Characterization of Si nanowaveguide line edge roughness and its effect on light transmission

The manufacture of low-loss silicon-on-insulator (SOI) photonic wires for telecommunication wavelengths (∼1.55 μm) is a challenge. Side wall and line edge roughness (LER) are the dominant sources of scattering loss. In this work the characterization of Si nanowaveguide (NW) LER was performed and its effect on light transmission was theoretically analyzed and measured. NW structures with a width of 0.5 μm and height of 0.22 μm were prepared using different thickness of hard-mask and plasma etch process technologies on SOI platforms. LER determination was performed by critical-dimension scanning electron microscope (CD-SEM), conventional atomic force microscope (AFM) and 3D-AFM. The characterization of LER quality was carried out by monitoring of its RMS deviation σ, main frequencies and correlation length Lc. The effect of sampling length was evaluated. The data for σ from 0.8 to 6 nm and Lc from 20 to 50 nm were found to be dependent mostly on etch process technology. Theoretical simulation of scattering loss due to LER based on the 2D analytical model for planar waveguides was performed. Finally, the correlation between NW optical transmission (scattering) losses (TL) and LER was obtained and can be applied to future technology development. Especially for the case of σ ∼ 0.85 nm and Lc ∼ 20 nm the lowest value of TL ∼ 1.2 dB/cm was measured at a wavelength of 1.55 μm. Good qualitative agreement between calculated and measured losses due to LER was found.