57 research outputs found
Modeling Strategies for Superconducting Microstrip Transmission Line Structures
Strategies are explored to reduce the electromagnetic simulation time of electrically large superconducting transmission line structures while retaining model accuracy. The complex surface reactance of an infinite thin-film superconducting sheet is evaluated with the BCS (Bardeen-Cooper-Schrieffer) theory and used as an input to model the phase velocity and characteristic impedance of finite width transmission line structures. Commercially available electromagnetic simulation software are employed for the calculations and the results are compared with limiting analytic forms from the literature. The influences of line width, metallization thickness, and substrate height on microstrip transmission line propagation are considered in detail and a scaling approach is presented to compensate for the leading order effect in numerical simulations. These findings are particularly important near the energy gap of the superconductor due to the influence of the kinetic inductance on the transmission line dispersion
A waveguide-coupled thermally-isolated radiometric source
The design and validation of a dual polarization source for waveguide-coupled
millimeter and sub-millimeter wave cryogenic sensors is presented. The thermal
source is a waveguide mounted absorbing conical dielectric taper. The absorber
is thermally isolated with a kinematic suspension that allows the guide to be
heat sunk to the lowest bath temperature of the cryogenic system. This approach
enables the thermal emission from the metallic waveguide walls to be
subdominant to that from the source. The use of low thermal conductivity Kevlar
threads for the kinematic mount effectively decouples the absorber from the
sensor cold stage. Hence, the absorber can be heated to significantly higher
temperatures than the sensor with negligible conductive loading. The kinematic
suspension provides high mechanical repeatability and reliability with thermal
cycling. A 33-50 GHz blackbody source demonstrates an emissivity of 0.999 over
the full waveguide band where the dominant deviation from unity arrises from
the waveguide ohmic loss. The observed thermal time constant of the source is
40 s when the absorber temperature is 15 K. The specific heat of the lossy
dielectric MF-117 is well approximated by C_v(T)=0.12\,T\,^{2.06} mJ g
K between 3.5 K and 15 K
Precision control of thermal transport in cryogenic single-crystal silicon devices
We report on the diffusive-ballistic thermal conductance of multi-moded
single-crystal silicon beams measured below 1 K. It is shown that the phonon
mean-free-path is a strong function of the surface roughness
characteristics of the beams. This effect is enhanced in diffuse beams with
lengths much larger than , even when the surface is fairly smooth, 5-10
nm rms, and the peak thermal wavelength is 0.6 m. Resonant phonon
scattering has been observed in beams with a pitted surface morphology and
characteristic pit depth of 30 nm. Hence, if the surface roughness is not
adequately controlled, the thermal conductance can vary significantly for
diffuse beams fabricated across a wafer. In contrast, when the beam length is
of order , the conductance is dominated by ballistic transport and is
effectively set by the beam area. We have demonstrated a uniformity of 8%
in fractional deviation for ballistic beams, and this deviation is largely set
by the thermal conductance of diffuse beams that support the
micro-electro-mechanical device and electrical leads. In addition, we have
found no evidence for excess specific heat in single-crystal silicon membranes.
This allows for the precise control of the device heat capacity with normal
metal films. We discuss the results in the context of the design and
fabrication of large-format arrays of far-infrared and millimeter wavelength
cryogenic detectors
Fabrication of Phononic-Isolated Kinetic Inductance Detectors
Kinetic inductance detectors (KID) have received increased interest due to their relative ease of fabrication, low noise, and scalability to large format arrays required by next generation of astronomical telescopes. The development of KIDs has progressed rapidly, with very low noise equivalent power demonstrated by several groups and KIDs arrays implemented in several ground-based and air-borne instruments. In this paper, we describe a new fabrication process which consists of a membrane isolated KID incorporating a phononic bandgap structure tuned to block recombination phonons from escaping to the thermal bath. This architecture is designed to increase the quasi-particle lifetime and results in increased responsivity to signal photons and lower noise. These devices have been fabricated as lumped-element resonators with hafnium inductors and Nb capacitors on low stress silicon nitride and silicon-on-insulator membranes. We discuss the fabrication process, which uses a combination of sub-micron laser based direct write lithography and nanoscale electron beam lithography
Development of a Robust, Efficient Process to Produce Scalable, Superconducting Kilopixel Far-IR Detector Arrays
The far-IR band is uniquely suited to study the physical conditions in the interstellar medium from nearby sources out to the highest redshifts. FIR imaging and spectroscopy instrumentation using incoherent superconducting bolometers represents a high sensitivity technology for many future suborbital and space missions, including the Origins Space Telescope. Robust, high sensitivity detector arrays with several 104 pixels, large focal plane filling factors, and low cosmic ray cross sections that operate over the entire far-IR regime are required for such missions. These arrays could consist of smaller sub-arrays, in case they are tileable. The TES based Backshort Under Grid array architecture which our group has fielded in a number of FIR cameras, is a good candidate to meet these requirements: BUGs are tileable, and with the integration of the SQUID multiplexer scaleable beyond wafer sizes; they provide high filling factors, low cosmic cross section and have been demonstrated successfully in far-infrared astronomical instrumentation. However, the production of BUGs with integrated readout multiplexers has many time and resource consuming process steps. In order to meet the requirement of robustness and efficiency on the production of future arrays, we have developed a new method to provide the superconducting connection of BUG detectors to the readout multiplexers or general readout boards behind the detectors. This approach should allow us to reach the goal to produce reliable, very large detector arrays for future FIR missions
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