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Illustration of the mechanism to which the optical loss is attributed. During RIE, sputtered metal atoms react with the silicon sidewall to form a dilute silicide. The silicide is difficult to remove and induces an optical loss that is comparable to loss induced by a metal. The metal hard mask was thoroughly stripped before optical characterization.
Scanning electron micrograph of the high-loss waveguide with the largest sidewall roughness observed in this study. This order of roughness is not sufficient to account for the observed loss. The waveguide was fabricated with a Co hard mask. The roughness on the top surface of the thick under-cladding is due to thin lift-off imperfections partially masking the waveguide surroundings during RIE until they were sputtered away.
STEM-EDS analysis of a Si waveguide etched in proximity of Pd and thoroughly cleaned with wet chemical processing. (a) Cross-sectional micrograph indicating where the EDS spectra were acquired. (b) EDS spectrum acquired from the waveguide sidewall showing no detectable Pd. [(c)–(e)] EDS spectra acquired from the waveguide sidewall showing, in average, about 5% atomic concentration of Pd in Si. The distribution of Pd along the waveguide sidewall is consistent with sputtering from the top metal mask.
Transparency of waveguides etched with various metal hard masks.
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