This is the second part of two articles reporting investigation on a thermal-detector-type far-infrared sensor operating in the 8–14 µm wavelength region using Low-Pressure Chemical Vapor Deposition (LPCVD) deposited low-stress Si-rich nitride (SiN) membrane as its thermal absorber. The use of the SixNy membrane material as a far-infrared radiation absorber has been reported in many papers, there are still few reports addressing its fundamental optical absorption property at thiswavelength region, in relation with its thermal properties, utilized in an infrared sensing device. After dealing with optical absorptivity of suchmembrane in the first paper, this second article deals with the thermal property, aswell as the overall sensitivity issue of the sensor. In this report, similarly, a combination of analytical modelling, numerical simulation, and experimental characterisation is carried out. The issue of scaling down is also addressed. Results show that the thermal sensitivity depends on the membrane size. A 1000µm×1000µm will achieve 80 K/(Wcm−2), while membrane as small as 10µm×10µm has only 0.15–0.3 K/(Wcm−2).