A Multi-Band Dual-Polarized Antenna-Coupled TES Bolometer

We are developing multi-band dual-polarized antenna-coupled transition edge sensor (TES) bolometers for observing Cosmic Microwave Background (CMB) polarization anisotropies. We have designed a prototype pixel that uses a dual-polarized log-periodic antenna on a silicon hemispherical lens. Each polarization is coupled onto a separate microstrip transmission line. Microstrip filters are used to divide the broadband output of this antenna into a set of narrow frequency bands centered at 90, 150, and 220 GHz with bandwidths of 20%. We report on the fabrication of these devices as well as the initial optical testing.      

Parameter comparison for low-noise Mo/Au TES bolometers

We describe a comparative investigation of the parameters of MoAu-bilayer TES bolometers designed for infrared detectors. A set of devices with variations in geometry were fabricated at the NASA/GSFC detector development facility. These detectors have different bilayer aspect ratios (providing differing normal state resistances and current densities), and have varieties of normal metal regions to study the effects of geometry on noise. These normal metal regions are oriented either parallel to or transverse to the direction of current flow, or both. The lowest noise detectors are found to have normal metal regions oriented transversely. For about a dozen different devices, we have measured a large set of parameters by means of a suite of tests. These include complex impedance measurements to derive time constants; IV curves to determine resistance and power; thermal conductance measurements; noise measurements as a function of device resistance; and direct resistance vs. temperature measurements.     

Design and Fabrication Highlights Enabling a 2 mm, 128 Element Bolometer Array for GISMO

The Backshort-Under-Grid (BUG) superconducting bolometer array architecture is intended to be highly versatile, operating over a large range of wavelengths and background conditions. To validate the basic array design and to demonstrate its applicability for future kilopixel arrays, we will demonstrate a 128-element bolometer array optimized for 2 mm wavelength using a new Goddard Space Flight Center instrument, GISMO (Goddard IRAM Superconducting 2-Millimeter Observer). The design considerations unique to GISMO and laboratory experimental results will be discussed.      

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.      

Indium Hybridization of Large Format TES Bolometer Arrays to Readout Multiplexers for Far-Infrared Astronomy

In our effort to develop large format kilo-pixel arrays of transition edge sensors, we are working toward demonstrating electrical connectivity of a hybridized 32×40 element detector to a NIST multiplexer. The geometry of large-scale arrays that attempt to maintain a high filling factor does not allow for typical fan-out wiring, thus the detector array is hybridized directly to the readout multiplexer. The two parts are bonded together with 10 micron tall indium bumps, which complete the electrical path and also provide mechanical strength. The indium hybridization was developed and performed at the Goddard Space Flight Center and has been shown to withstand tensile loading up to 0.26 gram-force per indium bump. A cavity is deep etched behind each pixel prior to bonding, leaving only 30 micron support walls, which maintains a high filling fraction. Electrical continuity has been shown through the indium path in room temperature measurements. We report on the fabrication process used to hybridize the parts and the subsequent electrical performance.      

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.      

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).      

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.      

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).      

Large Bolometer Arrays with Superconducting NbSi Sensors for Future Space Experiments

New techniques in microelectronics allow to build large arrays of bolometers filling the focal plane of submillimeter and millimeter telescopes. The expected sensitivity increase is the key for the next generation of space experiments in this wavelength range. Superconducting bolometers offer currently the best prospects in terms of sensitivity and multiplexed readout. We present here the developments led in France based on NbSi alloy thermometers. The manufacturing process of a 23 pixel array and the test setup are described.      

Norton-Corrected Measurement of Complex Impedances of a Large Format Bolometer Array

We report on our efforts to measure simultaneously a well-calibrated complex impedance of a large number of detectors in a long wavelength bolometer array. The array is described in other presentations. A method for correcting the complex impedance measurements of bolometers and calorimeters has been presented by Lindeman et al. (Rev. Sci. Instrum. 78:043105, [2007]) using a Thévenin equivalent circuit to represent the bias network. We have built on this method for superconducting bolometers with a Norton equivalent circuit and have used it to improve our impedance data. We further describe our method for extracting a Norton-corrected complex impedance as a function of frequency from a stream of multiplexed time-ordered data. This method is well-suited to producing simultaneous complex impedance measurements for a large number of detectors.      

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.      

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.      

Analyses on the operating point dependence of the energy resolution with a Ti/Au TES microcalorimeter

We examined the performance of a single pixel Ti/Au transition-edge sensor (TES) calorimeter for incident X-ray energies of Al-K, Cr-K, and Fe-K, as a function of the TES resistance. We find that the energy resolution does not always degrade with increasing energy. The best energy resolution of 5.7±0.9 eV at 6.4 keV is obtained, which is possibly even better than the baseline width of 6.5±0.2 eV. Assuming that the noise level is determined by the noise spectrum NS(f;RR+dR(E)) considering the resistance change of dR(E), instead of NS(f;R) at the operating point, these results may be explained by the fact that the noise decreases at the higher TES resistance. The pulse variation appears to have a minimum at a certain resistance of R+dR(E)48 mΩ, and the best energy resolution for each line is obtained at such an operating point, respectively. The pulse variation could be enhanced when the fluctuation of the TES sensitivity is large at R+dR(E).     

Superconducting Detectors and the Casimir Effect

Many types of superconducting detectors naturally form Casimir cavities (Superconducting Tunnel Junctions, some Transition Edge Sensor geometries). We show that by influencing the energy balance of the superconducting-to-normal (S–N) transition of the cavity boundaries the Casimir effect can have an effect on the microscopic parameters of the detector, such as the superconducting energy gap and critical field. In order to determine whether these effects are practically important a rigorous mathematical discussion of the energy balance of the detector’s transition is required. We discuss the basis for such a rigorous analysis as well as the type of electromagnetism required and the inadequacy of existing Casimir effect solutions when applied to the superconducting system.      

Development of Transition Edge Sensor Gamma-Ray Detectors for Positron-Annihilation-Spectroscopy

We are beginning development of a Transition Edge Sensor (TES) based gamma-ray detector for use in Coincidence-Doppler-Broadening (CDB) Positron-Annihilation-Spectroscopy (PAS) studies. We are specifically motivated to asses and study damage to nuclear Reactor Pressure Vessels (RPVs) which results from neutron irradiation during reactor operation. In this paper we provide a brief introduction to CDB-PAS and discuss our detector design.      

Characterization and modeling of transition edge sensors for high resolution X-ray calorimeter arrays

Characterizing and understanding, in detail, the behavior of a Transition Edge Sensor (TES) is required for achieving an energy resolution of 2 eV at 6 keV desired for future X-ray observatory missions. This paper will report on a suite of measurements (e.g. impedance and I–V among others) and simulations that were developed to extract a comprehensive set of TES parameters such as heat capacity, thermal conductivity, and R(T,I), (T,I), and β_i(T,I) surfaces. These parameters allow for the study of the TES calorimeter behavior at and beyond the small signal regime.     

Scope for replacing the prototype kilogram by an atomic mass unit

The main versions of the redefinition for the international prototype kilogram are examined for best accuracy and transmitting the mass unit. It is concluded that it is most promising to use Avogadro’s number and the atomic mass unit in order to transfer to a new standard for the kilogram, i.e., to introduce an atomic kilogram. __________ Translated from Izmeritel’naya Tekhnika, No. 10, pp. 3–5, October, 2006.     

Constructing models of random processes with specified properties by the Wiener method for identifying dynamic systems

It is shown that the use of the proposed models of random processes with specified statistical properties solves fundamental problems of identifying dynamic systems by Wiener’s method using testing actions of special form. Examples of the construction of these models in the time domain are presented. __________ Translated from Izmeritel’naya Tekhnika, No. 8, pp. 43–46, August, 2006.     

A time-marching scheme based on implicit Green’s functions for elastodynamic analysis with the domain boundary element method

The present work presents an alternative time-marching technique for boundary element formulations based on static fundamental solutions. The domain boundary element method (D-BEM) is adopted and the time-domain Green’s matrices of the elastodynamic problem are considered in order to generate a recursive relationship to evaluate displacements and velocities at each time-step. Taking into account the Newmark method, the Green’s matrices of the problem are numerically and implicitly evaluated, establishing the Green–Newmark method. At the end of the work, numerical examples are presented, verifying the accuracy and potentialities of the new methodology.