Impact of ALD grown passivation layers on silicon nitride based integrated optic devices for very-near-infrared wavelengths

A CMOS compatible post-processing method to reduce optical losses in silicon nitride (Si3N4) integrated optical waveguides is demonstrated. Using thin layer atomic layer deposition (ALD) of aluminum oxide (Al2O3) we demonstrate that surface roughness can be reduced. A 40 nm thick Al2O3 layer is deposited by ALD over Si3N4 based strip waveguides and its influence on the surface roughness and the waveguide loss is studied. As a result, an improvement in the waveguide loss, from very high loss (60 dB/cm) to low-loss regime (~5 dB/cm) is reported for a 220 nm x 500 nm Si3N4 wire at 900 nm wavelength. This opens prospects to implement very low loss waveguides.