Polycrystalline Si (poly-Si) waveguides offer design flexibility and multilayered structures in Si-integrated photonic devices. However, as-deposited poly-Si surfaces are rough compared with single-crystalline Si, and a rough surface causes significant waveguide scattering loss at the surface. In this study, surface smoothing of poly-Si waveguides with a gas-cluster ion beam (GCIB) was demonstrated as a new smoothing technique. As the GCIB process is a directional ion-beam process, in principle it can be applied not only to plane surfaces but also to three-dimensional or non-flat structures, such as waveguide ridges.
The initial average roughness of as-deposited poly-Si films (625°C, 1 μm thick) ranged from 15 nm to 22 nm, and the grain sizes were distributed from 0.2 to 0.4μm. This rough surface was dramatically smoothed to a roughness of 1.5 nm by Ar cluster ion irradiation. From the relation between the sputtered depth and the surface roughness, the sputtered depth must be greater than the height difference of the roughness (peak-to-valley) to obtain smooth surfaces. Optical transmission losses at λ =1.54 μm were measured using cutback measurement from samples before and after the smoothing by GCIB. After surface smoothing with GCIB, the optical loss decreased from 85 dB/cm to 54 dB/cm.