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AIP conference proceedings

Transition Measurements of a Micron-Sized Transition-Edge Hot-Electron Microbolometer

Emily M. Barrentine, Donald E. Brandl, Ari-David Brown, Nga T. Cao, Kevin L. Denis, W.‐T. Hsieh, Thomas R. Stevenson, Peter T. Timbie, K. U.-Yen, Edward J. Wollack, Betty Young, Blas Cabrera, Aaron Miller

Published Jan 1, 2009• 2 citations
Publisher

Abstract

We are developing a Transition‐Edge Hot‐electron Microbolometer (THM) for use in large detector array applications in millimeter‐wave astronomy. This bolometer detector consists of a superconducting bilayer TES with an overlapping thin‐film semi‐metal absorber. The detector is deposited directly on the substrate and thermal isolation of the bolometer is controlled by electron‐phonon scattering within the small volume of the detector. We present measurements characterizing the transition behavior of several micron‐sized THM test devices which are optimized for photon‐noise‐limited observations of the Cosmic Microwave Background (CMB). We interpret these measurements in terms of a lateral proximity effect between the TES and the superconducting Nb TES leads. We discuss possible modifications to the THM design to compensate for this effect while retaining the small detector volume necessary to obtain the desired value of electron‐phonon thermal conductivity.

DOI
10.1063/1.3292400

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IEEE Transactions on Applied Superconductivity•Mar 1, 2001•6 citations
We are developing a Transition‐Edge Hot‐electron Microbolometer (THM) for use in large detector array applications in millimeter‐wave astronomy. This bolometer detector consists of a superconducting bilayer TES with an overlapping thin‐film semi‐metal absorber. The detector is deposited directly on the substrate and thermal isolation of the bolometer is controlled by electron‐phonon scattering within the small volume of the detector. We present measurements characterizing the transition behavior of several micron‐sized THM test devices which are optimized for photon‐noise‐limited observations of the Cosmic Microwave Background (CMB). We interpret these measurements in terms of a lateral proximity effect between the TES and the superconducting Nb TES leads. We discuss possible modifications to the THM design to compensate for this effect while retaining the small detector volume necessary to obtain the desired value of electron‐phonon thermal conductivity.
Abstract In this paper, we report on the design, fabrication, and characterization of a 220 GHz superconducting transition edge sensor (TES) array developed for ground-based cosmic microwave background (CMB) experiments. Unlike conventional TES arrays adopting thermometers made of bilayer superconducting films, the TES array has thermometers made of simpler single-layer titanium (Ti) film deposited on a suspended silicon nitride (SiN x ) membrane. The thermal weak link is realized by releasing the TES’s thermal island including the Ti thermometer by dry etching with xenon difluoride (XeF2), giving a typical thermal conductance of 25 pW K−1. Its thermal-conductance mechanism is further studied by fitting the measured TES’s complex impedance with a three-block thermal model. The dark and optical noise equivalent power (NEP) of a superconducting TES are both measured. Despite existing lens reflection and dielectric loss in the superconducting microstrip line, its typical optical NEP reaches 100 aW Hz−0.5, which meets the sensitivity requirement for ground-based CMB experiments.
To improve both scalability and noise-filtering capability of a transition-edge sensor (TES), a new concept of a thin-film detector is suggested, which is based on embedding a microbridge TES into a high-Q planar GHz-range resonator weakly coupled to a 50-Ω-readout transmission line. Such a TES element is designed as a hot-electron microbolometer coupled to a THz-range antenna and as a load of the resonator at the same time. A weak THz signal coupled to the antenna heats the microbridge TES, thus reducing the quality factor of the resonator and leading to a power increment in the readout line. The power-to-power conversion gain, an essential figure of merit, is estimated to be above 10. To demonstrate the basic concept, we fabricated and tested a few submicrometer-sized devices from Nb thin films for operation temperature about 5 K. The dc and RF characterization of the new device is made at a resonator frequency about 5.8 GHz. A low-noise high-electron mobility transistor amplifier is used in our TES experiments without the need for a SQUID readout. The optical sensitivity to blackbody radiation within the frequency band 600-700 GHz ismeasured as (2.7 ± 0.9) × 10-14W/√Hz at Tc≈ 5 K at bath temperature ≈1.5 K.
We report on the measurement of a very low noise equivalent power of 3×10−19 W/Hz1/2 at 620 GHz in a superconducting antenna-coupled hot-electron bolometer. The sensing element is a micron-size titanium transition-edge sensor with NbTiN superconducting contacts fabricated on a sapphire substrate. The high sensitivity is due to the small device volume, low operating temperature, and weak electron-phonon coupling in titanium film. Measurements were done using a cryogenic black body emitter producing well-controlled femtowatt power levels. The achieved optical sensitivity is suitable for the low-background spectroscopy of molecular lines on next generation space telescopes.
A hot-electron transition-edge superconducting bolometer with adjustable thermal relaxation speed is proposed. The bolometer contacts are made from a superconductor with high critical temperature which blocks the thermal diffusion of hot carriers into the contacts. Thus electron-phonon interaction is the only mechanism for heat removal. The speed of thermal relaxation for hot electrons in a nanometre-size superconducting bolometer with T c = 100-300 mK is controlled by the elastic electron mean free path l . The relaxation rate behaves as T 4 l at subkelvin temperatures and can be reduced by a factor of 10-100 by decreasing l . Then an antenna- or waveguide-coupled bolometer with a time constant ~10 -3 -10 -4 s will exhibit photon-noise-limited performance at millimetre and submillimetre wavelengths. The bolometer will have a figure of merit NEP 1/2 = 10 -23 -10 -22 J -1 at 100 mK which is 10 3 -10 4 times better (i.e. smaller) than that of a state-of-the-art bolometer. A tremendous increase in speed and sensitivity will have a significant impact for observational mapping applications.
\nSuperconducting hot electron bolometer mixers are now a competitive alternative to Schottky diode mixers in the terahertz frequency range because of their ultra wideband (from millimeter waves to visible light), high conversion gain, and low intrinsic noise level. High Tc superconductor materials can be used to make hot electron bolometers and present some advantage in term of operating temperature and cooling. In this paper, we present first a model for the study of superconducting hot electron bolometers responsivity in direct detection mode, in order to establish a firm basis for the design of future THz mixers. Secondly, an original process to realize YBaCuO hot electron bolometer mixers will be described. Submicron YBaCuO superconducting structures are expitaxially sputter deposited on MgO substrates and patterned by using electron beam lithography in combination with optical lithography. Metal masks achieved by electron beam lithography are insuring a good bridge definition and protection during ion etching. Finally, detection experiments are being performed with a laser at 850 nm wavelength, in homodyne mode in order to prove the feasibility and potential performances of these devices.\n