Performance of cryogenic microbolometers and calorimeters with on-chip coolers

Astronomical observations of cosmic sources in the far-infrared and x-ray bands require extreme sensitivity. The most sensitive detectors are cryogenic bolometers and calorimeters operating typically at about 100 mK. The last stage of cooling (from 300 to 100 mK) often poses significant difficulties in space-borne experiments, both in system complexity and in reliability. We address the possibility of using refrigeration based on normal metal/insulator/superconductor (NIS) tunnel junctions as the last stage cooler for cryogenic thermal detectors. We compare two possible schemes: direct cooling of the electron gas of the detector with the aid of NIS tunnel junctions and the indirect cooling method, when the detector lattice is cooled by the refrigerating system, while the electron gas temperature is decreased by electron-phonon interaction. The latter method is found to allow at least an order of magnitude improvement in detector noise equivalent power compared to direct electron cooling.