Infrared integrated optical evanescent field sensor for gas analysis Part II.: Fabrication

A free-standing single crystal silicon integrated optical waveguide was developed for an integrated optical gas analysis sensor working in the middle infrared region. The cross section was designed to generate a pronounced fraction of evanescent optical power which interacts with the surrounding gas atmosphere field. The silicon waveguide has an optical window from about I to 6.3 mu m thus allowing the measurement of pollutants like CO, CO2, NO, N2O, NO2 emitted, e.g. in combustion processes as well as CH4 and H2O. The light at the waveguide output contains quantitative information about the gases present in a mixture. The high step in refractive index between silicon core and gas allows very small bending radii so that a sufficiently long waveguide can be realised on small chip area (about 1 m on 1 cm(2)). The waveguide is patterned as a spiral or a meander in the wafer plane. A suitable freestanding cross section profile is realized by a sequence of anisotropic dry etching, oxidation and isotropic dry etching of crystalline silicon bulk material. (c) 2004 Elsevier B.V. All rights reserved.