INTERFEROMETRIC, LOW THERMAL MASS IR-ABSORBER FOR THERMAL INFRARED DETECTORS

The absorption efficiency of thermal infrared detectors of various designs is theoretically evaluated and compared, with an emphasis on detectors possible to fabricate by surface micromachining. In particular, the requirement of low thermal mass is considered. An absorber consisting of a single resistive metal film is shown to give a maximum of 50% absorptance. By backing such a him with a perfect reflector located at a lambda/4 distance from it, nearly 100% absorptance can be attained in the broad wavelength range of 8-12 mu m. The simulations show that by adding a dielectric layer (membrane) onto the metal film, the absorptance remains nearly constant, provided that the proper values of n(d), d(1), R(s) and d(3) are chosen. For d(1) = 0.2 mu m, the maximum variation in d(3) and R(s), compatible with a decrease in mean absorptance from 100 to 95%, is 30 and 40%, respectively. If the dielectric film possesses absorptive behaviour the absorptance of the detector may still become large for previously selected structure parameters.