectra, taken in the usual planar diffraction geometry, are fitted. Our approach clearly reduces such parameter correlations, leading to a reliable localization of this interface.