Electrical Properties of Background-Limited Membrane-Isolation Transition-Edge Sensing Bolometers for Far-IR/Submillimeter Direct-Detection Spectroscopy

We built and measured the electrical properties of membrane-isolation transition-edge sensing bolometers (TESs) suitable for background-limited far-IR/submillimeter direct-detection spectroscopy. Each TES consists of a Mo/Au bilayer patterned onto a suspended, thermally isolated absorber that is connected to the substrate through four Si _x N _y beams deposited by low pressure chemical vapor deposition (LPCVD). We fabricated TESs with straight and meander support beams. The dimensions of the meander (straight) support beams are 700 μm (700 μm) long by 0.25 μm (0.5 μm) thick by 0.35 μm (0.5 μm) wide. We measured I–V characteristics for these TESs and determined that the thermal conductance G equals 72 fW/K (straight) and 19 fW/K (meander) for our best devices. The thermal conductance exhibits a T ^1/2 dependence with temperature which is evidence of effective elastic scattering of the acoustic phonon modes. The transition temperatures T _c for the same TESs are T _c =137 mK (straight) and T _c =71 mK (meander). If we assume the TESs are temperature-fluctuation noise limited, then the derived noise equivalent power (NEP) equals 1.9×10⁻¹⁹ W/Hz^1/2 (straight) and 6.1×10⁻²⁰ W/Hz^1/2 (meander), using our measured values for G and T _c . The meander-beam TES has a derived NEP that is close to two orders of magnitude lower than the state-of-the-art. Finally, we measured an effective time constant τ of about 300 ms (straight) and 400 ms (meander) using electrical and optical pulses. These values for the NEP and τ for the meander-beam TES meet the requirements for the Background-Limited far-IR/Submillimeter Spectrograph (BLISS), a proposed NASA instrument.      

Generation-Recombination Noise: The Fundamental Sensitivity Limit for Kinetic Inductance Detectors

We present measurements of quasiparticle generation-recombination noise in aluminium Microwave Kinetic Inductance Detectors, the fundamental noise source for these detectors. Both the quasiparticle lifetime and the number of quasiparticles can be determined from the noise spectra. The number of quasiparticles saturates to 10?μm^?3 at temperatures below 160?mK, which is shown to limit the quasiparticle lifetime to 4?ms. These numbers lead to a generation-recombination noise limited noise equivalent power (NEP) of 1.5×10^?19?W/Hz^1/2. Since NEP∝N _qp , lowering the number of remnant quasiparticles will be crucial to improve the sensitivity of these detectors. We show that the readout power now limits the number of quasiparticles and thereby the sensitivity.     

Distributed Antenna-Coupled TES for FIR Detector Arrays

We report progress toward large arrays of sensitive TES bolometers for submillimeter and far-infrared wavelengths with noise equivalent power (NEP) suitable for either imaging from a cooled space telescope or ground based spectroscopy. The arrays are based on a pixel design that makes use of a distributed transition edge sensor (TES) coupled to a slot antenna array. We have electrically characterized prototype detectors consisting of 256 TiN hot-electron TES microbolometers biased in parallel with T _c =50 mK. The measured electron-phonon thermal conductance of prototype devices is as low as 1.1 pW/K at 50 mK corresponding to an electrical NEP of 4×10⁻¹⁹ W/Hz^1/2. The time constant of two detectors with different geometries and transition widths was measured under a range of bias conditions. We have measured time constants ≳10⁻³ seconds, which is long enough for straightforward multiplexing with existing multiplexer technology.      

Performance of a Low-Noise Test Facility for the SAFARI TES Bolometer Arrays

We have constructed a test facility for characterizing the focal plane arrays of SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI’s three bolometer arrays are populated with extremely sensitive ( $\mathit{NEP}\sim 2\times 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}$ ) transition edge sensors with a transition temperature close to 100?mK. The extreme sensitivity and low saturation power (～4?fW) of SAFARI’s detectors present challenges to characterizing them. In optimizing the SAFARI Detector System Test Facility we have paid careful attention to stray-light exclusion as well as electrical, magnetic, and mechanical isolation. We present measurements verifying the facility’s performance and analyze them in terms of a two-fluid model of the TES current on the transition to investigate the background power level. We have measured a detector NEP of (5.1±0.4)×10^?19?W?Hz^?1/2, showing that the facility is ready to test the SAFARI prototype arrays and is approaching the performance needed for testing the flight arrays.     

Effect of Magnetic Fields on TiAu TES Bolometers for the SAFARI Instrument on the SPICA FIR Telescope

At the Netherlands Institute for Space Research (SRON) we are developing Transition Edge Sensors (TES) bolometers for the SAFARI Imaging Spectrometer on the SPICA telescope. With the current design of the SPICA telescope a background magnetic field of about a Gauss with fluctuations of a couple percent is expected. The effect of this external magnetic field on the bolometer needs to be well understood in order to design effective shielding. We present the results of an investigation of magnetic field on the performance of TES bolometers. For this experiment we used a Ti/Au TES bolometer which has a measured NEP of 5.0×10^?19 W/Hz^1/2. The TES is exposed to a parallel and perpendicular magnetic field of varying strength and the baseline and responsivity are determined. We see that the bolometers are sensitive to the perpendicular field but are relative insensitive to the parallel magnetic field. Fluctuations with a period of 30?mG are present as function of perpendicular magnetic field.     

A Superconducting Resonator with a Hafnium Microbridge at Temperatures of 50–350 mK

A high-quality superconducting resonator with a microbridge of hafnium film for use in a circuit for readout a terahertz-band imaging array with frequency division multiplexing is demonstrated experimentally. The variability of the impedance of the bridge at a frequency of 1.5 GHz, which is a key factor in the control of the quality of the resonator, is studied. The bridge, having a thickness of about 50 nm, a critical temperature T_C ≈ 380 mK, and a plan size of 2.5 × 2.5 μm, was connected as a load of a resonator made of niobium film with a thickness of about 100 nm (T_C ~ 9 K). It is shown that the bridge smoothly changes its impedance proportionally to the bias power in the entire temperature range. The effective thermal insulation of the bridge was measured in a dilution cryostat at temperatures of 50–300 mK. Thermal conductivity G of the bridge was calculated and found to be ~4 × 10^–13 W/K, which gives an estimate of the sensitivity of the structure in the bolometric mode NEP ≈ 8 × 10^–19 W/Hz^1/2 at a temperature of 150 mK.     

Signal and noise characteristics of a terahertz frequency-selective YBa2Cu3O7−x Josephson detector

The performance characteristics of a frequency-selective Josephson detector based on a symmetrical [001]-tilt YBa₂Cu₃O_7?x bicrystal junction have been studied in the terahertz frequency range. At an external radiation frequency of 0.692 THz and a detector temperature of 55 K, the volt-watt responsivity reaches (7 ± 2) × 10⁴ V/W, which corresponds to theoretical estimates calculated with allowance for thermal fluctuations in the junction. The noise equivalent power (2.9 ± 0.9) × 10^–13 W/Hz^1/2 and the power dynamic range D = 47 ± 3 dB of the detector are determined by the excess 1/f type noise in the junction. It is shown that NEP values up to 5 × 10^?15 W/Hz^1/2 and D above 60 dB can be reached with high-frequency modulators or pulsed sources of terahertz radiation.     

Optimizing Feedhorn-Coupled TES Polarimeters for Balloon and Space-Based CMB Observations

Maximizing the sensitivity of balloon-based and space-based observations of the cosmic microwave background (CMB) requires detectors with substantially lower saturation power and background noise than ground-based observations, because of reduced atmospheric loading and lower photon noise. We have fabricated and tested prototype transition-edge sensor (TES) bolometers that have architecture identical to that used in feedhorn-coupled TES polarimeter arrays developed for ground-based CMB observations, but have saturation power appropriate for balloon-based or space-based observations (0.5?pW–7?pW). The operating resistance of these bolometers (～3?mΩ) is appropriate for readout with time-division or gigahertz frequency-division SQUID multiplexers. Dark bolometer measurements show that the noise levels are near the expected thermal-fluctuation-noise background (<10^?17?W/Hz^1/2), that the thermal response times are faster than the observation requirements, and that low-frequency 1/f noise can be strongly suppressed to <10?mHz by pair differencing. We report on the performance of the prototype devices and progress towards optimizing them for balloon-based and spaced-based observations.     

Fabrication of Ultrasensitive TES Bolometric Detectors for HIRMES

The high-resolution mid-infrared spectrometer (HIRMES) is a high resolving power (R ~?100,000) instrument operating in the 25–122 μm spectral range and will fly on board the Stratospheric Observatory for Far-Infrared Astronomy in 2019. Central to HIRMES are its two transition edge sensor (TES) bolometric cameras, an 8 × 16 detector high-resolution array and a 64 × 16 detector low-resolution array. Both types of detectors consist of Mo/Au TES fabricated on leg-isolated Si membranes. Whereas the high-resolution detectors, with a noise equivalent power (NEP) ~ 1.5 × 10^?18 W/rt (Hz), are fabricated on 0.45 μm Si substrates, the low-resolution detectors, with NEP ~ 1.0 × 10^?17 W/rt (Hz), are fabricated on 1.40 μm Si. Here, we discuss the similarities and differences in the fabrication methodologies used to realize the two types of detectors.     

Development of Adiabatic Demagnetization Refrigerator for X-ray Microcalorimeter Operation

We are developing of adiabatic demagnetization refrigerator for X-ray microcalorimeter operation. By improving thermal environment in the cryostat, it achieved the lowest temperature of 80 mK, temperature stability of 9?μK rms at 100?mK, and a hold time of 7 hours at 100?mK. The excess heat load to the saltpill decreased from 10?μW to 3?μW. Using this system, we demonstrated a measurement of R–T curve of a TES microcalorimeter and ?–V curve of a SQUID.     

Experimental investigation of a matrix receiver for a 3-mm waveband imaging system

The characteristics of a planar 8 × 8 array of a 3-mm waveband receiver antenna elements loaded with low-barrier planar Mott diodes have been studied. Experimental data on the radiation-directivity patterns (RP) and sensitivity of antenna elements in the two-dimensional array are presented. The average RP width for central elements amounts to ～35° in the E plane and ～50° in the H plane. In the operating frequency range (90–100 GHz) of the matrix receiver, the average voltage responsivity is on the order of 9000 V/W at a noise equivalent power of about 5 × 10^?12 W/Hz^1/2.     

Noise and structural characteristics of high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductor films and the numerical simulation of bolometers based on such films

The structure, electrical properties, and noise characteristics of the epitaxial YBa₂Cu₃O_7?x high-T _c superconductor films grown by laser ablation and magnetron sputtering on CeO₂/Al₂O₃ and LaAlO₃ substrates have been studied. Experimental values of the film parameters (effective noise voltage V _n, bolometer resistance R _b, working temperature T _b, etc.) were used to calculate the main characteristics of bolometers based on such films, intended for the absolute radiometry of synchrotron radiation in the 150–3000 eV range. Numerical estimates of the equivalent noise power, NEP _Σ = 8 × 10^?11?1.3 × 10^?10 W/Hz^0.5, show that the proposed films can be used for the creation of a high-precision absolute radiometer capable of detecting soft x-ray (synchrotron) radiation in a broad frequency range with a power of about 1 μW at an error not exceeding 1%.     

Measurements of Torsional Oscillations and Thermal Conductivity in Solid 4He

Polycrystalline samples of hcp ⁴He of molar volume 19.5?cm³ with small amount of ³He impurities were grown in an annular container by the blocked-capillary method. Three concentrations of ³He, x ₃, were studied: isotopically purified ⁴He with the estimated x ₃≤10^?10, ‘well-grade’ helium with x ₃～3×10^?7 and a specially prepared mixture with x ₃=2.5×10^?6. The torsional oscillator response and thermal conductivity were investigated before and after annealing. The temperature and width of the torsional anomaly increase with increasing x ₃. Annealing resulted in an increased phonon mean free path but often in little change in the torsional oscillator response. While the magnitude of the torsional anomaly and phonon mean free path can be very different in different samples, no correlation was found between them; this implies that these two properties are controlled by different types of crystal defects. It seems plausible that the mean free path of thermal phonos at ～200?mK is controlled by vibrating dislocations while the magnitude of the frequency shift of torsional oscillations is governed by static defects such as pinned dislocations and grain boundaries.     

AC Magnetic Technique to Measure Specific Absorption Rate of Magnetic Nanoparticles

The electrodynamic method is applied to determine the specific absorption rate (SAR) of an assembly of superparamagnetic nanoparticles as a function of frequency and magnetic field amplitude. The home made frequency-adjustable electromagnet is used to create a nearly uniform magnetic field in a core gap of a volume 1×3×3 cm³ in the frequency range f=10–150 kHz and for magnetic field amplitudes up to H ₀=250 Oe. Two oppositely connected pick-up coils are used to record the electromotive force signal (EMF) generated by magnetic nanoparticles. By integrating the EMF signal one can determine the low-frequency hysteresis loops of the assembly and the assembly SAR. Using this method the measurement of SAR has been carried out for magnetite nanoparticles with an average diameter D=25 nm. The electrodynamic method is shown to be capable of measuring a small amount of magnetic nanoparticles, up to 5×10^?5 g, dispersed in a solid matrix. The maximal SAR ～?80 W/g has been obtained for the magnetite nanoparticle assembly investigated.     

The incommensurate solid solution phase Na5−4x Zr1+x(PO4)3:0.04 <x <0.15

The orthophosphate solid solution phase, Na_5?4x Zr_1+x(PO₄)₃:0.04 ? x ? 0.15 has trigonal symmetry with an apparent one dimensional incommensurate superstructure parallel to c_HEX. Using selected area electron diffraction patterns as a guide, an indexing scheme for the powder X-ray data has been devised. The parameter $\text{k = c}{}_{\mathrm{<mtext>supercell</mtext>}}\text{c}{}_{\mathrm{<mtext>subcell</mtext>}}$ varies smoothly with composition from ～ 10.4 at x = 0.04 to ～4.4 at x = 0.11 and is believed to originate in ordering of the extra interstitial Zr⁴⁺ ions. The Na⁺ ion conductivity increases gradually with x and for x = 0.108 varies from ～5×10^?8 ohm^?1 cm^?1 at 25°C to ～1×10^?3 ohm^?1 cm^?1 at 300°C.     

Ultra Low Background Cryogenic Test Facility for Far-Infrared Radiation Detectors

The next generation of far infrared radiation detectors is aimed to reach photon noise limited performance in space based observatories such as SPICA and BLISS. These detectors operate at loading powers of the astronomical signal of a few Attowatt (10^?18 W) or less, corresponding to a sensitivity expressed in noise equivalent power as low as $\mathrm{NEP} = 2\times10^{-20}\ \mbox{W}/\sqrt{\mathrm{Hz}}$ . We have developed a cryogenic test setup for microwave Kinetic Inductance Detectors (MKIDs) that aims to reach these ultra-low background levels. Stray light is stopped by using a box in a box design with a sample holder inside another closed box. Microwave signals for the MKID readout enters the outer box through custom made coax cable filters. The stray light loading per pixel is estimated to be less than 60×10^?18 W during nominal operation, a number limited by the intrinsic sensitivity of the MKIDs used to validate the system.     

Two-point calibration of standard capsule platinum resistance thermometers for the range 13.81 K to 273.15 K

A method of calibrating a standard capsule platinum resistance thermometer (prt) over the range 13.81 K to 273.15 K is described. Measurements of the resistance of the prt are needed at only two fixed points, the boiling point of He⁴ (4.2 K) and the ice-point (273.15 K), both of which are easy to realize. For a prt with a residual resistance ratio (R_{4.2 K}/R_{273.15 K}) of less than 4 × 10^?4, the method provides a calibration on ipts 68 with an uncertainty of 20 mK over the entire range 13.81 K to 273.15 K. For prts with residual resistance ratios between 4 × 10^?4 and 7 × 10^?4, the calibration uncertainty is 75 mK from 13.81 K to 40 K and 20 mK from 40 K to 273.15 K.     

Acoustic-wave-stimulated transformations of radiation defects in γ-irradiated n-type silicon crystals

The effect of an ultrasonic treatment (UST) in various regimes (f _UST = 4?30 MHz; W _UST = 0.1?2 W/cm²) on the electrical activity of radiation defects in γ-irradiated (D = 10⁸ and 10⁹ rad) n-type silicon crystals doped with oxygen (～10¹⁸ and <5 × 10¹⁵ cm^?3) has been studied. The energies and concentrations of the electrically active centers have been determined from an analysis of the temperature dependence (100?300 K) of the Hall effect characteristics, assuming a multilevel structure of these centers. The main types of the acoustically active defects, which change the properties of the semiconductor material upon UST, are the A-type centers (E _c ? 0.20 eV) and divacancies (E _c ? 0.26 eV) in Czochralski-grown single crystals and the divacancies and/or P _s?C _i complexes (E _c ? 0.23 eV) in floating-zone-melted samples.     

AC Bias Characterization of Low Noise Bolometers for SAFARI Using an Open-Loop Frequency Domain SQUID-based Multiplexer Operating Between 1 and 5 MHz

SRON is developing the Frequency Domain Multiplexing (FDM) read-out and the ultra low NEP TES bolometers array for the infra-red spectrometer SAFARI on board of the Japanese space mission SPICA. The FDM prototype of the instrument requires critical and complex optimizations. For single pixel characterization under AC bias we are developing a simple FDM system working in the frequency range from 1 to 5 MHz, based on the open loop read-out of a linearized two-stage SQUID amplifier and high Q lithographic LC resonators. We describe the details of the experimental set-up required to achieve low power loading (<1?fW) and low noise ( $\mathrm{NEP}\sim 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}$ ) in the TES bolometers. We conclude the paper by comparing the performance of a $4\times 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}$ TES bolometer measured under DC and AC bias.