Crystallographically limited submicrometer gratings in (100) and (211) silicon

We demonstrate the fabrication of submicrometer gratings in (100) and (211) Si with periodicities that are appropriate for normal-incidence coupling into Ge-Si waveguides. Gratings of periodicities in the 0.25-0.75microm range were fabricated by a standard holographic lithography technique. Crystallographically preferential wet etching that effectively terminates at the {111} family of planes facilitates the fabrication of symmetric gratings in (100) Si and of blazed gratings in (211) Si. The gratings used in this study are appropriate for the fabrication of integrated first- and second-order, normal-incidence grating couplers at a 1.30-microm wavelength. The blazed gratings show a preferential coupling of more than 80% of the total diffraction into the (+1) diffraction order at normal incidence. The maximum first-order diffraction efficiency obtained with these gratings was approximately 28% in (100) Si and 23% in (211) Si.