M. Kenyon

Antenna-Coupled TES Bolometer Arrays for CMB Polarimetry

We describe the design and performance of polarization selective antenna-coupled TES arrays that will be used in several upcoming Cosmic Microwave Background (CMB) experiments: SPIDER, BICEP-2/SPUD. The fully lithographic polarimeter arrays utilize planar phased-antennas for collimation (F/4 beam) and microstrip filters for band definition (25% bandwidth). These devices demonstrate high optical efficiency, excellent beam shapes, and well-defined spectral bands. The dual-polarization antennas provide well-matched beams and low cross polarization response, both important for high-fidelity polarization measurements. These devices have so far been developed for the 100 GHz and 150 GHz bands, two premier millimeter-wave atmospheric windows for CMB observations. In the near future, the flexible microstrip-coupled architecture can provide photon noise-limited detection for the entire frequency range of the CMBPOL mission. This paper is a summary of the progress we have made since the 2006 SPIE meeting in Orlando, FL.

Progress on background-limited membrane-isolated TES bolometers for far-IR/submillimeter spectroscopy

To determine the lowest attainable phonon noise equivalent power (NEP) for membrane-isolation bolometers, we fabricated and measured the thermal conductance of suspended Si3N4 beams with different geometries via a noise thermometry technique. We measured beam cross-sectional areas ranging from 0.35 × 0.5 μm2 to 135 × 1.0 μm2 and beam lengths ranging from 700 μm to 8300 μm. The measurements directly imply that membrane-isolation bolometers are capable of reaching a phonon noise equivalent power (NEP) of 4×10-20 W/Hz1/2. This NEP is adequate for the Background-Limited Infrared-Submillimeter Spectrograph (BLISS) proposed for the Japanese SPICA observatory, and adequate for NASAs SAFIR observatory, a 10-meter, 4 K telescope to be deployed at L2. Further, we measured the heat capacity of a suspended Si3N4 membrane and show how this result implies that one can make membrane-isolation bolometers with a response time which is fast enough for BLISS.

Antenna-coupled TES Bolometer Arrays for BICEP2/Keck and SPIDER

BICEP2/Keck and SPIDER are cosmic microwave background (CMB) polarimeters targeting the B-mode polarization induced by primordial gravitational waves from inflation. They will be using planar arrays of polarization sensitive antenna-coupled TES bolometers, operating at frequencies between 90 GHz and 220 GHz. At 150 GHz each array consists of 64 polarimeters and four of these arrays are assembled together to make a focal plane, for a total of 256 dual-polarization elements (512 TES sensors). The detector arrays are integrated with a time-domain SQUID multiplexer developed at NIST and read out using the multi-channel electronics (MCE) developed at the University of British Columbia. Following our progress in improving detector parameters uniformity across the arrays and fabrication yield, our main effort has focused on improving detector arrays optical and noise performances, in order to produce science grade focal planes achieving target sensitivities. We report on changes in detector design implemented to optimize such performances and following focal plane arrays characterization. BICEP2 has deployed a first 150 GHz science grade focal plane to the South Pole in December 2009.

Sensitive far-IR survey spectroscopy: BLISS for SPICA

We present a concept for BLISS, a sensitive far-IR-submillimeter spectrograph for SPICA. SPICA is a JAXA-led mission featuring a 3.5-meter telescope actively cooled to below 5K, envisioned for launch in 2017. The low-background platform is especially compelling for moderate-resolution survey spectroscopy, for which BLISS is designed. The BLISS / SPICA combination will offer line sensitivities below 10-20W m-2 in modest integrations, enabling rapid survey spectroscopy of galaxies out to redshift 5. The far-IR fine-structure and molecular transitions which BLISS / SPICA will measure are immune to dust extinction, and will unambiguously reveal these galaxies redshifts, stellar and AGN contents, gas properties, and heavy-element abundances. Taken together, such spectra will reveal the history of galaxies from 1 GY after the Big Bang to the present day. BLISS is comprised of five sub-bands, each with two R ~ 700 grating spectrometer modules. The modules are configured with polarizing and dichroic splitters to provide complete instantaneous spectral coverage in two sky positions. To approach background-limited performance, BLISS detectors must have sensitivities at or below 5 × 10-20W Hz-1/2, and the format is 10 arrays of several hundred pixels each. It is anticipated that these requirements can be met on SPICAs timescale with leg-isolated superconducting (TES) bolometers cooled with a 50 mK magnetic refrigerator.

Characterizing SixNy absorbers and support beams for far-infrared/submillimeter transition-edge sensors

We report on the characterization of SixNy (Si-N) optical absorbers and support beams for transition-edge sensors (TESs). The absorbers and support beams measured are suitable to meet ultra-sensitive noise equivalent power (NEP≤10-19W/√Hz) and effective response time (τ) requirements (τ≤100ms) for space-borne far-infrared( IR)/submillimeter(sub-mm) spectrometers, such as the Background Limited far-Infrared/Sub-mm Spectrograph (BLISS) and the SpicA FAR-infrared Instrument (SAFARI) for the SPace Infrared telescope for Cosmology and Astrophysics (SPICA). The thermal response time (τ0) of an absorber suspended by support beams from a lowtemperature substrate depends on the heat capacity (C) of the absorber and the thermal conductance (G) of the support beams (τ0=C/G). In membrane-isolated TESs for BLISS, the effective response time τ is expected to be a factor of 20 smaller than τ0 because of voltage-biased electrothermal feedback operation and assumption of a reasonable open-loop gain, LI≈20. We present design specifications for the arrays of membrane-isolated ultra-sensitive TESs for BLISS. Additionally, we measured G and τ0 for two Si-N noise thermometry device (NTD) architectures made using different fabrication processes: (1) a solid membrane Si-N absorber suspended by thin and long Si-N support beams and (2) a wire-mesh Si-N absorber suspended by long, and even thinner, Si-N support beams. The measurements of G and τ0 were designed to test suitability of the Si-N thermal performance to meet the demands of the two SPICA instruments. The solid membrane NTD architecture is similar to the TES architecture for SAFARI and the mesh membrane NTD is similar to that of BLISS TESs. We report measured values of G and C for several BLISS and SAFARI NTD devices. We observe that the heat capacity of the solid membrane devices can be reduced to the order of 1fJ/K at 65mK for devices that are wet etched by KOH. However, C for these devices is found to be on the order of 100fJ/K for a dry XeF2 process. The heat capacity is similarly large for the mesh devices produced with a dry XeF2 etch.Next generation cosmic microwave background (CMB) polarization anisotropy measurements will feature focal plane arrays with more than 600 millimeter-wave detectors. We make use of high-resolution photolithography and wafer-scale etch tools to build planar arrays of corrugated platelet feeds in silicon with highly symmetric beams, low cross-polarization and low side lobes. A compact Au-plated corrugated Si feed designed for 150 GHz operation exhibited performance equivalent to that of electroformed feeds: ∼ −0.2 dB insertion loss, < −20 dB return loss from 120 GHz to 170 GHz, < −25 dB side lobes and < −23 dB cross-polarization. We are currently fabricating a 50mm diameter array with 84 horns consisting of 33 Si platelets as a prototype for the SPTpol and ACTpol telescopes. Our fabrication facilities permit arrays up to 150mm in diameter.

Antenna-coupled TES bolometers for CMB polarimetry

We have developed a completely lithographic antenna-coupled bolometer for CMB polarimetry. The necessary components of a millimeter wave radiometer - a beam forming element, a band defining filter, and the TES detectors - are fabricated on a silicon chip with photolithography. The densely populated antennas allow a very efficient use of the focal plane area. We have fabricated and characterized a series of prototype devices. We find that their properties, including the frequency and angular responses, are in good agreement with the theoretical expectations. The devices are undergoing optimization for upcoming CMB experiments.

Antenna-coupled TES Arrays For The BICEP2/Keck and SPIDER polarimeters

The upcoming Cosmic Microwave Background (CMB) experiments BICEP2/Keck and SPIDER will be using planar arrays of polarization sensitive antenna‐coupled TES bolometers, operating at frequencies between 96 GHz and 220 GHz. At 145 GHz each array consists of 64 polarimeters (128 TES sensors) and four of these arrays are assembled together to make a focal plane. The detector arrays are integrated with a time‐domain SQUID multiplexer developed at NIST and read out using the Multi‐Channel Electronics (MCE) developed at the University of British Columbia. We present our progress in characterizing focal plane arrays and SQUID multiplexed readout for BICEP2 and SPIDER, describing testing procedures and giving a summary of dark measurements results, as well as preliminary optical measurements.

The background-limited infrared-submillimeter spectrograph (BLISS) for SPICA: a design study

We are developing the Background-Limited Infrared-Submillimeter Spectrograph (BLISS) for SPICA to provide a breakthrough capability for far-IR survey spectroscopy. SPICAs large cold aperture allows mid-IR to submm observations which are limited only by the natural backgrounds, and BLISS is designed to operate near this fundamental limit. BLISS-SPICA is 6 orders of magnitude faster than the spectrometers on Herschel and SOFIA in obtaining full-band spectra. It enables spectroscopy of dust-obscured galaxies at all epochs back to the rst billion years after the Big Bang (redshift 6), and study of all stages of planet formation in circumstellar disks. BLISS covers 35 - 433 microns range in ve or six wavelength bands, and couples two 2 sky positions simultaneously. The instrument is cooled to 50 mK for optimal sensitivity with an on-board refrigerators. The detector package is 4224 silicon-nitride micro-mesh leg-isolated bolometers with superconducting transition-edge-sensed (TES) thermistors, read out with a cryogenic time-domain multiplexer. All technical elements of BLISS have heritage in mature scientic instruments, and many have own. We report on our design study in which we are optimizing performance while accommodating SPICAs constraints, including the stringent cryogenic mass budget. In particular, we present our progress in the optical design and waveguide spectrometer prototyping. A companion paper in Conference 7741 (Beyer et al.) discusses in greater detail the progress in the BLISS TES bolometer development.

Ultra-sensitive transition-edge sensors (TESs) for far-IR/submm space-borne spectroscopy

We have built surface micromachined thin‐film metallized Si_xN_y optical absorbers for transition‐edge sensors (TESs) suitable for the Background‐Limited far‐IR/Submm Spectrograph (BLISS). BLISS is a broadband (38 μm–433 μm), grating spectrometer consisting of five wavebands each with a modest resolution of R ∼1000. Because BLISS requires the effective noise equivalent power (NEP) of the TES to be below 10^(-19) W/Hz^(1/2), our TESs consist of four long (1000 μm), narrow (0.4 μm), and thin (0.25 μm) Si_xN_y support beams that reduce the thermal conductance G between the substrate and the optical absorber. To reduce the heat capacity of the absorber and make the response time τ fast enough for BLISS, our absorbers are patterned into a mesh geometry with a fill factor of less than 10%. We use a bilayer of Ti/Au to make the effective impedance of the absorber match the impedance of the incoming radiation for each band. Measurements of the response time of the metallized absorbers to heat pulses show that their heat capacity exceeds the predictions of the Debye model. Our results are suggestive that the surface of the low pressure chemical vapor deposition (LPCVD) Si_xN_y used in the absorbers’ construction is the source of microstates that dominate the heat capacity.

Development of fast, background-limited transition-edge sensors for the Background-Limited Infrared/Sub-mm Spectrograph (BLISS) for SPICA

We report experimental progress toward demonstrating background-limited arrays of membrane-isolated transition-edge sensors (TESs) for the Background Limited Infrared/Sub-mm Spectrograph (BLISS). BLISS is a space-borne instrument with grating spectrometers for wavelengths λ= 35-435 μm and with R= λ/Δλ~500. The goals for BLISS TESs are: noise equivalent power (NEP) = 5×10^-20 W/Hz^1/2 and response time τ<30ms. We expect background-limited performance from bilayers TESs with T_C=65mK and G=15fW/K. However, such TESs cannot be operated at 50mK unless stray power on the devices, or dark power P_D, is less than 200aW. We describe criteria for measuring P_D that requires accurate knowledge of T_C. Ultimately, we fabricated superconducting thermistors from Ir (T_C≥135mK) and Mo/Cu proximitized bilayers, where T_C is the thermistor transition temperature. We measured the Ir TES arrays in our 45mK base temperature adiabatic demagnetization refrigerator test system, which can measure up to eight 1x32 arrays simultaneously using a time-division multiplexer, as well as our single-pixel test system which can measure down to 15mK. In our previous Ir array measurements our best reported performance was NEP=2.5×10^-19 W/Hz^1/2 and τ~5ms for straight-beam TESs. In fact, we expected NEP 1.5×10^-19W/Hz^1/2for meander beam TESs, but did not achieve this previously due to 1/f noise. Here, we detail improvements toward measuring the expected NEP and demonstrate NEP=(1.3±0.2)×10^-19W/Hz^1/2 in our single-pixel test system and NEP=(1.6±0.3)×10^-19W/Hz^1/2 in our array test system.