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We report on a novel MEMS (microelectromechanical system) based on a Fabry–Perot spectrometer with an ultrawide wavelength range (3.2 to 8.4 µm) compared to the previously reported ones (typically 2.8 to 5.8 µm). The wavelength range of a Fabry–Perot spectrometer is known to be increased by increasing the ratio of the refractive indexes of the multilayer mirrors. Thus, a novel mirror structure was proposed, replacing a low‐refractive‐index layer of SiO2 (nL = 1.44) by air (nL = 1) for a wider wavelength range. The proposed device was fabricated by HF sacrificial layer etching of SiO2 between four ultrathin 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 so as not to deform the mirror. (2) A supercritical drying process was employed after the sacrificial etching to prevent possible sticking issues. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(5): 50–57, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10433