This is the first part of the two articles reporting investigation of a thermal detector-type far-infrared sensor operating in the 8–14µm wavelength, using Low-Pressure-Chemical-Vapor-Deposition (LPCVD)- deposited low-stress Si-rich nitride (SiN)membrane as the thermal absorber. Although the use of the SixNy membrane material as a far-infrared radiation absorber or as supporting membrane has been reported in many papers, there are still few reports addressing its fundamental optical absorption property at this wavelength region, in relation with its thermal properties, utilized in an infrared sensing device. Although such properties are very process-dependent,we are trying to devise a general rule andmethod of combining both theoretical simulations and experimentally observed data, for obtaining such properties, which in turn, enabling better design and optimization of the Infrared device, especially when scaling down is an issue. This first article of the report deals with the optical absorptivity of the membrane in the 8–14µmwavelength regions. Results show that the absorptivity depends on the thickness of the SiN thin film, yet all the samples show absorbance selectivity at 8–14µm, with up to 50% absorbance.