Characterization of a High-Performance Ti/Au TES Microcalorimeter with a Central Cu Absorber

We are developing X-ray microcalorimeters based on Ti/Au transition-edge sensors (TES). Among sensors we have fabricated, one with a Cu absorber at the center of the TES shows a particularly good X-ray energy resolution: 1.56 eV at 250 eV and 2.5 eV at 5.9 keV. In this paper, a detailed study of its impedance and noise is presented. The noise is not explained by a sum of known sources. The magnitude of unexplained noise is largest when the sensitivity of the TES on temperature (α) and on current (β) are the highest. The observed relation between the noise level and sensitivity suggests a source of thermal fluctuations inside the TES or between the TES and the absorber. We also found that β is linearly correlated to the product of α and current, which limits the effective sensitivity that is expressed as α/(1+β).      

Heat Sinking and Crosstalk for Large, Close-Packed Arrays of Microcalorimeters

We have studied crosstalk on our 8×8 transition-edge sensor (TES) x-ray microcalorimeter arrays. The shapes of thermal crosstalk pulses are well reproduced as a convolution of heat input from the source pixel and decay in the receiver pixel. We have measured the amount of thermal crosstalk as a function of distance between the source and receiver pixels. Using the results, we have estimated the degradation of energy resolution due to the thermal crosstalk as a function of count rate to determine the level of thermal crosstalk that is needed to satisfy the requirements of NASA’s Constellation-X mission.      

A High Energy Resolution Gamma-Ray TES Microcalorimeter with Fast Response Time

We report on the development of the fast response gamma-ray TES microcalorimeter composed of a bulk Sn absorber coupled to a Ti/Au TES. In order to realize a TES microcalorimeter with a large absorber and a fast response time, besides taking saturation and linearity into account, study of the effect of thermal diffusion in the absorber on energy resolution is essential. Therefore, we performed 3 dimensional simulations using SPICE to calculate the effect of thermal diffusion in the absorber on energy resolution. By fabricating a device with the optimized heat capacity of the absorber and the thermal conductance between the absorber and the TES based on the simulation result, we have demonstrated an energy resolution of 38.4±0.9 eV at 60 keV with a time constant of 0.5 ms.      

Performance of TES X-ray Microcalorimeters with a Novel Absorber Design

Superconducting transition-edge sensor (TES) microcalorimeters have demonstrated the Constellation-X requirements for spectral resolution, speed, and pixel size in a close-packed geometry. We will present our recent breakthrough energy resolution with sensors that have all gold and bismuth-gold absorbers. This has been enabled by cantilevered absorbers that make contact to the TES only in regions that are not part of the active thermometer. With this approach, rapid thermalization of the x-ray energy is achieved and interaction between the absorber and TES sensor films is avoided. This design allows us to obtain uniform high performance and is compatible with large-format, high fill-factor arrays. We will discuss this design, the results we have achieved in 8×8 arrays of these pixels, and the dependence of the performance on the geometry of the absorber contact area and on stress within the sensor.      

Analysis of Nuclear Material by Alpha Spectroscopy with a Transition-Edge Microcalorimeter

We present measurements from a cryogenic microcalorimeter designed to detect alpha particles. The enhanced resolution of microcalorimeter alpha detectors will provide new capabilities for actinide analysis. We demonstrate a spectral resolution of 2.4 keV full width at half maximum (FWHM) for 5.3 MeV alpha particles from a ²¹⁰Po source. In addition, we present an alpha spectrum from ²⁰⁹Po showing the first direct measurement of decay into the ²⁰⁵Pb ground state. Finally, measurements of 100 keV gamma-rays from a Gd source show an ultimate alpha particle resolution of 159 eV to be achievable which may provide an avenue for investigating ion energy loss mechanisms in bulk materials.      

Advances in Fabrication of TES μ-Calorimeter Arrays and Associated Filter Structures for AC-Biased Read Out

In this paper we describe microfabrication developments for a 1024 pixel calorimeter array, to be used in future X-ray spectroscopic space missions. We report on an optimization study of high packing factor, mushroom shaped, Cu/Bi absorbers, for which a sputter deposition technique into a new type of photoresist mould has been developed. We have fabricated a prototype 32×32 array where 98 representable pixels are potentially functioning TESs. To gain more control on the detector pixel performance a new procedure was developed to couple absorber and thermometer, utilizing a thin insulating layer. Advances are reported in the fabrication of superconducting LC resonating circuits, needed for frequency domain multiplexed read out. A quality factor Q∼4000 @ 2.6 MHz has been measured, which was limited by critical current in the circuit.      

A Fabrication Process for Microstrip-Coupled Superconducting Transition Edge Sensors Giving Highly Reproducible Device Characteristics

Astronomical instruments for measuring Cosmic Microwave Background polarisation, such as CLOVER, require large arrays of Superconducting Transition Edge Sensors (TESs). We report recent results from a processing route development aimed at high yield fabrication of microstrip-coupled TESs. The incoming signal is delivered onto a silicon nitride membrane by means of a superconducting microstrip transmission line. This transmission line is then terminated with a thin-film load resistor. The wafer-based fabrication route of the Mo/Cu TESs gives highly reproducible device characteristics in terms of superconducting transition temperature, electrical and thermal characteristics. An overall device yield of 65% has been achieved for a multi-wafer processing run.      

Effect of On-Chip Magnetic Shielding for TES Microcalorimeters

We investigated the magnet field dependence of the X-ray pulse height and the critical current of a Ti/Au bilayer TES micro-calorimeter. The pulse height was strongly affected by the magnetic field intensity applied perpendicularly to the TES surface. We found that the critical current at zero temperature, I _c0, decreased by a factor of two by applying a magnet field of ∼10 μT. Our data are consistent with a TES sensitivity proportional to (I/I _c0)^−2/3, as predicted by the Ginzburg-Landau theory. This fact implies that the shape of the R–T curve of the TES is partly determined by the critical current of the superconductor. In order to make our TES microcalorimeters less sensitive to the external magnetic field, we fabricated devices equipped with on-chip magnetic shielding. One device has a turn-around style electrical lead made of Al, in which the return line is laid beneath the Ti/Au TES. Another device has a Nb layer deposited between a multi-layer membrane. We demonstrated that the devices were usable below 200 μT with small degradation of the pulse height and energy resolution.      

Performance of resistive microcalorimeters and bolometers

Despite the impressive results achieved by microcalorimeters and bolometers, their performance is still significantly worse than that predicted by Mather's ideal model (Appl. Opt. 21 (1982) 1125). The difference is due both to non-ideal effects and to excess noise of unknown origin. The non-ideal effects have been recently quantified and include hot-electron effect, absorber decoupling, thermometer non-ohmic behavior, and all related extra noise sources. The excess noise affects primarily Transition Edge Sensors (TES) and is currently under experimental and theoretical investigation. This paper reviews the origin of non-ideal effects in microcalorimeters and bolometers and their effect on energy resolution and noise equivalent power. It also reviews the results on the characterization and suppression of the excess noise in TES, and the recent theoretical investigations to explain its origin in relation to fundamental physics in superconductors.     

Characterization of Iridium Thin Films for TES Microcalorimeters

We are developing transition edge sensors (TES) using single layer iridium (Ir) as the detector and deposited tin (Sn) as the absorber. We obtained good Ir devices with reproducible and uniform transition temperature around 120 mK, transition widths of 1–2 mK, residual resistivity ratio (RRR) between 1.8–3 and high sensitivity α. Our Ir films are obtained using radio frequency (RF) magnetron sputtering and photolithography. In this paper we present a full low temperature characterization of the Ir films.      

Phonon-Mediated Distributed Transition-Edge-Sensor X-Ray Detector with Deep Trenches

We continue our development of a phonon-mediated distributed-TES X-ray detector. X-rays are absorbed in a large silicon or germanium crystal, and the energy is read out by four distributed TESs. This design takes advantage of existing TES technology while overcoming the difficulties of designing spatially large arrays. In this paper, we discuss three detector designs. First, a silicon detector with 220 μm deep trenches through a 350 μm crystal. Second, a germanium detector with 275 μm deep trenches through a 550 μm crystal. Finally another silicon detector with 330 μm deep trenches through a 350 μm crystal. We discuss energy loss mechanisms in the detector and propose a reason for the energy resolution that we observe.      

Mo/Au Bilayers Deposited by Sputtering at Room Temperature for Transition Edge Sensors Fabrication

We report on the fabrication and characterisation of Mo/Au bilayers which are intended to be used as high resolution cryogenic imaging spectrometers for next generation of X-ray Astrophysical observatories in space. Mo/Au bilayers have been deposited by sputtering at room temperature on Si₃N₄ bulk and membranes. The composition of the bilayers as well as their interfaces have been characterised by Rutherford Backscattering Spectroscopy (RBS). Results indicate absence of interdiffusion. Further structural characterization has been performed by X-ray diffraction and Atomic Force Microscopy. The transition temperature T _c of the Mo/Au bilayers has been tuned by changing the Au layer thickness. Very sharp superconducting transitions have been observed with transition widths as small as 0.3 mK. These films look therefore extremely promising for the intended detectors, as well as for other applications in materials science and nanotechnology.      

Comparative Study of TiAu-Based TES Microcalorimeters with Different Geometries

Seven microcalorimeters with different geometries have been tested and their performance is compared. The study, for TiAu TESs with a Cu absorber, indicates the presence of so-called constant voltage noise and internal thermal fluctuation noise. The constant voltage noise is not changed by a normal metal pattern on the TES, or by a magnetic field. The energy resolution of the detectors, having different heat capacities, is 2.5 and 5.0 eV (at 5.9 keV).      

Iridium thin films deposited via pulsed laser deposition for future applications as transition-edge sensors

The University of Miami has recently started developing and studying high-resolution microcalorimeters operating near 100 mK for X-ray and particle physics and astrophysics. These detectors will be based on Transition Edge Sensors technology fabricated using iridium thin films deposited via the Pulsed Laser Deposition technique. We report here the preliminary result of the room temperature characterization of the Ir thin films, and an overview of future plans to use the films as transition edge sensors.     

Intrinsic noise sources in superconductors near the transition temperature

The performance of Transition Edge Sensors (TES) is limited by excess noise that is not predicted by the current theory of microcalorimeters and bolometers. The nature of this noise is currently unknown, but is likely to be dominated by fundamental physics of supeconductors. The University of Miami has recently started a joint effort between the microcalorimeter group and the superconductivity group to study and characterize the noise in TES. In particular, we plan to investigate the effect of flux motion due to self-field and external field and the effect of fluctuating order parameter by measuring the para-conductivity due to fluctuations in the number of Cooper pairs near the transition. We also plan to characterize the fundamental physical parameters of the TES to better predict their properties. In this paper we report our preliminary qualitative assessment of the problem, based on the literature, and we illustrate the experimental techniques that we plan to use for the investigation.     

Optimization of the TES-Bias Circuit for a Multiplexed Microcalorimeter Array

A transition-edge-sensor (TES) microcalorimeter’s shunt resistor (R _sh) and thermal conductance to the cryogenic bath (G) are often considered to be interchangeable knobs with which to control detector speed. Indeed, for otherwise-identical TES-parameter models, there are many combinations of R _sh and G that give the same decay time-constant (τ _crit). However, our previous work showed that with time- or code-division-multiplexed readout, the distribution of signal-to-noise ratio with frequency, which depends strongly on R _sh and G, is just as important as τ _crit. Here, we present a set of calculations to select the optimal values of R _sh and G, given a linear TES model and count-rate and energy-resolution requirements. Lower G and lower R _sh make multiplexing easier. Our calculations also determine the allowed combination of SQUID-readout noise (S _Φ ) and multiplexer row-period (t _row) and row-count (N _rows). Recent improvements to S _Φ and t _row in the NIST time-division-multiplexing architecture have allowed a NIST eight-pixel TES array to be read out with 2.70?eV (full-width at half-maximum) average energy resolution at 6?keV. The improvements make the X-ray Microcalorimeter Spectrometer co-proposed by NASA and NIST for ESA’s Athena X-ray observatory straightforwardly achievable, including engineering margin, with N _rows=16.     

TES Bolometer Array for the APEX-SZ Camera

We will report on the APEX-SZ bolometer camera which houses a 320 element Transition-Edge Sensor (TES) bolometer array designed to survey for galaxy clusters using the 12-meter diameter APEX telescope sited in Chile. Design and fabrication of the TES bolometer array will be discussed, as well as its integration with a frequency-domain SQUID multiplexed readout system. The full configuration of the APEX-SZ camera was deployed in April 2007. A preliminary galaxy cluster map from this deployment will be presented.      

Microcalorimeter Instruments for the Spectrum-R(X)G and NeXT Missions

X-ray spectrometers utilizing a microcalorimeter array are presently under study for the Russian Spectrum R-G (or Spectrum-X-Gamma) mission, which is to be launched in 2011, and for the Japanese NeXT (New X-ray Telescope or Non-thermal energy eXploration Telescope) mission, whose launch is expected to be in 2012 to 2015. The primary instrument of Spectrum R-G is eROSITA, which will make an all sky survey in the 0.1–10 keV range using an array of seven telescopes and X-ray CCD cameras. The mission also carries smaller instruments, a wide-field monitor (Lobster) and a hard X-ray telescope (ART). We are proposing SXC—the Spectrum-X Calorimeter—to obtain spatially-resolved precision spectra of a number of nearby massive clusters of galaxies during an initial 6-month pointed phase, and to obtain a detailed spectral map of the soft X-ray diffuse background during the 4-year survey phase. The NeXT mission is a combination of wide band X-ray spectroscopy provided by multi-layer coating, focusing X-ray mirrors and pixel detectors, and high resolution soft X-ray spectroscopy by microcalorimeter instrument, SXS—the Soft X-ray Spectrometer. The effective area of the SXS is about 20 times larger than that of SXC at the iron K line energy (6.7 keV) while the solid angle of the field of view is by a factor of 15 smaller. One of the major scientific objectives of SXS is to determine turbulent and/or macroscopic velocities in the hot gas of distant clusters of galaxies. Both of the instruments will use 6×6 microcalorimeter array similar to the one launched on Suzaku, while both will adopt a ³He Joule Thomson cooler and two-stage Stirling cycle in the cryogenic systems. The ³He Joule Thomson cooler provides a thermal guard to liquid He but it can also work as a 1.8 K heat bath for the adiabatic demagnetization refrigerator.      

The non-equilibrium response of a high-resolution Ti/Au X-ray microcalorimeter

We have studied the response of a high resolution Ti/Au transition edge sensor (TES) microcalorimeter with an energy resolution of 6.2±0.7 eV at 1.5 keV and 7.8±0.9 eV at 6.4 keV. We find that the sensitivity ≡R/T dR/dT of the TES varies significantly along the superconducting-to-normal transition that the bias point traverses during a pulse. Furthermore, is reduced significantly during a pulse compared to its equilibrium value calculated from the DC characteristics. This leads to an overestimate of the expected energy resolution when basing the prediction on the equilibrium sensitivity.     

Introduction to TES Physics

Superconducting Transition Edge sensors (TES) devices are now in use across a broad range of applications including dark matter searches, gamma ray and x-ray spectroscopy, UV-optical-IR spectroscopy, IR bolometry and CMB bolometers. We summarize the effects that must be taken into account in the design of TES devices. These include determining time constants, maintaining voltage bias, avoiding electrothermal oscillations, critical current limitations, saturation and preventing normal-superconducting phase separation. We summarize our understanding of excess noise seen in most TES devices.