Superconducting bolometers for millimeter and submillimeter wave astronomy

Abstract

Bolometers are simple and robust incoherent continuum detectors which nevertheless can reach sensitivities close to the fundamental noise limit. This thesis describes the theory, design, fabrication and testing of the superconducting bolometers, developed in collaboration between the Max Planck Institute for Radio Astronomy (MPIfR), Bonn and the Institute for Photonic Technology (IPHT), Jena, Germany. The voltage biased superconducting bolometer (VSB) offers various advantages over the traditional semiconducting bolometer; it is faster, more sensitive, has a higher dynamic range, allows complete microlithographic fabrication and can be multiplexed with Superconducting Quantum Interference Devices (SQUIDs). The low noise SQUID amplifiers operate at bolometer temperatures and have very low power dissipation. The low impedance characteristics of VSBs and SQUIDs makes them less sensitive to microphonic pickup and it is possible to achieve very low noise equivalent power (NEP) levels. The fabrication of bolometers with integrated SQUIDs and the multiplexing electronics will allow the production of bolometer arrays with several hundred or more pixels. <br /> The superconducting thermistor, deposited on the low stress silicon nitride membrane, is a bilayer of gold-palladium and molybdenum and is designed for a transition temperature of 450 mK. Bolometers for the 1.2 mm atmospheric window were designed, built and tested. Different test arrays with seven bolometers were fabricated to study the properties of the thermistor and the silicon nitride membrane. The thermal conductance (G) of the bolometer is tuned by structuring the silicon nitride membrane into spider-like geometries. The bolometers are divided into three different categories, High-G, Medium-G and Low-G, depending on their thermal conductance. The silicon nitride membrane is continuous for the High-G and it is structured into a spider-like geometry for Medium-G and Low-G bolometers. The thermal conductance of Low-G bolometers is too low for operating with a 300 K background, because under this condition, the bolometer will be driven into the normal conducting state by the radiation alone. The thermal conductance of Medium-G bolometers is appropriate for the operation with a 300 K background and for the experimental purposes the silicon nitride membrane of the Medium-G bolometer is structured into 8-legs, 16-legs and 32-legs spider geometries.<br /> The incident radiation is absorbed by crossed dipoles made from gold-palladium (Au-Pd) alloy with a surface resistance of 10 Ohms. The base temperature of 300 mK is provided by a liquid 4He cryostat with integrated 3He stage. The time constant of the bolometer is derived by measuring the modulated signal of a blackbody using a lock-in amplifier. The noise is measured as a timeseries and analyzed using National Instruments’ LabVIEW package. A bolometer model has been developed to understand the physics of the bolometer. Using the COSMOS finite element analysis (FEA) package, the thermal conductance is obtained for the bolometers of different geometries. The ideal performance of the bolometer is derived from VSB theory and the results from the bolometer model are compared with experimental results. <br /> FEA simulations showed that the deposition of a gold (Au) ring around the absorbing area could increase the sensitivity of the bolometer. Therefore, a new Medium-G layout was fabricated, with a gold ring around the absorbing center patch of the silicon nitride membrane. For the Medium-G bolometer without the gold ring, the measured optical noise equivalent power (NEP) is 1.9 × 10−16 W/√Hz and the time constant is in the range between 0.2 and 0.38 ms. For the Medium-G bolometer with gold ring, the measured NEP is 1.7 × 10−16 W/√Hz and the time constant is in the range between 1.4 and 2 ms. The gold ring increases the heat capacity, and this is a way to increase the time constant of the bolometer. This will be useful for time domain multiplexed arrays. The performance of Medium-G bolometers is close to the 300 K background limit in the 1.2 mm atmospheric window.