Fabrication of high Aspect-Ratio Si Pillar-based Hybrid plasmonic-photonic Crystal Waveguides for ultra-sensitive Infrared Gas-sensing Applications

The sensing of ambient gases is of growing interest for many applications as for example indoor air quality monitoring, gas leakage or fire detection. We present the fabrication of high aspect-ratio silicon (Si) pillar-based hybrid plasmonic-photonic crystal waveguides. This type of waveguide is suitable for ultra-sensitive gas sensing applications by providing enhanced light-matter interaction due to low group velocities (slow light) in photonic crystal waveguides and high evanescent field ratio. Eight-inch silicon substrate wafers are processed by means of lithography and deep reactive ion etch (DRIE, aka. Bosch etch), which took place in the industrial clean-room facilities of Infineon Technologies Austria AG in Villach. The fabrication processes were optimized to a level such that aspect ratios of around seven for pillars of 510 nm diameter could be reached. The discrepancy of measured diameters and diameters given by the design was found to be lower than 50 nm, with a standard deviation for the measured diameters lower than 20 nm.