Ultra-low-noise MoCu transition edge sensors for space applications

Abstract

A study of ultra-low-noise MoCu transition edge sensors (TESs) has been performed in the context of realizing the highly sensitive far infrared imaging arrays needed for the next generation of space telescopes. More than 50 TESs, on four different chips, cut out of two different wafers were characterized. The TESs were in the form of 16-element arrays and were read out using superconducting quantum interference device (SQUID) time division multiplexing. The devices were fabricated on 200-nm-thick silicon nitride membranes, with leg widths and lengths covering the ranges of 1–4 μm and 160–960 μm, respectively. The apparent critical temperatures varied over 110–127 mK, but it is shown that much of the variation was due to differential loading by stray light, amounting to 2 ± 2 fW across the array. The measured thermal conductances to the heat bath spanned the range 0.12–1.1 pW/K, with the lowest values being typical of those needed for ultra-low-noise operation. We also studied the inherent variation in the conductances of 15 nominally identical TESs on the same chip and found a value of ±10%, which is higher than that seen on our high-conductance devices designed for ground-based operation. We measured and modeled the electrical input impedance of a subset of these TESs, and studied their step responses. The models, based on previously determined material parameters, are in excellent agreement with the measurements. Dark noise spectra were recorded and compared with the same electrothermal models using the same parameters as the dynamical simulations. The measured noise is reasonably well described by the sum of the contributions from phonon noise in the legs, Johnson noise in the bilayer, and SQUID readout noise. Dark noise equivalent powers as low as 4.2 × 10−19 W/Hz were measured. The NEP was higher than the theoretical limit by a factor of about 1.6.