Paper PS-MoM6
Optimization of the Optical Transmission of Submicron Silicon-on-Insulator Rib Waveguides

Monday, November 10, 2014, 10:00 am, Room 308

Optical interconnects have largely replaced copper for long distance data transmission and are gaining interest on shorter distances. At the intra-chip level, silicon waveguides are foreseen to relieve copper wire bottlenecks in the BEOL layers. The strong light confinement allowed by the large refractive index difference between Si and SiO₂ permits the building of submicron silicon on insulator (SOI) waveguides with small bending radii and thus of compact photonic circuits. However, the strong light confinement also results in large propagation losses due to the scattering of the guided light on the rough etched sidewalls of the silicon core. Modeling shows that the transmission loss mostly depends on the line edge roughness (LER) of the guide and on its correlation length.

In the present work, we apply our silicon processing and sidewall roughness metrology know-how developed for transistor gates to optimize the fabrication process of submicron rib waveguides on 200 mm SOI wafer in order to reduce their optical loss. First SiO₂ and photoresist etch masks are evaluated. In both cases, the eventual benefit of an HBr plasma cure treatment, originally developed for FEOL processing, on the photoresist is also assessed. Second, we investigate the impact of several silicon smoothing strategies on the patterned waveguides: thermal oxidation and hydrogen annealing. Oxidations are performed in pure O₂ at 1000°C, above the SiO₂ viscous transition. The following thicknesses are tested: 5, 10, 30 and 3 × 10 nm. Oxidizing in three steps compared to a single longer step is believed to increase LER reduction because smoothing is faster in the initial reaction limited regime than in the subsequent diffusion limited regime. Hydrogen annealing is performed in pure H₂ for 2 min at several temperatures between 850 and 1000°C. After each process step the LER is measured by CD-SEM. In order to obtain LER values freed from instrumental noise and their correlation length, CD-SEM data are treated by spectral analysis . At the end of the process, the sidewall roughness of the rib waveguides is also characterized by AFM on a tilted sample. In addition, their profile is measured by cross-sectional SEM and their transmission loss on an optical test bench.

Measurements show that a resist mask is better than the SiO₂ mask for minimizing optical attenuation at the price of a degraded profile while the photoresist cure treatment does not have much influence. Further experiments are ongoing to evaluate the impact of the silicon smoothing processes (hydrogen annealing or thermal oxidation) on the roughness and to correlate it with optical loss measurements.