Transition-edge Bolometers Research Articles

Superconducting detectors and mixers for millimeter and submillimeter astrophysics

Superconducting detectors will play an increasingly significant role in astrophysics, especially at millimeter through far-IR wavelengths, where the scientific opportunities include key problems in astronomy and cosmology. Superconducting detectors offer many benefits: outstanding sensitivity, lithographic fabrication, and large array sizes, especially through the recent development of multiplexing techniques. This paper describes the scientific opportunities, the basic physics of these devices, the techniques for radiation coupling, and reviews the recent progress in direct detectors, such as transition-edge bolometers, and the work on tunnel junction (superconductor-insulator-superconductor) and hot-electron mixers.

Superconducting bolometer array with SQUID readout for submillimetre wavelength detection

We have tested a fully microfabricated superconducting bolometer array for astrophysical observations at submillimetre wavelengths. The system is cooled by a 3He sorption refrigerator operating in a pumped 4He cryostat. The transition edge bolometer uses a bilayer of molybdenum and a gold–palladium alloy with a transition temperature of around 500 mK as the thermometer. The bolometer is voltage biased, and the current is measured by a superconducting quantum interference device (SQUID) ammeter.

YBCO mid-infrared bolometer arrays

A 2/spl times/2 array of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBaCuO) transition edge bolometers has been fabricated on an MgO substrate. The bolometers are of the meander type with line width and line spacing of 40 /spl mu/m. The meander length of a single bolometer is 12.5 mm, covering an area of 1 mm/spl times/1 mm. The voltage responsivity of each pixel, at 10 /spl mu/m wavelength, has been studied as a function of modulation frequency. The array exhibits a uniform response and an electrical noise equivalent power (NEP) of 4.2 /spl times/10/sup -12/ W//spl radic/Hz or an optical NEP of 1 /spl times/10/sup -9/ W//spl radic/Hz. Thermal crosstalk between adjacent pixels has been also studied and first imaging results are presented.

NOISE PROPERTIES OF HIGH TEMPERATURE SUPERCONDUCTING BOLOMETERS

The review describes the noise properties of the high temperature superconducting (HTS) bolometers developed for the applications in the optical electronic devices of infrared and submillimeter wave-lengths. The principle of high-Tc transition edge bolometer operation and bolometer noise theory are considered, taking into account the peculiarities of constant bias current and constant bias voltage modes. The published results of bolometer noise modeling are discussed. Various sources of the excess 1/f-noise in HTS films as temperature sensitive element for bolometer are reviewed, including the experimental data and modern noise models. Comparative analysis of noise characteristics of the most developed HTS bolometers for application (antenna-coupled microbolometers and bolometers based on silicon micromachining technology) is reported.

Effects of electrothermal feedback on operation of high- Tc superconducting transition edge bolometers

Computer simulations of the operation of high- T c superconducting bolometers in the constant voltage and constant current biasing modes were carried out. Thermophysical parameters of practically employed membrane bolometers and microbolometers were used for the calculations.

Model for excess noise in voltage-biased superconducting bolometers

We are developing superconducting transition-edge bolometers for far-infrared and millimeter wavelengths. The bolometers described here are suspended by thin legs of silicon nitride for thermal isolation. At frequencies between 200 mHz and 10-50 Hz these devices show white noise at their thermal fluctuation limit (NEP approximately 10(-17) W/ radicalHz). At higher frequencies a broad peak appears in the noise spectrum, which we attribute to a combination of thermal fluctuations in complex thermal circuits and electrothermal feedback. Detailed noise calculations fit the noise measured in three different devices that were specifically designed to test the model. We discuss how changes in bolometer materials can shift the noise peak above the frequency range of interest for most applications.

Superconducting transition-edge bolometer in a resistive and in an inductive mode

We analyze the energy resolution of a superconducting transition-edge bolometer (STEB) biased with an ac current. We will show that the ac readout method enables us to utilize STEB either monitoring the change in dissipation due to the change in temperature or the change in inductance owing to the increased in number of quasiparticles.

Nonlinearity and electrothermal feedback of high Tc transition edge bolometers

The effects of electrothermal feedback on the performance of a micromachined superconducting high Tc transition edge bolometer, over a temperature range 85–95 K, have been investigated. The system behaves nonlinearly, due to the variation of the resistance-temperature coefficient β with temperature. Optimum operating points of constant current mode (CCM) and constant voltage mode (CVM) modes vary with temperature and biasing conditions. In CCM, effective response time τeff varies little with temperature. The optimum output signal of CVM occurs at approximately 3–5 K lower temperature within the tail region, where β is maximum. The biasing voltage also displays a maximum, above which the bolometric performance degrades. The CCM exhibits a comparatively limited linear dynamic range. Negative thermal feedback causes a decrease of τeff of up to 2 orders of magnitude in CVM. CCM requires precise thermal stabilization at midtransition. Noise in both modes is limited by the 1/f contribution. Maximum D* values are slightly higher for CVM at higher frequencies. Bolometer operation in the CVM mode requires cooling the heatsink below 77 K, but does not need accurate temperature control.

Performance of superconducting transition edge bolometers for infrared photometry and X-ray micro-calorimetry

The performance of voltage biased superconducting transition edge bolometers for infrared photometry and X-ray micro-calorimetry is described. Experimental results for the resistance–temperature curve of the superconducting phase thermometer, the current–voltage characteristic of the bolometer, and its current noise are presented and evaluated. Excess current noise turns out to be present when the bolometers are biased in the superconducting transition. The possible origin of this excess noise is shortly discussed. For the infrared bolometer an NEP of 2 ×10 −17 W/ Hz at 270 mK is obtained. The X-ray micro-calorimeter resolution is 60 eV FWHM at 155 mK for 5.9 keV X-rays, a factor 2.6 above the thermodynamical limit.

High-T/sub c/ bolometers with silicon-nitride spiderweb suspension for far-infrared detection

High-T/sub c/ GdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//(GBCO) superconducting transition edge bolometers with operating temperatures near 90 K have been made with both closed silicon-nitride membranes and patterned silicon-nitride (SiN) spiderweb-like suspension structures. As a substrate silicon-on-nitride (SON) wafers are used which are made by fusion bonding of a silicon wafer to a silicon wafer with a silicon-nitride top layer. The resulting monocrystalline silicon top layer on the silicon-nitride membranes enables the epitaxial growth of GBCO. By patterning the silicon-nitride the thermal conductance G is reduced from about 20 to 3 /spl mu/W/K. The noise of both types of bolometers is dominated by the intrinsic noise from phonon fluctuations in the thermal conductance G. The optical efficiency in the far infrared is about 75% due to a goldblack absorption layer. The noise equivalent power NEP for FIR detection is 1.8 pW//spl radic/Hz, and the detectivity D* is 5.4/spl times/10/sup 10/ cm /spl radic/Hz/W. Time constants are 0.1 and 0.6 s, for the closed membrane and the spiderweb like bolometers respectively. The effective time constant can be reduced with about a factor 3 by using voltage bias. Further reduction necessarily results in an increase of the NEP due to the 1/f noise of the superconductor.

A fully lithographed voltage-biased superconducting spiderweb bolometer

We describe the fabrication and characterization of superconducting transition-edge bolometers for astrophysical applications at far-infrared and mm wavelengths. The sensor is voltage biased and the current is measured with a superconducting quantum interference ammeter. Strong negative electrothermal feedback keeps the sensor temperature nearly constant, reduces the response time significantly, and improves linearity. It also makes the responsivity relatively insensitive to changes in optical background loading and refrigerator temperature. The bolometers are made using standard microlithographic techniques suitable for fabrication of large scale arrays. Detailed measurements of optical response are presented for a range of bias conditions and are compared with theory. Measured noise spectra are shown and a model for the noise is presented.

Voltage-biased superconducting transition-edge bolometer with strong electrothermal feedback operated at 370 mK

We present an experimental study of a composite voltage-biased superconducting bolometer (VSB). The tested VSB consists of a Ti-film superconducting thermometer (T(c) ~375 mK) on a Si substrate suspended by NbTi superconducting leads. A resistor attached to the substrate provides calibrated heat input into the bolometer. The current through the bolometer is measured with a superconducting quantum interference device ammeter. Strong negative electrothermal feedback fixes the bolometer temperature at T(c) and reduces the measured response time from 2.6 s to 13 ms. As predicted, the measured current responsivity of the bolometer is equal to the inverse of the bias voltage. A noise equivalent power of 5 x 10(-17) W/ radicalHz was measured for a thermal conductance G ~ 4.7 x 10(-10) W/K, which is consistent with the expected thermal noise. Excess noise was observed for bias conditions for which the electrothermal feedback strength was close to maximum.

Analysis of Undoped and Ag-doped High-Tc YBCO Superconducting Bolometers Fabricated Using a Novel Anti-Reflective Coating and Photolithographic Technique

AbstractSuperconducting transition edge bolometers have been fabricated on Ag-doped and undoped YBCO layers on Si(100) substrates using a YSZ buffer layer. We have also fabricated bolometer devices using a stacked-structure of undoped and Ag-doped YBCO material. These thin films were deposited using pulsed laser deposition (PLD). These devices were fabricated using a novel photolithographic/anti-reflective coating (ARC) process. The use of the anti-reflective coating shows no degradation in the overall quality of the superconducting film, but rather it serves as a barrier to moisture, and improves the overall optical collection efficiency of the bolometer device. At a bias of ImA, and chopping frequency of 100Hz, we have measured photoresponse as a function of device temperature, calculated responsivity, and estimated noise equivalent power (NEP) for each of the three device structures. Peak photoresponse, responsivity, and estimated NEP for the undoped bolometer device are 12μV, 0.6mV(W/cm2), and 2.03×103 W/cmHz1/2 respectively. Peak photoresponse, responsivity, and estimated NEP for the Ag-doped bolometer device are 50μV, 2.5mV(W/cm2), and 9.40×10−8 W/cmHz1/2 respectively. Peak photoresponse, responsivity, and estimated NEP for the stacked structure bolometer device are 70μV, 3.5mV(W/cm2), and 1.02× 10−7 W/cmHz1/2 respectively. These results are the first for Ag-doped YBCO bolometers, as well as for superconducting bolometers fabricated using our novel photoresist/anti-reflective coating method.

A Third Sound Resonator with Cesium Boundaries

We have created a two dimensional third sound resonator with cesium boundaries on a PMMA-coated glass substrate. At chemical potential values below those necessary for pre-wetting, the cesium boundaries act as non-wet edges, effectively confining the helium to the region enclosed by the cesium. Aluminum transition-edge bolometers and silver electrodes are used as detectors and thermal sources of third sound, respectively. A first-generation resonator is described and preliminary results of third sound attenuation and 4 He - Cs reflection coefficient measurements made in the resonator are presented.

Antireflective coating for BiPbSrCaCuO sensors

The applications of high T/sub c/ superconductors as transition edge bolometers or room temperature thermopiles appear most promising at the long infrared wavelengths. However, the sensor performance at these wavelengths is impeded by the large reflectance of superconductor films. In this work, effects of antireflective coating on the performance of BiPbSrCaCuO sensors were investigated. Thermal evaporation was used to deposit semiconductor CdS onto sputtered BiPbSrCaCuO films. It was found that, in order to achieve transparent and uniform CdS coating, it was necessary to maintain the substrate temperature at 180/spl deg/C during the evaporation. The superconductivity of BiPbSrCaCuO appeared to be not degraded by the deposition of CdS. After the deposition of CdS with a nearly optimum thickness, the reflectance of BiPbSrCaCuO sensor was reduced from 0.8 to below 0.4 for wavelengths above 10 /spl mu/m. In addition, the temperature coefficient of the film resistance was seen to increase in the transition region. The increases of film absorptance and resistance were consistent with the change of bolometric responsivity of the coated sensor. The use of Ge semiconductor for antireflective coating was also studied. A reflectance of 0.16 was achieved at wavelengths near 10 /spl mu/m on Ge coated sensors. This small reflectance was in good agreement with predictions derived from the computed optical constants of BiPbSrCaCuO films.

Receiver measurements at 1267 GHz using a diffusion-cooled superconducting transition-edge bolometer

We report receiver noise temperature measurements using a quasioptically coupled diffusion-cooled hot-electron bolometer mixer at a local oscillator frequency of 1267 GHz, and an intermediate frequency of 1.4 GHz. A best double-sideband (DSB) receiver noise temperature of 1880 K was measured, with an estimated mixer noise of 945 K (DSB), and a conversion efficiency including coupling losses of -19 dB (DSB). The coupled local oscillator power requirement for this device is estimated at roughly 6 nW. The mixer noise temperature is less than twice that measured earlier in a waveguide receiver at 530 GHz, which shows that the sensitivity of this type of mixer does not degrade sharply with increasing frequency in this regime.

A High-Tc Superconductor Bolometer for Remote Sensing of Atmospheric OH

The technological feasibility is being investigated of a high-Tc superconductor transition edge bolometer for far-infrared detection, which can meet the requirements of a Fabry-Perot based satellite instrument designed for remote sensing of atmospheric OH. These include a time constant τ<0.3 s, an operating temperature above 35 K, a diameter of 1.1 mm, and a noise equivalent power (NEP) smaller than about 4.0·10-12 W Hz-1/2 for radiation with λ approximately equals 85 μm. Presently, no other sensor can meet these requirements. A NEP value of 3·10-11 W/Hz1/2 and τ approximately equals 0.4 ms has been realized with high-Tc bolometers on Si membranes with a receiving area of 0.85×0.85 mm2. By replacing the Si by Si3N4 we expect that the thermal conductance G can be reduced by more than a factor 20. This should result in a NEP less than 4·10-12 W/Hz-1/2 and a time constant <0.1 s. A bond-and-etch-back technique is used to prepare a mono crystalline silicon top layer on the Si3N4 membrane, which is necessary for the epitaxial growth of the superconductor. An absorption layer will be added to the detector to enhance the efficiency. Promising candidates for use as an absorption layer are metal black films with an efficiency η around 80% at 85 μm wavelength.

An assessment of synthesis effects on the high-Tc superconducting transition: With application to thin-film bolometer devices

Abstract The characteristics of the transition to superconductivity in R Ba 2 Cu 3 O (7−δ) , where R includes: Y, Gd, Eu and Dy, have been investigated, for applications to transition-edge thin-film bolometers. Devices were fabricated from thin, epitaxial films, which were formed by pulsed-laser deposition (PLD). Bulk ceramic samples (pellets) were synthesized for use as the ablation targets. A study of the (DC) resistive transition width ( ΔT c ) of the bulk samples, reveals that the oxidation annealing cycle plays a dominant role in determining ΔT c . Peak temperatures of the oxygenation must be very carefully controlled. A slightly too high processing temperature causes changes in the grainsurface morphology and an increase in ΔT c . Rare earth ( R ) substitution has a significant effect. Residual phase impurity, off stoichiometry, lower density, pressing-induced orientation and smaller pellet sizes, were all found to be less deleterious to ΔT c . Proper synthesis improves superconductivity, narrowing ΔT c and affecting grain-boundary (GB) morphology, as revealed by XRD, SEM and AC magnetic-susceptibility analyses of the bulk ceramics. The occurrence of a substantial susceptibility-loss (χ″) peak correlates well with (undesired) larger ΔT c transitions. In bulk ceramics this may be accompanied by a toe feature in the resistive transition, i.e. additional width to the lower-half of the transition. Thin, epitaxial films, made by PLD from these same bulk ceramics, indicate that certain types of non-optimal features of the bulk transition are not present in the fabricated films. In particular, the toe feature is removed due to the complete restructuring that occurs during the epitaxial film-growth process by PLD. For films to have narrow transitions, we find that it is necessary for the ablation target to have a sharp onset to its transition. The relationship to infrared bolometer detectors is discussed and examples are reported.

Extended function of a high-T c transition edge bolometer on a micromachined Si membrane

The bolometric performance of a high-Tc transition edge bolometer has been evaluated within the temperature range 80 K&amp;lt;T&amp;lt;300 K. The detectivity D* of the device is peaking at transition midpoint and remains at moderate levels up to room temperature. The bolometric time constant of the device increases from 0.33 ms at transition midpoint to 1.55 ms at ambient temperature. The noise pattern displays 1/f behavior at low frequency and is scaling with bias current and the thermal resistance coefficient β of the superconducting film.

Combined photodiode and high-T/sub c/ transition edge bolometer function on zirconia buffered silicon membranes

We report on a novel photoreceiver on silicon membranes, comprising both, an efficient photodiode or quantum detector and a transition edge bolometer on the same substrate material. The photodiode action of the device originates from the silicon substrate and delivers a fast optical response, with a limited sensitivity range at wavelength <1.1 /spl mu/m. The bolometer response, in contrast, is broadband, but rather slow with response times in the ms regime. The combination of both detectors thus represents a new optical receiver concept, and should be useful for a broad range of optical applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>