Infrared Absorption Sensor for Multiple Gas Sensing

—Development of Fabry-Perot Spectrometer with Ultra Wide Wavelength Range—

Abstract

We report on a novel MEMS (Micro Electro Mechanical Systems) based Fabry-Perot spectrometer with ultra wide wavelength range (3.2-8.4 μm) compared to the previously reported ones (typically 2.8-5.8 μm). The wavelength range of a Fabry-Perot spectrometer is known to increase by increasing the ratio of the refractive indexes of the multilayer mirrors. Thus, a novel mirror structure was proposed replacing low refractive index layer of SiO₂ (n_L=1.44) by “air (n_L=1)” for wider wavelength range. The proposed device was fabricated by HF sacrificial layer etching of SiO₂ between four ultra-thin polysilicon films (ca. 320 nm). The following two ideas were adopted to fabricate this delicate structure properly. (1) Electrostatic force was generated only in the outer region surrounding the air mirror not to deform the mirror. (2) Supercritical drying process was employed after the sacrificial etching to prevent the possible sticking issue.