The kinetic inductance detector (KID) is an exciting new device that promises high-sensitivity, large-format, submillimetre to x-ray imaging arrays for astrophysics. KIDs comprise a superconducting thin-film microwave resonator capacitively coupled to a probe transmission line. By exciting the electrical resonance with a microwave probe signal, the transmission phase of the resonator can be monitored, allowing the deposition of energy or power to be detected. We describe the fabrication and low-temperature testing, down to 26 mK, of a number of devices, and confirm the basic principles of operation. The KIDs were fabricated on r-plane sapphire using superconducting niobium and aluminium as the resonator material, and tantalum as the x-ray absorber. KID quality factors of up to Q = (741 ± 15) × 10 3 were measured for niobium at 1 K, and quasiparticle effective recombination times of τ * R = 30 µs after x-ray absorption. Al/Ta quasiparticle traps were combined with resonators to make complete detectors. These devices were operated at 26 mK with quality factors of up Q = (187.7 ± 3.5) × 10 3 and a phase-shift responsivity of ∂θ/∂N qp = (5.06 ± 0.23) × 10 −6 degrees per quasiparticle. Devices were characterized both at thermal equilibrium and as x-ray detectors. A range of different x-ray pulse types was observed. Low phase-noise readout measurements on Al/Ta KIDs gave a minimum NEP = 1.27 × 10 −16 W Hz −1/2 at a readout frequency of 550 Hz and NEP = 4.60 × 10 −17 W Hz −1/2 at 95 Hz, for effective recombination times τ * R = 100 µs and τ * R = 350 µs respectively. This work demonstrates that high-sensitivity detectors are possible, encouraging further development and research into KIDs.
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.
We have fabricated Transition Edge Sensors (TESs) whose thermal characteristics are completely characterised by few-mode ballistic phonon exchange with the heat bath. These TESs have extremely small amorphous SiN x support legs: 0.2 µm thick, 0.7 to 1.0 µm wide and 1.0 to 4.0 µm long. We show, using classical elastic wave theory, that it is only necessary to know the geometry and bulk elastic constants of the material to calculate the thermal conductance and fluctuation noise. Our devices operate in the few-mode regime, between 5 and 7 modes per leg, and have noise equivalent powers (NEPs) of 1.2 aW Hz −1/2 . The NEP is dominated by the thermal fluctuation noise in the legs, which itself is dominated by phonon shot-noise. Thus TESs have been demonstrated whose thermal characteristics are fully accounted for by an elastic noise-wave model. Our current devices, and second-generation devices based on patterned phononic filters, can be used to produce optically compact, mechanically robust, highly sensitive TES imaging arrays, circumventing many of the problems inherent in conventional long-legged designs.
Detectors based on transition edge sensors (TESs) must achieve theoretically predicted noise levels if they are to be suitable for the next generation of space-borne astronomical telescopes. The noise of an ideal detector is determined by the sum of three contributions: (i) thermal-fluctuation noise in the heat link to the bath, (ii) Johnson noise in the sensor itself, and (iii) noise in the electrical read-out circuit. Many groups have reported TESs with noise levels significantly above the theoretical predictions. We use two well-defined experimental configurations to measure the read-out noise spectra of Mo–Cu TESs with transition temperatures of 370 and 200mK. The TESs are geometrically simple, comprising superconducting and normal metal films on a silicon nitride (SiNx) membrane. The measurements are compared with a multiparameter noise model, which is based on a physical model of the thin-film devices. Taking into consideration separate, accurate measurements of the heat capacity of identical SiNx membranes, we are able to provide a good account of both the magnitude and frequency dependences of the measured current-noise spectra. We find that an important excess noise mechanism involves the random exchange of heat between the heat capacity of the bilayer and the heat capacity of the nitride membrane, with either the thermal conductance of the membrane, or in some cases the thermal conductance of the bilayer, being the mediating path. Clear design recommendations are given to achieve the best possible noise performance.
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