<p>A complete design of a silicon-on-silica integrated CO₂ optical sensor is proposed. The detection relies on the strong optical absorption of the gas at wavelength of 4.28 um. Typically, silica is not employed in such applications since it exhibits strong optical losses in the mid-infrared range. Alternative materials with low optical losses, such as sapphire and silicon nitride, are usually employed instead. In the proposed design, and unlike formerly proposed sensors, silica is utilized as the buried layer to be compatible with the cheap and mature CMOS technology. The proposed design circumvents the silica absorption losses by optimizing the waveguide confinement factor. The sensor features a detection sensitivity of 6.4 ppm and a device length of 4.68 cm. The design includes a grating coupler offering 3.5 dB coupling efficiency with an incident Gaussian beam from an InF₃ single mode fiber. The study shows both analytical and numerical calculations with good agreement. The presented work pushes the utilization of the mature CMOS technology deeper into mid-infrared applications.</p>