We are constructing the Primordial Inflation Polarization Explorer (PIPER) to measure the polarization of the cosmic
microwave background (CMB) and search for the imprint of gravity waves produced during an inflationary epoch in the
early universe. The signal is faint and lies behind confusing foregrounds, both astrophysical and cosmological, and so
many detectors are required to complete the measurement in a limited time. We will use four of our matured 1,280 pixel,
high-filling-factor backshort-under-grid bolometer arrays for efficient operation at the PIPER CMB wavelengths. All
four arrays observe at a common wavelength set by passband filters in the optical path. PIPER will fly four times to
observe at wavelengths of 1500, 1100, 850, and 500 μm in order to separate CMB from foreground emission. The arrays
employ leg-isolated superconducting transition edge sensor bolometers operated at 128 mK; tuned resonant backshorts
for efficient optical coupling; and a second-generation superconducting quantum interference device (SQUID)
multiplexer readout. We describe the design, development, and performance of PIPER bolometer array technology to
achieve background-limited sensitivity for a cryogenic balloon-borne telescope.
The Primordial Inflation Polarization Explorer (PIPER) is a balloon-borne instrument designed to search for
the faint signature of inflation in the polarized component of the cosmic microwave background (CMB). Each
flight will be configured for a single frequency, but in order to aid in the removal of the polarized foreground
signal due to Galactic dust, the filters will be changed between flights. In this way, the CMB polarization at a
total of four different frequencies (200, 270, 350, and 600 GHz) will be measured on large angular scales. PIPER
consists of a pair of cryogenic telescopes, one for measuring each of Stokes Q and U in the instrument frame.
Each telescope receives both linear orthogonal polarizations in two 32 × 40 element planar arrays that utilize
Transition-Edge Sensors (TES). The first element in each telescope is a variable-delay polarization modulator
(VPM) that fully modulates the linear Stokes parameter to which the telescope is sensitive. There are several
advantages to this architecture. First, by modulating at the front of the optics, instrumental polarization is
unmodulated and is therefore cleanly separated from source polarization. Second, by implementing this system
with the appropriate symmetry, systematic effects can be further mitigated. In the PIPER design, many of the
systematics are manifest in the unmeasured linear Stokes parameter for each telescope and thus can be separated
from the desired signal. Finally, the modulation cycle never mixes the Q and U linear Stokes parameters, and
thus residuals in the modulation do not twist the observed polarization vector. This is advantageous because
measuring the angle of linear polarization is critical for separating the inflationary signal from other polarized
components.
We have successfully fabricated a superconducting transition edge sensor (TES), bolometer that centers on the use of
electron-phonon decoupling (EPD) for thermal isolation. We have selected a design approach that separates the two
functions of far-infrared and THz radiative power absorption and temperature measurement, allowing separate
optimization of the performance of each element. We have integrated molybdenum/gold (Mo/Au) bilayer TES and ion
assisted thermally evaporated (IAE) bismuth (Bi) films as radiation absorber coupled to a low-loss microstripline from
niobium (Nb) ground plane to a twin-slot antenna structure. The thermal conductance (G) and the time constant for the
different geometry device have been measured. For one such device, the measured G is 1.16×10-10 W/K (± 0.61×10-
10 W/K) at 60 mK, which corresponds to noise equivalent power (NEP) = 1.65×10-18W/ √Hz and time constant of ~5 μs.
KEYWORDS: Bolometers, Silicon, Gold, Finite element methods, Signal to noise ratio, Superconductors, Resistance, Vestigial sideband modulation, Sensors, Temperature metrology
We present the experimental results and a bolometer model of the voltage-biased superconducting bolometer
on the low stress silicon nitride (Si3N4) membrane, developed in collaboration between the Max-Planck-Institut
fur Radioastronomie (MPIfR), Bonn and the Institute for Photonic Technology (IPHT), Jena, Germany. The
superconducting thermistor, deposited on the low stress silicon nitride membrane, is a bilayer of gold-palladium
and molybdenum and is designed for a transition temperature of 450 mK. Bolometers for the 1.2 mm atmospheric
window were designed, built and tested. The thermal conductance of the bolometer is tuned by structuring the
silicon nitride membrane into spider-like geometries. The incident radiation is absorbed by crossed dipoles
made from gold-palladium alloy with a surface resistance of 10 Ω/square. Using the COSMOS finite element analysis
package, the thermal conductance is obtained for the bolometers of different geometries. FEA simulations showed
that the deposition of a gold ring around the absorbing area could increase the sensitivity of the bolometer.
Therefore, a gold ring is deposited around the center absorbing patch of the silicon nitride membrane. For the
bolometer with a gold ring, the measured NEP is 1.7 X (see manuscript for formula)
Hz and the time constant is in the range
between 1.4 and 2 ms.
A new facility instrument, the Large APEX Bolometer Camera (LABOCA), developed by the Max-Planck-Institut f&diaeru;r Radioastronomie (MPIfR, Bonn, Germany), has been commissioned in May 2007 for operation on the Atacama Pathfinder Experiment telescope (APEX), a 12 m submillimeter radio telescope located at 5100 m altitude on Llano de Chajnantor in northern Chile. For mapping, this 295-bolometer camera for the 870 micron atmospheric window operates in total power mode without wobbling the secondary mirror. One LABOCA beam is 19 arcsec FWHM and the field of view of the complete array covers 100 square arcmin. Combined with the high efficiency of APEX and the excellent atmospheric transmission at the site, LABOCA offers unprecedented capability in large scale mapping of submillimeter continuum emission. Details of design and operation are
presented.
Ever since the first proposal of the voltage-biased transition-edge bolometer the astrophysics community desired bolometer arrays with as many pixels as possible. With respect to the technical problem due to the need of lots of readout SQUID sensors only with multiplexing it is possible to go beyond a few hundred pixel. A technology which allows the manufacture of detector and readout on one chip would simplify this task substantially. Here we demonstrate the fabrication of a transition edge sensor based on a thermistor out of a molybdenum / gold-palladium bilayer. The alloy of gold-palladium (Au-Pd), which allows the tuning of molybdenum's critical temperature by one order of magnitude, is taken from our foundry process for SQUID manufacturing. Au-Pd can further be used for shunt resistances, absorber patterns and bond pads, and, therefore, it is a good choice for a combined technology. The thermistor is placed on a moderately patterned silicon nitride membrane in the shape of an 8-legged spider. The radiation band of interest is coupled via a conical feed horn to a simple grid of dipole-like antenna patterns. This removes the need for the poorly reproducible high-resistance absorption films for the matching of the free space impedance. The simple detector technology is compatible with the SQUID manufacturing. Hence, some of the SQUID layers can be merged with the corresponding detector layer, i.e. the thermistor wiring and the SQUID washer are made in a single niobium layer. The concept of feed horn coupling eases the design requirements, consequently the SQUID can be placed close to the detector, thereby allowing a simpler wiring to be used and in theory a better performance to be obtained.
We present the experimental results of voltage-biased superconducting bolometers (VSB) on silicon nitride
(Si3N4) membranes with niobium wiring developed in collaboration between the Institut fur Physikalische
Hochtechnologie (IPHT), Jena, Germany and the Max-Planck-Institut fur Radioastronomie (MPIfR), Bonn,
Germany. The bolometer current is measured with the superconducting quantum interference device (SQUID),
and as expected, the current responsivity is proportional to the inverse of the bias voltage. The experiments
were performed with bilayer gold-palladium molybdenum thermistor at 300 mK 3He cooled cryostat and the
desired transition temperature of Tc = 450 mK is achieved. The strong negative electro-thermal feedback of
the VSB maintains the constant bolometer temperature and reduces the response time from 4 ms to 100 μs. We
have tested thermistors of various size and shape on a continuous membrane and achieved a noise equivalent
power (NEP) of 3.5 × 10-16 W/√Hz. The measured NEP is relatively high due to the comparatively high
background and high thermal conductance of the unstructured silicon nitride (Si3N4) membrane. We have
fabricated 8-leg spider structured membranes in three different geometries and the relation between the
geometry and the thermal conductance (G) is studied. Using the COSMOS finite element analysis tool,
we have modeled the TES bolometers to determine the thermal conductance for different geometries and
calculated the various parameters. Due to the demands of large number pixel bolometer camera we plan to
implement multiplex readout with integrated SQUIDs in our design.
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