This picture shows the latest version of the "Spiderweb" Voltage-Biased Superconducting Bolometer (VSB). It was designed for balloon and space borne experiments and it can be made entirely by standard microfabrication techniques.
The bolometer is supported by a 1µm thick silicon nitride (Si3N4) mesh patterned to resemble a spiderweb to reduce the cosmic-ray cross section. The 3.5mm diameter mesh has 7µm wide members and ~150µm spaces. The mesh is metallized to absorb far-infrared and mm-waves. It is supported by eight 7µm wide and 1mm long legs which thermally isolate the mesh from the heat sink. The 200µm × 200µm superconducting thermistor is defined on a continuous region of Si3N4 at the center of the mesh. Both the absorber and the thermistor consist of a trilayer of 500Å of Ti, 500Å of Al, and 500Å of Ti. This ``proximity sandwich'' was chosen to achieve a useful transition temperature of Tc=380mK. The width of the resistive transition was 2.0mK (10%-90%) and the maximum value of (d log R)/(d log T), which measures the steepness of the transition, was 800. In future devices the two upper layers can be removed from the absorber to reduce its heat capacity and to increase its impedance, thereby optimizing the optical efficiency. Superconducting contact to the thermistor is made by 3µm wide leads with Tc~1K, which are a sandwich of 1000Å of Al on top of the trilayer. These leads dominate the G=1.0 × 10-10W/K thermal conductance of the bolometer.

Last modified: Sep 22, 1998