High-T-c bolometers with silicon-nitride spiderweb suspension for far-infrared detection

High-T-c GdBa2Cu3O7-delta (GBCO) superconducting transition edge bolometers with operating temperatures near 90 K have been made with both closed sili con-nitride membranes and patterned silicon-nitride (SiN) spiderweb-like su spension structures, As a substrate silicon-on-nitride (SON) wafers are use d which are made by fusion bonding of a silicon wafer to a silicon wafer wi th a silicon-nitride top layer. The resulting monocrystalline silicon top l ayer on the silicon-nitride membranes enables the epitaxial growth of GBCO, By patterning the silicon-nitride the thermal conductance G is reduced fro m about 20 to 3 mu W/K. The noise of both types of bolometers is dominated by the intrinsic noise from phonon fluctuations in the thermal conductance G, The optical efficiency in the far infrared is about 75% due to a goldbla ck absorption layer. The noise equivalent power NEP for FIR detection is 1. 8 pW/root Hz, and the detectivity D* is 5.4x10(10) cm root Hz/W. Time const ants are 0.1 and 0.6 s, for the closed membrane and the spiderweb like bolo meters respectively. The effective time constant can be reduced with about a factor 3 by using voltage bias. Further reduction necessarily results in an increase of the NEP due to the 1/f noise of the superconductor.