Commercialization of Micro-fabrication of Antenna-Coupled Transition Edge Sensor Bolometer Detectors for Studies of the Cosmic Microwave Background

We report on the development of commercially fabricated multichroic antenna-coupled transition edge sensor (TES) bolometer arrays for cosmic microwave background (CMB) polarimetry experiments. CMB polarimetry experiments have deployed instruments in stages. Stage II experiments deployed with O(1000) detectors and reported successful detection of B-mode (divergence-free) polarization pattern in the CMB. Stage III experiments have recently started observing with O(10,000) detectors with wider frequency coverage. A concept for a stage IV experiment, CMB-S4, is emerging to make a definitive measurement of CMB polarization from the ground with O(400,000) detectors. The orders of magnitude increase in detector count for CMB-S4 require a new approach in detector fabrication to increase fabrication throughput and reduce the cost. We report on collaborative efforts with two commercial micro-fabrication foundries to fabricate antenna-coupled TES bolometer detectors. The detector design is based on the sinuous antenna-coupled dichroic detector from the POLARBEAR-2 experiment. The TES bolometers showed the expected I–V response, and the RF performance agrees with the simulation. We will discuss the motivation, design consideration, fabrication processes, test results, and how industrial detector fabrication could be a path to fabricate hundreds of detector wafers for future CMB polarimetry experiments.     

Electromagnetic Design of Feedhorn-Coupled Transition-Edge Sensors for Cosmic Microwave Background Polarimetry

Observations of the cosmic microwave background (CMB) provide a powerful tool for probing the evolution of the early universe. Specifically, precision measurement of the polarization of the CMB enables a direct test for cosmic inflation. A?key technological element on the path to the measurement of this faint signal is the capability to produce large format arrays of background-limited detectors. We describe the electromagnetic design of feedhorn-coupled, TES-based sensors. Each linear orthogonal polarization from the feedhorn is coupled to a superconducting microstrip line via a symmetric planar orthomode transducer (OMT). The symmetric OMT design allows for highly-symmetric beams with low cross-polarization over a wide bandwidth. In addition, this architecture enables a single microstrip filter to define the passband for each polarization. Care has been taken in the design to eliminate stray coupling paths to the absorbers. These detectors will be fielded in the Cosmology Large Angular Scale Surveyor (CLASS).     

An All Silicon Feedhorn-Coupled Focal Plane for Cosmic Microwave Background Polarimetry

Upcoming experiments aim to produce high fidelity polarization maps of the cosmic microwave background. To achieve the required sensitivity, we are developing monolithic, feedhorn-coupled transition edge sensor polarimeter arrays operating at 150?GHz. We describe this focal plane architecture and the current status of this technology, focusing on single-pixel polarimeters being deployed on the Atacama B-mode Search (ABS) and an 84-pixel demonstration feedhorn array backed by four 10-pixel polarimeter arrays. The feedhorn array exhibits symmetric beams, cross-polar response <?23?dB and excellent uniformity across the array. Monolithic polarimeter arrays, including arrays of silicon feedhorns, will be used in the Atacama Cosmology Telescope Polarimeter (ACTPol) and the South Pole Telescope Polarimeter (SPTpol) and have been proposed for upcoming balloon-borne instruments.     

Cosmic Microwave Background Polarization Detector with High Efficiency, Broad Bandwidth, and Highly Symmetric Coupling to Transition Edge Sensor Bolometers

We describe a prototype detector system designed for precise measurements of Cosmic Microwave Background polarization. The design combines a quasi-optical polarization modulator, a metal feedhorn, a superconducting planar microwave circuit, and a pair of transition-edge sensor (TES) bolometers operating at <100 mK. The circular feedhorn produces highly symmetric beams with very low cross-polarization. The planar circuit preserves symmetry in coupling to bolometers measuring orthogonal polarizations. We implement the circuit with superconducting niobium transmission lines. Three-dimensional interfaces between the planar circuit and waveguides leading to feedhorn and backshort have been carefully designed with electromagnetic simulations. Power is thermalized in resistors and conducted to bolometers via normal electrons. Our system is designed for a 29–43 GHz signal band. We have tested individual circuit elements in this frequency range. Fabrication of a full single-pixel system is underway.      

Fabrication of Microstripline Wiring for Large Format Transition Edge Sensor Arrays

We have developed a process to integrate microstripline wiring with transition edge sensors (TES). The process includes additional layers for metal-etch stop and dielectric adhesion to enable recovery of parameters achieved in non-microstrip pixel designs. We report on device parameters in close-packed TES arrays achieved with the microstrip process including R _n , G, and T _c uniformity. Further, we investigate limits of this method of producing high-density, microstrip wiring including critical current to determine the ultimate scalability of TES arrays with two layers of wiring.     

Fabrication of Antenna-Coupled KID Array for Cosmic Microwave Background Detection

Kinetic inductance detectors (KIDs) have become an attractive alternative to traditional bolometers in the sub-mm and mm observing community due to their innate frequency multiplexing capabilities and simple lithographic processes. These advantages make KIDs a viable option for the O(500,000) detectors needed for the upcoming Cosmic Microwave Background-Stage 4 experiment. We have fabricated an antenna-coupled MKID array in the 150 GHz band optimized for CMB detection. Our design uses a twin-slot antenna coupled to an inverted microstrip made from a superconducting Nb/Al bilayer as the strip, a Nb ground plane and a SiN\(_x\) dielectric layer in between, which is then coupled to an Al KID grown on high-resistivity Si. We present the fabrication process and measurements of SiN\(_x\) microstrip resonators.     

Thin Gold Covered Titanium Transition Edge Sensor for Optical Measurement

A thin-gold-film-covered titanium transition edge sensor is newly developed for highly reliable optical photon detection. The aim of the gold film is to prevent a formation of a surface oxidation layer (typically 2.8?nm) on titanium that causes severe degradation of the titanium superconductivity. Optical properties for the gold-covered titanium TES embedded in an optical cavity are calculated, and we find that the maximum absorptance and absorption bandwidth will be reduced with increasing a thickness of the gold film. However, more than 99% absorptance can be possible for the gold (10?nm in thickness) and titanium (30?nm) if 11 dielectric layers are used in an anti-reflection coating. A depth profile of a chemical state for the fabricated device was analysed by an X-ray photoelectron spectroscopy. The profile shows no evidence of TiO₂ existence in photoelectron spectrum. Superconducting critical temperature covered with the 10?nm gold were in the range of 200?mK to 320?mK depending on the titanium thickness of 18?nm to 26?nm.     

Large Area Transition Edge Sensor X-ray Microcalorimeters for Diffuse X-ray Background Studies

We are developing transition edge sensor (TES) mirocalorimeters with large area (0.72 mm \(^2\) ) absorbers to study the keV diffuse X-ray background. The goal is to develop a 2 cm \(^2\) array of 256 pixels for a sounding rocket payload. We present a pixel design which includes a Mo/Au bilayer TES coupled to a large (850 x 850 x 0.2 \(\upmu \) m \(^3\) ) gold absorber. Our simulations indicate that such a design can achieve energy resolution as good as 1.6 eV FWHM in our target bandpass of 0.05–1 keV. Additionally, thermal modelling shows that for typical gold layers, the position-dependent variation of the pulse shape over the large area of the absorber is not expected to significantly degrade this energy resolution. An array of devices will be fabricated in late 2013 to test this design.     

Bolometric Detectors for Measurements of the Cosmic Microwave Background

Measurements of the Cosmic Microwave Background radiation provide the strongest support for the standard model of Inflationary Big Bang Cosmology. This paper sketches the impact of past and future CMB measurements on this rapidly developing field. Cryogenic millimeter wave bolometric detectors contribute strongly to this important experimental effort. The first such detectors were developed in the laboratory of Professor Michael Tinkham in the Berkeley Physics Department in the late 1950s. This development was first driven by the study of the superconducting energy gap, and other spectroscopy of other condensed matter systems. Later, it was driven very strongly, by the requirements for measurements of the CMB. This interaction between bolometer developments and the requirements of specific measurements is described. Until the past few years, the most useful bolometers had semiconductor thermistors and JFET readout amplifiers. The new superconducting voltage biased Transition Edge Sensor (TES) bolometers with SQUID readouts are beginning to have an impact and are expected to be the technology of choice in the future.     

Multi-Chroic Dual-Polarization Bolometric Detectors for Studies of the Cosmic Microwave Background

We are developing multi-chroic antenna-coupled Transition Edge Sensor (TES) bolometer detectors for Cosmic Microwave Background (CMB) polarimetry. Multi-chroic detectors increase focal plane area efficiency, and thus the mapping speed per focal plane area, and provide greater discrimination against polarized galactic foregrounds with no increase in weight or cryogenic cost. In each pixel, a silicon lens-coupled dual-polarized sinuous antenna collects photons over a two-octave frequency band. The antenna couples the broadband millimeter wave signal into microstrip transmission lines, and on-chip filter banks split the broadband signal into multiple frequency bands. Separate TES bolometers detect the power in each frequency band and linear polarization state. We will describe the design and performance of these devices and present optical data taken. Our measurements of dual-polarization pixels in multiple frequency bands show beams with percent-level ellipticity, and percent-level cross-polarization leakage. We will also describe the development of large arrays of these multi-chroic pixels. Finally, we will describe kilo-pixel arrays of these detectors planned for the future CMB experiments that will achieve unprecedented mapping speed.     

A Fabrication Route for Arrays of Ultra-low-Noise MoAu Transition Edge Sensors on Thin Silicon Nitride for Space Applications

We describe a process route to fabricate arrays of Ultra-low-Noise MoAu Transition Edge Sensors (TESs). The low thermal conductance required for space applications is achieved using 200?nm-thick Silicon Nitride (SiN _x ) patterned to form long-thin legs with widths of 2.1?μm. Using bilayers formed on SiN _x islands from films with 40 nm-thick Mo and Au thicknesses in the range 30 to 280 nm deposited by dc-sputtering in ultra-high vacuum we can obtain tunable transition temperatures in the range 700 to 70?mK. The sensors use large-area absorbers fabricated from high resistivity, thin-film β-phase Ta to provide impedance-matching to incident radiation. The absorbers are patterned to reduce the heat capacity associated with the nitride support structure and include Au thermalizing features to assist the heat flow into the TES. Arrays of 400 detectors at the pixel spacing required for the long-wavelength band of the far-infrared instrument SAFARI are now being fabricated. Device yields approaching 99% are achieved.     

Site-Specific Magnetic Assembly of Nanowires for Sensor Arrays Fabrication

The effect of variation in local magnetic field on magnetic assembly of 30 and 200 nm diameter Ni nanowires synthesized by template directed electrodeposition was investigated with different materials (Ni–Ni and Ni–Au) and shapes of electrodes. Ni–Au paired electrodes improved confinement of the assembled Ni nanowires across the electrode gap because of the narrower distribution of magnetic field around the gap between the two electrodes. Simulation results indicated a local magnetic field strength at the electrode tip increased by a factor of 2.5 with the use of a needle-shape electrode as compared to rectangular-shape electrode. The resistance of nanowire interconnects increased as the applied voltage was raised, and under the same applied voltage, the increase in resistance is further enhanced at lower temperatures because of higher current density.      

Multi-chroic Dual-Polarization Bolometric Focal Plane for Studies of the Cosmic Microwave Background

We are developing multi-chroic antenna-coupled Transition Edge Sensor (TES) focal planes for Cosmic Microwave Background (CMB) polarimetry. In each pixel, a dual polarized sinuous antenna collects light over a two-octave frequency band. Each antenna couples to the telescope with a contacting silicon lens. The antenna couples the broadband RF signal to microstrip transmission lines, and then filter banks split the broadband signal into several frequency bands. A TES bolometer detects the power in each band and polarization. We will describe the design of this device and demonstrate its performance with optical data measured using prototype pixels. Our measurements show low ellipticity beams, low cross-polarization, and properly partitioned bands in banks of 2, 3, and 7 filters. Finally, we will describe how we will upgrade the Polarbear CMB experiment using the focal planes of these detectors to increase the experiment’s mapping speed and its ability to discriminate between the CMB and polarized foregrounds.     

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.      

Modelling the Resistive State in a Transition Edge Sensor

We have developed a model for the resistive transition in a transition edge sensor (TES) based on the model of a resistively shunted junction, taking into account phase-slips of a superconducting system across the barriers of the tilted washing board potential. We obtained analytical expressions for the resistance of the TES, R(T,I), and its partial logarithmic derivatives α _I and β _I as functions of temperature and current. We have shown that all the major parameters describing the resistive state of the TES are determined by the dependence on temperature of the Josephson critical current, rather than by intrinsic properties of the S-N transition. The complex impedance of a pristine TES exhibits two-pole behaviour due to its own intrinsic reactance.     

TES用超导薄膜制备及特性研究

超导转变边沿传感器(TES)是光强单位坎[德拉]量子化所需的单光子探测器,金属超导薄膜物理特性的研究是TES研究的基础.采用不同电源功率磁控溅射制备高纯Al、Ti超导纳米薄膜,分析其薄膜生长速率随气压和功率的变化.利用表面应力仪及电学测量仪,分析薄膜表面应力及电学特征.最后将薄膜放入商用稀释制冷机中,进行超低温测试,获...     

Gamma-Ray Spectrometer based on a Transition Edge Sensor for Nuclear Materials Analysis

We designed and fabricated a hard X-ray and gamma-ray TES spectrometer for nuclear materials analysis. The superconducting tin absorber is coupled to an Ir/Au TES by using a gold post to improve the thermal contact between the absorber and the TES. The reported energy resolution is 156 eV FWHM at 59.5 keV and 166 eV FWHM at 122 keV gamma-rays. We performed measurement of a Pu sample and clearly separated the \(^{239}\) Pu (56.828 keV) and the \(^{241}\) Am (59.5 keV) peaks by this TES microcalorimeter which cannot be resolved by the HPGe detector.