LOW-LOSS POLYCRYSTALLINE SILICON WAVE-GUIDES FOR SILICON PHOTONICS

Photonic integrated circuits in silicon require waveguiding through a material compatible with silicon very large scale integrated circuit t echnology. Polycrystalline silicon (poly-Si), with a high index of ref raction compared to SiO2 and air, is an ideal candidate for use in sil icon optical interconnect technology. In spite of its advantages, the biggest hurdle to overcome in this technology is that losses of 350 dB /cm have been measured in as-deposited bulk poly-Si structures, as aga inst 1 dB/cm losses measured in waveguides fabricated in crystalline s ilicon. We report methods for reducing scattering and absorption, whic h are the main sources of losses in this system. To reduce surface sca ttering losses we fabricate waveguides in smooth recrystallized amorph ous silicon and chemomechanically polished poly-Si, both of which redu ce losses by about 40 dB/cm. Atomic force microscopy and spectrophotom etry studies are used to monitor surface roughness, which was reduced from an rms value of 19-20 nm down to about 4-6 nm. Bulk absorption/sc attering losses can depend on size, structure, and quality of grains a nd grain boundaries which we investigate by means of transmission elec tron microscopy. Although the lowest temperature deposition has twice as large a grain size as the highest temperature deposition, the losse s appear to not be greatly dependent on grain size in the 0.1-0.4 mu m range. Additionally, absorption/scattering at dangling bonds is inves tigated before and after a low temperature electron-cyclotron resonanc e hydrogenation step. After hydrogenation, we obtain the lowest report ed poly-Si loss values at lambda=1.54 mu m of about 15 dB/cm. (C) 1996 American Institute of Physics.