128 research outputs found
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
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
Scalable background-limited polarization-sensitive detectors for mm-wave applications
We report on the status and development of polarization-sensitive detectors
for millimeter-wave applications. The detectors are fabricated on
single-crystal silicon, which functions as a low-loss dielectric substrate for
the microwave circuitry as well as the supporting membrane for the
Transition-Edge Sensor (TES) bolometers. The orthomode transducer (OMT) is
realized as a symmetric structure and on-chip filters are employed to define
the detection bandwidth. A hybridized integrated enclosure reduces the
high-frequency THz mode set that can couple to the TES bolometers. An
implementation of the detector architecture at Q-band achieves 90% efficiency
in each polarization. The design is scalable in both frequency coverage, 30-300
GHz, and in number of detectors with uniform characteristics. Hence, the
detectors are desirable for ground-based or space-borne instruments that
require large arrays of efficient background-limited cryogenic detectors.Comment: 7 pages, 3 figures, Presented at SPIE Astronomical Telescopes and
Instrumentation 2014: Millimeter, Submillimeter, and Far-Infrared Detectors
and Instrumentation for Astronomy VII. To be published in Proceedings of SPIE
Volume 915
Silicon-Based Antenna-Coupled Polarization-Sensitive Millimeter-Wave Bolometer Arrays for Cosmic Microwave Background Instruments
We describe feedhorn-coupled polarization-sensitive detector arrays that
utilize monocrystalline silicon as the dielectric substrate material.
Monocrystalline silicon has a low-loss tangent and repeatable dielectric
constant, characteristics that are critical for realizing efficient and uniform
superconducting microwave circuits. An additional advantage of this material is
its low specific heat. In a detector pixel, two Transition-Edge Sensor (TES)
bolometers are antenna-coupled to in-band radiation via a symmetric planar
orthomode transducer (OMT). Each orthogonal linear polarization is coupled to a
separate superconducting microstrip transmission line circuit. On-chip
filtering is employed to both reject out-of-band radiation from the upper band
edge to the gap frequency of the niobium superconductor, and to flexibly define
the bandwidth for each TES to meet the requirements of the application. The
microwave circuit is compatible with multi-chroic operation. Metalized silicon
platelets are used to define the backshort for the waveguide probes. This
micro-machined structure is also used to mitigate the coupling of out-of-band
radiation to the microwave circuit. At 40 GHz, the detectors have a measured
efficiency of 90%. In this paper, we describe the development of the 90 GHz
detector arrays that will be demonstrated using the Cosmology Large Angular
Scale Surveyor (CLASS) ground-based telescope
The Cosmology Large Angular Scale Surveyor
The Cosmology Large Angular Scale Surveyor (CLASS) is a four telescope array
designed to characterize relic primordial gravitational waves from inflation
and the optical depth to reionization through a measurement of the polarized
cosmic microwave background (CMB) on the largest angular scales. The
frequencies of the four CLASS telescopes, one at 38 GHz, two at 93 GHz, and one
dichroic system at 145/217 GHz, are chosen to avoid spectral regions of high
atmospheric emission and span the minimum of the polarized Galactic
foregrounds: synchrotron emission at lower frequencies and dust emission at
higher frequencies. Low-noise transition edge sensor detectors and a rapid
front-end polarization modulator provide a unique combination of high
sensitivity, stability, and control of systematics. The CLASS site, at 5200 m
in the Chilean Atacama desert, allows for daily mapping of up to 70\% of the
sky and enables the characterization of CMB polarization at the largest angular
scales. Using this combination of a broad frequency range, large sky coverage,
control over systematics, and high sensitivity, CLASS will observe the
reionization and recombination peaks of the CMB E- and B-mode power spectra.
CLASS will make a cosmic variance limited measurement of the optical depth to
reionization and will measure or place upper limits on the tensor-to-scalar
ratio, , down to a level of 0.01 (95\% C.L.)
Fabrication of Silicon Backshorts with Improved Out-of-Band Rejection for Waveguide-Coupled Superconducting Detectors
The Cosmology Large Angular Scale Surveyor (CLASS) is a ground-based instrument that will measure the polarization of the cosmic microqave background to search for gravitational waves form a posited epoch of inflation early in the universe's history. This measurement will require integration of superconducting transition-edge sensors with microwave waveguide inputs with good conrol of systematic errors, such as unwanted coupling to stray signals at frequencies outside of a precisely defined microwave band. To address these needs we will present work on the fabrication of silicon quarter-wave backshorts for the CLASS 40GHz focal plane. The 40GHz backshort consists of three degeneratively doped silicon wafers. Two spacer wafers are micromachined with through wafer vins to provide a 2.0mm long square waveguide. The third wafer acts as the backshort cap. The three wafers are bonded at the wafer level by Au-Au thermal compression bonding then aligned and flip chip bonded to the CLASS detector at the chip level. The micromachining techniques used have been optimized to create high aspect ratio waveguides, silicon pillars, and relief trenches with the goal of providing improved out of band signal rejection. We will discuss the fabrication of integrated CLASS superconducting detectors with silicon quarter wave backshorts and present current measurement results
- …