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

The absorption efficiency of thermal infrared detectors of various des igns is theoretically evaluated and compared, with an emphasis on dete ctors possible to fabricate by surface micromachining. In particular, the requirement of low thermal mass is considered. An absorber consist ing 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 a t 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 abso rptance 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 v ariation in d(3) and R(s), compatible with a decrease in mean absorpta nce from 100 to 95%, is 30 and 40%, respectively. If the dielectric fi lm possesses absorptive behaviour the absorptance of the detector may still become large for previously selected structure parameters.