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Coherence effects in narrow trench measurements using white light interferometry

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Abstract

White light interferometry (WLI)1 and optical coherence tomography (OCT) for micro-roughness and profile measurements are both based on the principle of low coherence peak sensing. Additional coherence peaks not corresponding to the surface topology exist due to diffraction from narrow structures; these additional coherence levels must be properly interpreted by WLI and OCT algorithms. We observed additional coherence peaks for a silicon surface with etched trenches only a few microns wide and investigated the amplitude variation of the coherence peaks versus the trench width and depth. Effects from changes in the optical system such as the numerical aperture (NA), magnification, polarization and wavelength spectra were also studied. For example, for an objective with a given numerical aperture the amplitude of the additional coherence level increases with the decreasing trench width. The amplitude of the fringes at the bottom of trench decreases making the amplitude of the fringes representing the real surface much weaker than the additional coherence levels created from strong complex diffraction effects. The experimental data obtained with optical profiler NT 8000 was compared with simulated results obtained from rigorous coupled-wave analysis2 (RCWA) and Hopkins image-formation theory that traces different polarization components in the diffraction image. We have found very good agreement between experimental and simulation results for the case of circular polarization and object structure in the form of a trench 5 microns wide and about 20 microns deep. The additional coherence level can be a problem in i.e. MEMS measurement if not interpreted correctly.

© 2003 Optical Society of America

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