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This work is funded in part by the National Science Foundation, Grant #s ???? and ???? and the National Aeronautics and Space Administration, Grant # ????


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A Bolometer is simply a device that measures heat input from its surroundings and converts it to a measurable quantity such as voltage or current. It consists of an absorber of heat capacity C that has a weak thermal link, G, to a bath temperature T₀. A thermometer is attached directly to the absorber. The absorber will see the power load from the signal, P_signal, and a power load from the thermometer’s electrical bias, P_bias. Therefore, the bolometer’s temperature is simply T=T₀ + (P_signal + P_bias)/G. As long as the bias power remains constant, any change in signal power will result in a change in temperature of the absorber and be detected by the thermometer.

Figure: (a) General Bolometer concept. P_signal is the power load from the source you are trying to detect.
P_bias represents the power load from the bias readout electronics.
T0 is the bath temperature. C and T are the bolometer’s heat capacity and temperature, respectively.
(b) Example of a single “spider-web” bolometer. The NTD Ge thermistor is located at the center of the web.

Bolocam’s detector array consists of 144 Si₃N₄ micromesh “spider-web” bolometers with neutron-transmutation-doped (NTD) germanium thermistors (resistors) bonded to the absorbers. An example of such a bolometer is shown in figure (b). The “spider-web” mesh has several advantages over a solid absorber, including a higher resonant vibration frequency (reduced susceptibility to microphonics), smaller heat capacity, and a lower cross section to cosmic rays. All 144 bolometers are fabricated onto a single silicon wafer by photolithographic means. The thermistors are then indium bump bonded to the mesh absorber before the silicon substrate is etched away from beneath the absorber, leaving just a “web” that is suspended by thin support legs. The thermal conductance, G, of each bolometer is dictated by metal leads that are deposited on one of the web support legs connecting the mesh to the silicon wafer substrate (at bath temperature). For optimal performance, G should be tuned with respect to the expected background loading and bath temperature. However, since Bolocam will use the same array to observe at three different wavelengths (and hence, three different background loads), a compromise must be made on the thermal conductance that accommodates all bandpasses.

For further information on BOLOCAM II, contact Grant Wilson or Jay Austermann.